Biosynthesis

Template:Short description Template:Multiple issues Biosynthesis, i.e., chemical synthesis occurring in biological contexts, is a term most often referring to multi-step, enzyme-catalyzed processes where chemical substances absorbed as nutrients (or previously converted through biosynthesis) serve as enzyme substrates, with conversion by the living organism either into simpler or more complex products. Examples of biosynthetic pathways include those for the production of amino acids, lipid membrane components, and nucleotides, but also for the production of all classes of biological macromolecules, and of acetyl-coenzyme A, adenosine triphosphate, nicotinamide adenine dinucleotide and other key intermediate and transactional molecules needed for metabolism. Thus, in biosynthesis, any of an array of compounds, from simple to complex, are converted into other compounds, and so it includes both the catabolism and anabolism (building up and breaking down) of complex molecules (including macromolecules). Biosynthetic processes are often represented via charts of metabolic pathways. A particular biosynthetic pathway may be located within a single cellular organelle (e.g., mitochondrial fatty acid synthesis pathways), while others involve enzymes that are located across an array of cellular organelles and structures (e.g., the biosynthesis of glycosylated cell surface proteins).

Elements of biosynthesis

{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= Template:Ambox }} }} Elements of biosynthesis include: precursor compounds, chemical energy (e.g. ATP), and catalytic enzymes which may need coenzymes (e.g. NADH, NADPH). These elements create monomers, the building blocks for macromolecules. Some important biological macromolecules include: proteins, which are composed of amino acid monomers joined via peptide bonds, and DNA molecules, which are composed of nucleotides joined via phosphodiester bonds.

Properties of chemical reactions

Biosynthesis occurs due to a series of chemical reactions. For these reactions to take place, the following elements are necessary:<ref name=isbn0-8153-3218-1 />

• Precursor compounds: these compounds are the starting molecules or substrates in a reaction. These may also be viewed as the reactants in a given chemical process.
• Chemical energy: chemical energy can be found in the form of high energy molecules. These molecules are required for energetically unfavourable reactions. Furthermore, the hydrolysis of these compounds drives a reaction forward. High energy molecules, such as ATP, have three phosphates. Often, the terminal phosphate is split off during hydrolysis and transferred to another molecule.
• Catalysts: these may be for example metal ions or coenzymes and they catalyze a reaction by increasing the rate of the reaction and lowering the activation energy.

In the simplest sense, the reactions that occur in biosynthesis have the following format:<ref name=Zumdahl>Template:Cite book</ref>

<chem> Reactant ->[][enzyme] Product </chem>

Some variations of this basic equation which will be discussed later in more detail are:<ref name=Voet>Template:Cite book</ref>

1. Simple compounds which are converted into other compounds, usually as part of a multiple step reaction pathway. Two examples of this type of reaction occur during the formation of nucleic acids and the charging of tRNA prior to translation. For some of these steps, chemical energy is required:

   <chem> {Precursor~molecule} + ATP <=> {product~AMP} + PP_i</chem>

2. Simple compounds that are converted into other compounds with the assistance of cofactors. For example, the synthesis of phospholipids requires acetyl CoA, while the synthesis of another membrane component, sphingolipids, requires NADH and FADH for the formation the sphingosine backbone. The general equation for these examples is:

   <chem> {Precursor~molecule} + Cofactor ->[][enzyme] macromolecule </chem>

3. Simple compounds that join to create a macromolecule. For example, fatty acids join to form phospholipids. In turn, phospholipids and cholesterol interact noncovalently in order to form the lipid bilayer. This reaction may be depicted as follows:

   <chem> {Molecule~1} + Molecule~2 -> macromolecule </chem>

Lipid

File:The lipid and lipid bilayer.png

Lipid membrane bilayer

Many intricate macromolecules are synthesized in a pattern of simple, repeated structures.<ref name="Molecular Cell Biology">Template:Cite book</ref> For example, the simplest structures of lipids are fatty acids. Fatty acids are hydrocarbon derivatives; they contain a carboxyl group "head" and a hydrocarbon chain "tail".<ref name="Molecular Cell Biology" /> These fatty acids create larger components, which in turn incorporate noncovalent interactions to form the lipid bilayer.<ref name="Molecular Cell Biology" /> Fatty acid chains are found in two major components of membrane lipids: phospholipids and sphingolipids. A third major membrane component, cholesterol, does not contain these fatty acid units.<ref name=Lehninger>Template:Cite book</ref>

