Editing Active site (section)

==Examples of enzyme catalysis mechanisms==
In reality, most enzyme mechanisms involve a combination of several different types of catalysis.

===[[Glutathione reductase]]===
[[File:GSR Catalytic Cycle.PNG|thumb|the mechanism of glutathione reductase|400px]]

The role of [[glutathione]](GSH) is to remove accumulated reactive oxygen species which may damage cells. During this process, its [[thiol]] side chain is [[oxidation|oxidised]] and two glutathione molecules are connected by a [[disulphide bond]] to form a [[dimer (chemistry)|dimer]](GSSG). In order to regenerate glutathione the disulphide bond has to be broken, In human cells, this is done by [[glutathione reductase]](GR).{{citation needed|date=June 2024}}

Glutathione reductase is a dimer that contains two identical subunits. It requires one [[Nicotinamide adenine dinucleotide phosphate|NADP]] and one [[Flavin adenine dinucleotide|FAD]] as the [[cofactor (biochemistry)|cofactor]]s. The active site is located in the linkage between two subunits. The NADPH is involved in the generation of FADH-. In the active site, there are two [[cysteine]] residues besides the FAD cofactor and are used to break the disulphide bond during the catalytic reaction. NADPH is bound by three positively charged residues: Arg-218, His-219 and Arg-224.{{citation needed|date=June 2024}}

The catalytic process starts when the FAD is [[Organic redox reaction|reduced]] by NADPH to accept one electron and from FADH<sup>−</sup>. It then attacks the disulphide bond formed between 2 cysteine residues, forming one SH bond and a single S<sup>−</sup> group. This S<sup>−</sup> group will act as a nucleophile to attack the disulphide bond in the oxidised glutathione(GSSG), breaking it and forming a cysteine-SG complex. The first SG<sup>−</sup> anion is released and then receives one proton from adjacent SH group and from the first glutathione monomer. Next the adjacent S<sup>−</sup> group attack disulphide bond in cysteine-SG complex and release the second SG<sup>−</sup> anion. It receives one proton in solution and forms the second glutathione monomer.

<ref name=":0" />{{Rp|137–9}}

===Chymotrypsin===
[[File:Mechanism of peptide bond cleavage in a-chymotrypsin.svg|thumb|Mechanism of peptide bond cleavage by chymotrypsin.|400px]]
[[Chymotrypsin]] is a [[serine protease|serine endopeptidase]] that is present in [[pancreatic juice]] and helps the [[hydrolysis]] of [[proteins]] and [[polypeptides|peptide]].<ref name=":0" />{{Rp|84–6}} It  catalyzes the hydrolysis of peptide bonds in [[Stereoisomerism|L-isomers]] of [[tyrosine]], [[phenylalanine]], and [[tryptophan]]. In the active site of this enzyme, three amino acid residues work together to form a [[catalytic triad]] which makes up the catalytic site. In chymotrypsin, these residues are Ser-195, His-57 and Asp-102.

The mechanism of chymotrypsin can be divided into two phases. First, Ser-195 nucleophilically attacks the [[peptide bond]] carbon in the substrate to form a tetrahedral intermediate. The nucleophilicity of Ser-195 is enhanced by His-57, which abstracts a proton from Ser-195 and is in turn stabilised by the negatively charged [[carboxylate]] group (RCOO<sup>−</sup>) in Asp-102. Furthermore, the tetrahedral [[oxyanion]] intermediate generated in this step is stabilised by [[hydrogen bonds]] from Ser-195 and Gly-193.

In the second stage, the R'NH group is protonated by His-57 to form [[Amine|R'NH<sub>2</sub>]] and leaves the intermediate, leaving behind the [[acyl group|acylated]] Ser-195. His-57 then acts as a base again to abstract one proton from a water molecule. The resulting [[hydroxide]] anion nucleophilically attacks the acyl-enzyme complex to form a second tetrahedral oxyanion intermediate, which is once again stabilised by H bonds. In the end, Ser-195 leaves the tetrahedral intermediate, breaking the CO bond that connected the enzyme to the peptide substrate. A proton is transferred to Ser-195 through His-57, so that all three amino acid return to their initial state.