In structural biology, a protein subunit is a polypeptide chain or single protein molecule that assembles (or "coassembles") with others to form a protein complex.<ref name="Stoker">Template:Cite book</ref><ref name="Smith">Template:Cite book</ref><ref name="Alberts">Template:Cite book</ref> Large assemblies of proteins such as viruses often use a small number of types of protein subunits as building blocks.<ref name="Kumar">Template:Cite book</ref><ref>Template:Cite journal</ref>
A subunit is often named with a Greek or Roman letter, and the numbers of this type of subunit in a protein is indicated by a subscript.<ref name="Lesieur">Template:Cite book</ref> For example, ATP synthase has a type of subunit called α. Three of these are present in the ATP synthase molecule, leading to the designation α3. Larger groups of subunits can also be specified, like α3β3-hexamer and c-ring.<ref name="Ahmad">Template:Cite journal</ref>
Naturally occurring proteins that have a relatively small number of subunits are referred to as oligomeric.<ref name=IUPAC1966>Template:Cite journalQuote: Oligomer molecule: A molecule of intermediate relative molecular mass, the structure of which essentially comprises a small plurality of units derived, actually or conceptually, from molecules of lower relative molecular mass.</ref> For example, hemoglobin is a symmetrical arrangement of two identical α-globin subunits and two identical β-globin subunits.<ref name="Alberts"/><ref name="Liu">Template:Cite book</ref> Longer multimeric proteins such as microtubules and other cytoskeleton proteins may consist of very large numbers of subunits. For example, dynein is a multimeric protein complex involving two heavy chains (DHCs), two intermediate chains (ICs), two light-intermediate chains (LICs) and several light chains (LCs).<ref name="Dharan">Template:Cite journal</ref>
The subunits of a protein complex may be identical, homologous or totally dissimilar and dedicated to disparate tasks.<ref name="Stoker"/> In some protein assemblies, one subunit may be a "catalytic subunit" that enzymatically catalyzes a reaction, whereas a "regulatory subunit" will facilitate or inhibit the activity.<ref name="Søberg">Template:Cite journal</ref> Although telomerase has telomerase reverse transcriptase as a catalytic subunit, regulation is accomplished by factors outside the protein.<ref name="pmid22381618">Template:Cite journal</ref>
An enzyme composed of both regulatory and catalytic subunits when assembled is often referred to as a holoenzyme. For example, class I phosphoinositide 3-kinase is composed of a p110 catalytic subunit and a p85 regulatory subunit.<ref>Template:Cite journal</ref> One subunit is made of one polypeptide chain. A polypeptide chain has one gene coding for it – meaning that a protein must have one gene for each unique subunit.