Eukaryotic phospholipids

Template:See also The foundation of all biomembranes consists of a bilayer structure of phospholipids.<ref name=Hanin>Template:Cite book</ref> The phospholipid molecule is amphipathic; it contains a hydrophilic polar head and a hydrophobic nonpolar tail.<ref name="Molecular Cell Biology" /> The phospholipid heads interact with each other and aqueous media, while the hydrocarbon tails orient themselves in the center, away from water.<ref name=Vance /> These latter interactions drive the bilayer structure that acts as a barrier for ions and molecules.<ref name=Katsaras>Template:Cite book</ref>

There are various types of phospholipids; consequently, their synthesis pathways differ. However, the first step in phospholipid synthesis involves the formation of phosphatidate or diacylglycerol 3-phosphate at the endoplasmic reticulum and outer mitochondrial membrane.<ref name=Vance /> The synthesis pathway is found below:

Phosphatidic acid synthesis

The pathway starts with glycerol 3-phosphate, which gets converted to lysophosphatidate via the addition of a fatty acid chain provided by acyl coenzyme A.<ref name="Stryer">Template:Cite book</ref> Then, lysophosphatidate is converted to phosphatidate via the addition of another fatty acid chain contributed by a second acyl CoA; all of these steps are catalyzed by the glycerol phosphate acyltransferase enzyme.<ref name=Stryer /> Phospholipid synthesis continues in the endoplasmic reticulum, and the biosynthesis pathway diverges depending on the components of the particular phospholipid.<ref name=Stryer />

Sphingolipids

Like phospholipids, these fatty acid derivatives have a polar head and nonpolar tails.<ref name=Lehninger /> Unlike phospholipids, sphingolipids have a sphingosine backbone.<ref>Template:Cite book</ref> Sphingolipids exist in eukaryotic cells and are particularly abundant in the central nervous system.<ref name=Vance /> For example, sphingomyelin is part of the myelin sheath of nerve fibers.<ref name=Siegel />

Sphingolipids are formed from ceramides that consist of a fatty acid chain attached to the amino group of a sphingosine backbone. These ceramides are synthesized from the acylation of sphingosine.<ref name=Siegel>Template:Cite book</ref> The biosynthetic pathway for sphingosine is found below:

Sphingosine synthesis

As the image denotes, during sphingosine synthesis, palmitoyl CoA and serine undergo a condensation reaction which results in the formation of 3-dehydrosphinganine.<ref name=Vance /> This product is then reduced to form dihydrospingosine, which is converted to sphingosine via the oxidation reaction by FAD.<ref name=Vance>Template:Cite book</ref>

Cholesterol

This lipid belongs to a class of molecules called sterols.<ref name=Lehninger /> Sterols have four fused rings and a hydroxyl group.<ref name=Lehninger /> Cholesterol is a particularly important molecule. Not only does it serve as a component of lipid membranes, it is also a precursor to several steroid hormones, including cortisol, testosterone, and estrogen.<ref name=Harris>Template:Cite book</ref>

Cholesterol is synthesized from acetyl CoA.<ref name=Harris /> The pathway is shown below:

Cholesterol synthesis pathway

More generally, this synthesis occurs in three stages, with the first stage taking place in the cytoplasm and the second and third stages occurring in the endoplasmic reticulum.<ref name=Stryer /> The stages are as follows:<ref name=Harris />

1. The synthesis of isopentenyl pyrophosphate, the "building block" of cholesterol

2. The formation of squalene via the condensation of six molecules of isopentenyl phosphate

3. The conversion of squalene into cholesterol via several enzymatic reactions

Nucleotides

The biosynthesis of nucleotides involves enzyme-catalyzed reactions that convert substrates into more complex products.<ref name="isbn0-8153-3218-1">Template:Cite book</ref> Nucleotides are the building blocks of DNA and RNA. Nucleotides are composed of a five-membered ring formed from ribose sugar in RNA, and deoxyribose sugar in DNA; these sugars are linked to a purine or pyrimidine base with a glycosidic bond and a phosphate group at the 5' location of the sugar.<ref name="Watson's Molecular Biology of the Gene" />

Purine nucleotides

File:Nucleotides syn1.svg

The synthesis of IMP.

The DNA nucleotides adenosine and guanosine consist of a purine base attached to a ribose sugar with a glycosidic bond. In the case of RNA nucleotides deoxyadenosine and deoxyguanosine, the purine bases are attached to a deoxyribose sugar with a glycosidic bond. The purine bases on DNA and RNA nucleotides are synthesized in a twelve-step reaction mechanism present in most single-celled organisms. Higher eukaryotes employ a similar reaction mechanism in ten reaction steps. Purine bases are synthesized by converting phosphoribosyl pyrophosphate (PRPP) to inosine monophosphate (IMP), which is the first key intermediate in purine base biosynthesis.<ref name="Kappock/Purine">Template:Cite journal</ref> Further enzymatic modification of IMP produces the adenosine and guanosine bases of nucleotides.

