Instructural biology, a protein subunit is a polypeptide chain or single protein molecule that assembles (or "coassembles") with others to form a protein complex.[1][2][3] Large assemblies of proteins such as viruses often use a small number of types of protein subunits as building blocks.[4]
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.[5] 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.[6]
Naturally occurring proteins that have a relatively small number of subunits are referred to as oligomeric.[7] For example, hemoglobin is a symmetrical arrangement of two identical α-globin subunits and two identical β-globin subunits.[3][8] 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).[9]
The subunits of a protein complex may be identical, homologous or totally dissimilar and dedicated to disparate tasks.[1] 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.[10] Although telomerase has telomerase reverse transcriptase as a catalytic subunit, regulation is accomplished by factors outside the protein.[11]
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.[12] 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.