1. The first step in purine biosynthesis is a condensation reaction, performed by glutamine-PRPP amidotransferase. This enzyme transfers the amino group from glutamine to PRPP, forming 5-phosphoribosylamine. The following step requires the activation of glycine by the addition of a phosphate group from ATP.
2. GAR synthetase<ref>Template:Cite journal</ref> performs the condensation of activated glycine onto PRPP, forming glycineamide ribonucleotide (GAR).
3. GAR transformylase adds a formyl group onto the amino group of GAR, forming formylglycinamide ribonucleotide (FGAR).
4. FGAR amidotransferase<ref>Template:Cite journal</ref> catalyzes the addition of a nitrogen group to FGAR, forming formylglycinamidine ribonucleotide (FGAM).
5. FGAM cyclase catalyzes ring closure, which involves removal of a water molecule, forming the 5-membered imidazole ring 5-aminoimidazole ribonucleotide (AIR).
6. N5-CAIR synthetase transfers a carboxyl group, forming the intermediate N5-carboxyaminoimidazole ribonucleotide (N5-CAIR).<ref>Template:Cite journal</ref>
7. N5-CAIR mutase rearranges the carboxyl functional group and transfers it onto the imidazole ring, forming carboxyamino- imidazole ribonucleotide (CAIR). The two step mechanism of CAIR formation from AIR is mostly found in single celled organisms. Higher eukaryotes contain the enzyme AIR carboxylase,<ref>Template:Cite journal</ref> which transfers a carboxyl group directly to AIR imidazole ring, forming CAIR.
8. SAICAR synthetase forms a peptide bond between aspartate and the added carboxyl group of the imidazole ring, forming N-succinyl-5-aminoimidazole-4-carboxamide ribonucleotide (SAICAR).
9. SAICAR lyase removes the carbon skeleton of the added aspartate, leaving the amino group and forming 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR).
10. AICAR transformylase transfers a carbonyl group to AICAR, forming N-formylaminoimidazole- 4-carboxamide ribonucleotide (FAICAR).
11. The final step involves the enzyme IMP synthase, which performs the purine ring closure and forms the inosine monophosphate intermediate.<ref name=Lehninger />

Pyrimidine nucleotides

File:Nucleotides syn2.png

Uridine monophosphate (UMP) biosynthesis

Other DNA and RNA nucleotide bases that are linked to the ribose sugar via a glycosidic bond are thymine, cytosine and uracil (which is only found in RNA). Uridine monophosphate biosynthesis involves an enzyme that is located in the mitochondrial inner membrane and multifunctional enzymes that are located in the cytosol.<ref name="Srere/Metabolic enzymes">Template:Cite journal</ref>

1. The first step involves the enzyme carbamoyl phosphate synthase combining glutamine with CO₂ in an ATP dependent reaction to form carbamoyl phosphate.
2. Aspartate carbamoyltransferase condenses carbamoyl phosphate with aspartate to form uridosuccinate.
3. Dihydroorotase performs ring closure, a reaction that loses water, to form dihydroorotate.
4. Dihydroorotate dehydrogenase, located within the mitochondrial inner membrane,<ref name="Srere/Metabolic enzymes" /> oxidizes dihydroorotate to orotate.
5. Orotate phosphoribosyl hydrolase (OMP pyrophosphorylase) condenses orotate with PRPP to form orotidine-5'-phosphate.
6. OMP decarboxylase catalyzes the conversion of orotidine-5'-phosphate to UMP.<ref name="Broach/Yeast nucleotide">Template:Cite book</ref>

After the uridine nucleotide base is synthesized, the other bases, cytosine and thymine are synthesized. Cytosine biosynthesis is a two-step reaction which involves the conversion of UMP to UTP. Phosphate addition to UMP is catalyzed by a kinase enzyme. The enzyme CTP synthase catalyzes the next reaction step: the conversion of UTP to CTP by transferring an amino group from glutamine to uridine; this forms the cytosine base of CTP.<ref name="O'Donovan/ pyrimidine microorg">Template:Cite journal</ref> The mechanism, which depicts the reaction UTP + ATP + glutamine ⇔ CTP + ADP + glutamate, is below:

'Thymidylate synthase reaction: dUMP + 5,10-methylenetetrahydrofolate ⇔ dTMP + dihydrofolate

Ctp synthase mechanism: UTP + ATP + glutamine ⇔ CTP + ADP + glutamate

Cytosine is a nucleotide that is present in both DNA and RNA. However, uracil is only found in RNA. Therefore, after UTP is synthesized, it is must be converted into a deoxy form to be incorporated into DNA. This conversion involves the enzyme ribonucleoside triphosphate reductase. This reaction that removes the 2'-OH of the ribose sugar to generate deoxyribose is not affected by the bases attached to the sugar. This non-specificity allows ribonucleoside triphosphate reductase to convert all nucleotide triphosphates to deoxyribonucleotide by a similar mechanism.<ref name="O'Donovan/ pyrimidine microorg" />

In contrast to uracil, thymine bases are found mostly in DNA, not RNA. Cells do not normally contain thymine bases that are linked to ribose sugars in RNA, thus indicating that cells only synthesize deoxyribose-linked thymine. The enzyme thymidylate synthetase is responsible for synthesizing thymine residues from dUMP to dTMP. This reaction transfers a methyl group onto the uracil base of dUMP to generate dTMP.<ref name="O'Donovan/ pyrimidine microorg" /> The thymidylate synthase reaction, dUMP + 5,10-methylenetetrahydrofolate ⇔ dTMP + dihydrofolate, is shown to the right.

DNA

File:DNA replication en.svg

As DNA polymerase moves in a 3' to 5' direction along the template strand, it synthesizes a new strand in the 5' to 3' direction

Although there are differences between eukaryotic and prokaryotic DNA synthesis, the following section denotes key characteristics of DNA replication shared by both organisms.

DNA is composed of nucleotides that are joined by phosphodiester bonds.<ref name="Molecular Cell Biology" /> DNA synthesis, which takes place in the nucleus, is a semiconservative process, which means that the resulting DNA molecule contains an original strand from the parent structure and a new strand.<ref name="Karp's Cell and Molecular Biology">Template:Cite book</ref> DNA synthesis is catalyzed by a family of DNA polymerases that require four deoxynucleoside triphosphates, a template strand, and a primer with a free 3'OH in which to incorporate nucleotides.<ref name=Griffiths>Template:Cite book</ref>

In order for DNA replication to occur, a replication fork is created by enzymes called helicases which unwind the DNA helix.<ref name=Griffiths /> Topoisomerases at the replication fork remove supercoils caused by DNA unwinding, and single-stranded DNA binding proteins maintain the two single-stranded DNA templates stabilized prior to replication.<ref name="Watson's Molecular Biology of the Gene">Template:Cite book</ref>

DNA synthesis is initiated by the RNA polymerase primase, which makes an RNA primer with a free 3'OH.<ref name=Griffiths /> This primer is attached to the single-stranded DNA template, and DNA polymerase elongates the chain by incorporating nucleotides; DNA polymerase also proofreads the newly synthesized DNA strand.<ref name=Griffiths />

During the polymerization reaction catalyzed by DNA polymerase, a nucleophilic attack occurs by the 3'OH of the growing chain on the innermost phosphorus atom of a deoxynucleoside triphosphate; this yields the formation of a phosphodiester bridge that attaches a new nucleotide and releases pyrophosphate.<ref name=Stryer />

Two types of strands are created simultaneously during replication: the leading strand, which is synthesized continuously and grows towards the replication fork, and the lagging strand, which is made discontinuously in Okazaki fragments and grows away from the replication fork.<ref name="Karp's Cell and Molecular Biology" /> Okazaki fragments are covalently joined by DNA ligase to form a continuous strand.<ref name="Karp's Cell and Molecular Biology" /> Then, to complete DNA replication, RNA primers are removed, and the resulting gaps are replaced with DNA and joined via DNA ligase.<ref name="Karp's Cell and Molecular Biology" />

Amino acids

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} A protein is a polymer that is composed from amino acids that are linked by peptide bonds. There are more than 300 amino acids found in nature of which only twenty two, known as the proteinogenic amino acids, are the building blocks for protein.<ref name="AA structure">Template:Cite journal</ref> Only green plants and most microbes are able to synthesize all of the 20 standard amino acids that are needed by all living species. Mammals can only synthesize ten of the twenty standard amino acids. The other amino acids, valine, methionine, leucine, isoleucine, phenylalanine, lysine, threonine and tryptophan for adults and histidine, and arginine for babies are obtained through diet.<ref>Template:Cite book</ref>

Amino acid basic structure

File:L-amino acid general.svg

L-amino acid

The general structure of the standard amino acids includes a primary amino group, a carboxyl group and the functional group attached to the α-carbon. The different amino acids are identified by the functional group. As a result of the three different groups attached to the α-carbon, amino acids are asymmetrical molecules. For all standard amino acids, except glycine, the α-carbon is a chiral center. In the case of glycine, the α-carbon has two hydrogen atoms, thus adding symmetry to this molecule. With the exception of proline, all of the amino acids found in life have the L-isoform conformation. Proline has a functional group on the α-carbon that forms a ring with the amino group.<ref name="AA structure" />

Glutamine oxoglutarate aminotransferase and glutamine synthetase

Nitrogen source

One major step in amino acid biosynthesis involves incorporating a nitrogen group onto the α-carbon. In cells, there are two major pathways of incorporating nitrogen groups. One pathway involves the enzyme glutamine oxoglutarate aminotransferase (GOGAT) which removes the amide amino group of glutamine and transfers it onto 2-oxoglutarate, producing two glutamate molecules. In this catalysis reaction, glutamine serves as the nitrogen source. An image illustrating this reaction is found to the right.

The other pathway for incorporating nitrogen onto the α-carbon of amino acids involves the enzyme glutamate dehydrogenase (GDH). GDH is able to transfer ammonia onto 2-oxoglutarate and form glutamate. Furthermore, the enzyme glutamine synthetase (GS) is able to transfer ammonia onto glutamate and synthesize glutamine, replenishing glutamine.<ref name="AA Metabolism Miflin">Template:Cite journal</ref>

The glutamate family of amino acids

The glutamate family of amino acids includes the amino acids that derive from the amino acid glutamate. This family includes: glutamate, glutamine, proline, and arginine. This family also includes the amino acid lysine, which is derived from α-ketoglutarate.<ref name="Amino Families">Template:Cite journal</ref>

The biosynthesis of glutamate and glutamine is a key step in the nitrogen assimilation discussed above. The enzymes GOGAT and GDH catalyze the nitrogen assimilation reactions.

In bacteria, the enzyme glutamate 5-kinase initiates the biosynthesis of proline by transferring a phosphate group from ATP onto glutamate. The next reaction is catalyzed by the enzyme pyrroline-5-carboxylate synthase (P5CS), which catalyzes the reduction of the ϒ-carboxyl group of L-glutamate 5-phosphate. This results in the formation of glutamate semialdehyde, which spontaneously cyclizes to pyrroline-5-carboxylate. Pyrroline-5-carboxylate is further reduced by the enzyme pyrroline-5-carboxylate reductase (P5CR) to yield a proline amino acid.<ref>Template:Cite journal</ref>

In the first step of arginine biosynthesis in bacteria, glutamate is acetylated by transferring the acetyl group from acetyl-CoA at the N-α position; this prevents spontaneous cyclization. The enzyme N-acetylglutamate synthase (glutamate N-acetyltransferase) is responsible for catalyzing the acetylation step. Subsequent steps are catalyzed by the enzymes N-acetylglutamate kinase, N-acetyl-gamma-glutamyl-phosphate reductase, and acetylornithine/succinyldiamino pimelate aminotransferase and yield the N-acetyl-L-ornithine. The acetyl group of acetylornithine is removed by the enzyme acetylornithinase (AO) or ornithine acetyltransferase (OAT), and this yields ornithine. Then, the enzymes citrulline and argininosuccinate convert ornithine to arginine.<ref>Template:Cite journal</ref>

File:Lysine Biosynthesis.png

The diaminopimelic acid pathway

There are two distinct lysine biosynthetic pathways: the diaminopimelic acid pathway and the α-aminoadipate pathway. The most common of the two synthetic pathways is the diaminopimelic acid pathway; it consists of several enzymatic reactions that add carbon groups to aspartate to yield lysine:<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

1. Aspartate kinase initiates the diaminopimelic acid pathway by phosphorylating aspartate and producing aspartyl phosphate.
2. Aspartate semialdehyde dehydrogenase catalyzes the NADPH-dependent reduction of aspartyl phosphate to yield aspartate semialdehyde.
3. 4-hydroxy-tetrahydrodipicolinate synthase adds a pyruvate group to the β-aspartyl-4-semialdehyde, and a water molecule is removed. This causes cyclization and gives rise to (2S,4S)-4-hydroxy-2,3,4,5-tetrahydrodipicolinate.
4. 4-hydroxy-tetrahydrodipicolinate reductase catalyzes the reduction of (2S,4S)-4-hydroxy-2,3,4,5-tetrahydrodipicolinate by NADPH to yield Δ'-piperideine-2,6-dicarboxylate (2,3,4,5-tetrahydrodipicolinate) and H₂O.
5. Tetrahydrodipicolinate acyltransferase catalyzes the acetylation reaction that results in ring opening and yields N-acetyl α-amino-ε-ketopimelate.
6. N-succinyl-α-amino-ε-ketopimelate-glutamate aminotransaminase catalyzes the transamination reaction that removes the keto group of N-acetyl α-amino-ε-ketopimelate and replaces it with an amino group to yield N-succinyl-L-diaminopimelate.<ref>Template:Cite journal</ref>
7. N-acyldiaminopimelate deacylase catalyzes the deacylation of N-succinyl-L-diaminopimelate to yield L,L-diaminopimelate.<ref>Template:Cite journal</ref>
8. DAP epimerase catalyzes the conversion of L,L-diaminopimelate to the meso form of L,L-diaminopimelate.<ref>Template:Cite journal</ref>
9. DAP decarboxylase catalyzes the removal of the carboxyl group, yielding L-lysine.

The serine family of amino acids

The serine family of amino acid includes: serine, cysteine, and glycine. Most microorganisms and plants obtain the sulfur for synthesizing methionine from the amino acid cysteine. Furthermore, the conversion of serine to glycine provides the carbons needed for the biosynthesis of the methionine and histidine.<ref name="Amino Families" />

During serine biosynthesis,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> the enzyme phosphoglycerate dehydrogenase catalyzes the initial reaction that oxidizes 3-phospho-D-glycerate to yield 3-phosphonooxypyruvate.<ref>Template:Cite journal</ref> The following reaction is catalyzed by the enzyme phosphoserine aminotransferase, which transfers an amino group from glutamate onto 3-phosphonooxypyruvate to yield L-phosphoserine.<ref>Template:Cite journal</ref> The final step is catalyzed by the enzyme phosphoserine phosphatase, which dephosphorylates L-phosphoserine to yield L-serine.<ref>Template:Cite journal</ref>

There are two known pathways for the biosynthesis of glycine. Organisms that use ethanol and acetate as the major carbon source utilize the glyconeogenic pathway to synthesize glycine. The other pathway of glycine biosynthesis is known as the glycolytic pathway. This pathway converts serine synthesized from the intermediates of glycolysis to glycine. In the glycolytic pathway, the enzyme serine hydroxymethyltransferase catalyzes the cleavage of serine to yield glycine and transfers the cleaved carbon group of serine onto tetrahydrofolate, forming 5,10-methylene-tetrahydrofolate.<ref>Template:Cite journal</ref>

Cysteine biosynthesis is a two-step reaction that involves the incorporation of inorganic sulfur. In microorganisms and plants, the enzyme serine acetyltransferase catalyzes the transfer of acetyl group from acetyl-CoA onto L-serine to yield O-acetyl-L-serine.<ref>Template:Cite journal</ref> The following reaction step, catalyzed by the enzyme O-acetyl serine (thiol) lyase, replaces the acetyl group of O-acetyl-L-serine with sulfide to yield cysteine.<ref>Template:Cite journal</ref>

The aspartate family of amino acids

The aspartate family of amino acids includes: threonine, lysine, methionine, isoleucine, and aspartate. Lysine and isoleucine are considered part of the aspartate family even though part of their carbon skeleton is derived from pyruvate. In the case of methionine, the methyl carbon is derived from serine and the sulfur group, but in most organisms, it is derived from cysteine.<ref name="Amino Families" />

The biosynthesis of aspartate is a one step reaction that is catalyzed by a single enzyme. The enzyme aspartate aminotransferase catalyzes the transfer of an amino group from aspartate onto α-ketoglutarate to yield glutamate and oxaloacetate.<ref>Template:Cite journal</ref> Asparagine is synthesized by an ATP-dependent addition of an amino group onto aspartate; asparagine synthetase catalyzes the addition of nitrogen from glutamine or soluble ammonia to aspartate to yield asparagine.<ref>Template:Cite journal</ref>

File:Lysine Biosynthesis.png

The diaminopimelic acid lysine biosynthetic pathway

The diaminopimelic acid biosynthetic pathway of lysine belongs to the aspartate family of amino acids. This pathway involves nine enzyme-catalyzed reactions that convert aspartate to lysine.<ref name="Lysine pathways">Template:Cite journal</ref>

1. Aspartate kinase catalyzes the initial step in the diaminopimelic acid pathway by transferring a phosphoryl from ATP onto the carboxylate group of aspartate, which yields aspartyl-β-phosphate.<ref>Template:Cite journal</ref>
2. Aspartate-semialdehyde dehydrogenase catalyzes the reduction reaction by dephosphorylation of aspartyl-β-phosphate to yield aspartate-β-semialdehyde.<ref>Template:Cite journal</ref>
3. Dihydrodipicolinate synthase catalyzes the condensation reaction of aspartate-β-semialdehyde with pyruvate to yield dihydrodipicolinic acid.<ref>Template:Cite journal</ref>
4. 4-hydroxy-tetrahydrodipicolinate reductase catalyzes the reduction of dihydrodipicolinic acid to yield tetrahydrodipicolinic acid.<ref>Template:Cite journal</ref>
5. Tetrahydrodipicolinate N-succinyltransferase catalyzes the transfer of a succinyl group from succinyl-CoA on to tetrahydrodipicolinic acid to yield N-succinyl-L-2,6-diaminoheptanedioate.<ref>Template:Cite journal</ref>
6. N-succinyldiaminopimelate aminotransferase catalyzes the transfer of an amino group from glutamate onto N-succinyl-L-2,6-diaminoheptanedioate to yield N-succinyl-L,L-diaminopimelic acid.<ref>Template:Cite journal</ref>
7. Succinyl-diaminopimelate desuccinylase catalyzes the removal of acyl group from N-succinyl-L,L-diaminopimelic acid to yield L,L-diaminopimelic acid.<ref>Template:Cite journal</ref>
8. Diaminopimelate epimerase catalyzes the inversion of the α-carbon of L,L-diaminopimelic acid to yield meso-diaminopimelic acid.<ref>Template:Cite journal</ref>
9. Siaminopimelate decarboxylase catalyzes the final step in lysine biosynthesis that removes the carbon dioxide group from meso-diaminopimelic acid to yield L-lysine.<ref>Template:Cite journal</ref>

Proteins

File:Peptide syn.svg

The tRNA anticodon interacts with the mRNA codon in order to bind an amino acid to growing polypeptide chain.

File:Charge tRNA.png

The process of tRNA charging

Protein synthesis occurs via a process called translation.<ref name=Weaver>Template:Cite book</ref> During translation, genetic material called mRNA is read by ribosomes to generate a protein polypeptide chain.<ref name=Weaver /> This process requires transfer RNA (tRNA) which serves as an adaptor by binding amino acids on one end and interacting with mRNA at the other end; the latter pairing between the tRNA and mRNA ensures that the correct amino acid is added to the chain.<ref name=Weaver /> Protein synthesis occurs in three phases: initiation, elongation, and termination.<ref name="Watson's Molecular Biology of the Gene" /> Prokaryotic (archaeal and bacterial) translation differs from eukaryotic translation; however, this section will mostly focus on the commonalities between the two organisms.

Additional background

Before translation can begin, the process of binding a specific amino acid to its corresponding tRNA must occur. This reaction, called tRNA charging, is catalyzed by aminoacyl tRNA synthetase.<ref name=Copper>Template:Cite book</ref> A specific tRNA synthetase is responsible for recognizing and charging a particular amino acid.<ref name=Copper /> Furthermore, this enzyme has special discriminator regions to ensure the correct binding between tRNA and its cognate amino acid.<ref name=Copper /> The first step for joining an amino acid to its corresponding tRNA is the formation of aminoacyl-AMP:<ref name=Copper />

<chem> {Amino~acid} + ATP <=> {aminoacyl-AMP} + PP_i</chem>

This is followed by the transfer of the aminoacyl group from aminoacyl-AMP to a tRNA molecule. The resulting molecule is aminoacyl-tRNA:<ref name=Copper />

<chem> {Aminoacyl-AMP} + tRNA <=> {aminoacyl-tRNA} + AMP</chem>

The combination of these two steps, both of which are catalyzed by aminoacyl tRNA synthetase, produces a charged tRNA that is ready to add amino acids to the growing polypeptide chain.

In addition to binding an amino acid, tRNA has a three nucleotide unit called an anticodon that base pairs with specific nucleotide triplets on the mRNA called codons; codons encode a specific amino acid.<ref name=Jackson>Template:Cite journal</ref> This interaction is possible thanks to the ribosome, which serves as the site for protein synthesis. The ribosome possesses three tRNA binding sites: the aminoacyl site (A site), the peptidyl site (P site), and the exit site (E site).<ref name=Green>Template:Cite journal</ref>

There are numerous codons within an mRNA transcript, and it is very common for an amino acid to be specified by more than one codon; this phenomenon is called degeneracy.<ref name=Weissbach>Template:Cite book</ref> In all, there are 64 codons, 61 of each code for one of the 20 amino acids, while the remaining codons specify chain termination.<ref name=Weissbach />

Translation in steps

As previously mentioned, translation occurs in three phases: initiation, elongation, and termination.

File:TRNA ribosomes diagram en.svg

Translation

Step 1: Initiation

The completion of the initiation phase is dependent on the following three events:<ref name="Watson's Molecular Biology of the Gene" />

1. The recruitment of the ribosome to mRNA

2. The binding of a charged initiator tRNA into the P site of the ribosome

3. The proper alignment of the ribosome with mRNA's start codon

Step 2: Elongation

Following initiation, the polypeptide chain is extended via anticodon:codon interactions, with the ribosome adding amino acids to the polypeptide chain one at a time. The following steps must occur to ensure the correct addition of amino acids:<ref name=Frank>Template:Cite journal</ref>

1. The binding of the correct tRNA into the A site of the ribosome

2. The formation of a peptide bond between the tRNA in the A site and the polypeptide chain attached to the tRNA in the P site

3. Translocation or advancement of the tRNA-mRNA complex by three nucleotides

Translocation "kicks off" the tRNA at the E site and shifts the tRNA from the A site into the P site, leaving the A site free for an incoming tRNA to add another amino acid.

Step 3: Termination

The last stage of translation occurs when a stop codon enters the A site.<ref name=isbn0-8153-3218-1 /> Then, the following steps occur:

1. The recognition of codons by release factors, which causes the hydrolysis of the polypeptide chain from the tRNA located in the P site<ref name=isbn0-8153-3218-1 />

2. The release of the polypeptide chain<ref name=Weissbach/>

3. The dissociation and "recycling" of the ribosome for future translation processes<ref name=Weissbach/>

A summary table of the key players in translation is found below:

Key players in Translation	Translation Stage	Purpose
tRNA synthetase	before initiation	Responsible for tRNA charging
mRNA	initiation, elongation, termination	Template for protein synthesis; contains regions named codons which encode amino acids
tRNA	initiation, elongation, termination	Binds ribosomes sites A, P, E; anticodon base pairs with mRNA codon to ensure that the correct amino acid is incorporated into the growing polypeptide chain
ribosome	initiation, elongation, termination	Directs protein synthesis and catalyzes the formation of the peptide bond

Diseases associated with macromolecule deficiency

File:Xanthelasma palpebrarum.jpg

Familial hypercholesterolemia causes cholesterol deposits

Errors in biosynthetic pathways can have deleterious consequences including the malformation of macromolecules or the underproduction of functional molecules. Below are examples that illustrate the disruptions that occur due to these inefficiencies.

• Familial hypercholesterolemia: this disorder is characterized by the absence of functional receptors for LDL.<ref name="Bandeali- Familial hypercholesterolemia">Template:Cite journal</ref> Deficiencies in the formation of LDL receptors may cause faulty receptors which disrupt the endocytic pathway, inhibiting the entry of LDL into the liver and other cells.<ref name="Bandeali- Familial hypercholesterolemia" /> This causes a buildup of LDL in the blood plasma, which results in atherosclerotic plaques that narrow arteries and increase the risk of heart attacks.<ref name="Bandeali- Familial hypercholesterolemia" />
• Lesch–Nyhan syndrome: this genetic disease is characterized by self- mutilation, mental deficiency, and gout.<ref name="Kang- Lesch-Nyhan syndrome">Template:Cite journal</ref> It is caused by the absence of hypoxanthine-guanine phosphoribosyltransferase, which is a necessary enzyme for purine nucleotide formation.<ref name="Kang- Lesch-Nyhan syndrome" /> The lack of enzyme reduces the level of necessary nucleotides and causes the accumulation of biosynthesis intermediates, which results in the aforementioned unusual behavior.<ref name="Kang- Lesch-Nyhan syndrome" />
• Severe combined immunodeficiency (SCID): SCID is characterized by a loss of T cells.<ref name="Janeway's Immunobiology">Template:Cite book</ref> Shortage of these immune system components increases the susceptibility to infectious agents because the affected individuals cannot develop immunological memory.<ref name="Janeway's Immunobiology" /> This immunological disorder results from a deficiency in adenosine deaminase activity, which causes a buildup of dATP. These dATP molecules then inhibit ribonucleotide reductase, which prevents of DNA synthesis.<ref name="Janeway's Immunobiology" />
• Huntington's disease: this neurological disease is caused from errors that occur during DNA synthesis.<ref name="Hughes_Huntington's">Template:Cite book</ref> These errors or mutations lead to the expression of a mutant huntingtin protein, which contains repetitive glutamine residues that are encoded by expanding CAG trinucleotide repeats in the gene.<ref name="Hughes_Huntington's" /> Huntington's disease is characterized by neuronal loss and gliosis. Symptoms of the disease include: movement disorder, cognitive decline, and behavioral disorder.<ref name="Biglan- Huntington's">Template:Cite journal</ref>

See also

• Lipids
• Phospholipid bilayer
• Nucleotides
• DNA
• DNA replication
• Proteinogenic amino acid
• Codon table
• Prostaglandin
• Porphyrins
• Chlorophylls and bacteriochlorophylls
• Vitamin B₁₂

References

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