In
structural biology
Structural biology is a field that is many centuries old which, and as defined by the Journal of Structural Biology, deals with structural analysis of living material (formed, composed of, and/or maintained and refined by living cells) at every le ...
, a protein subunit is a
polypeptide chain
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
...
or single
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
molecule that assembles (or "''coassembles''") with others to form a
protein complex
A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multienzyme complexes, in which multiple catalytic domains are found in a single polypeptide chain.
Protein ...
.
Large assemblies of proteins such as
viruses
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's ...
often use a small number of types of protein subunits as building blocks.
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.
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 α
3. Larger groups of subunits can also be specified, like α
3β
3-hexamer and c-ring.
Naturally-occurring proteins that have a relatively small number of subunits are referred to as
oligomer
In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
ic.
[Quote: ''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.''] For example,
hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
is a symmetrical arrangement of two identical α-globin subunits and two identical β-globin subunits.
Longer
multimeric
In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
proteins such as
microtubules and other
cytoskeleton
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is com ...
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).
The subunits of a protein complex may be identical,
homologous or totally dissimilar and dedicated to disparate tasks.
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.
Although
telomerase
Telomerase, also called terminal transferase, is a ribonucleoprotein that adds a species-dependent telomere repeat sequence to the 3' end of telomeres. A telomere is a region of repetitive sequences at each end of the chromosomes of most euka ...
has
telomerase reverse transcriptase
Telomerase reverse transcriptase (abbreviated to TERT, or hTERT in humans) is a catalytic subunit of the enzyme telomerase, which, together with the telomerase RNA component (TERC), comprises the most important unit of the telomerase complex.
T ...
as a catalytic subunit, regulation is accomplished by factors outside the protein.
An enzyme composed of both regulatory and catalytic subunits when assembled is often referred to as a
holoenzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...
. For example,
class I phosphoinositide 3-kinase is composed of a p110 catalytic subunit and a p85 regulatory subunit.
One subunit is made of one
polypeptide chain. A polypeptide chain has one
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
coding for it – meaning that a protein must have one gene for each unique subunit.
See also
*
Fusion protein
*
Subunit vaccine
*
Protein quaternary structure
*
Allostery
In biochemistry, allosteric regulation (or allosteric control) is the regulation of an enzyme by binding an effector molecule at a site other than the enzyme's active site.
The site to which the effector binds is termed the ''allosteric sit ...
*
Cooperativity
*
Monomer
In chemistry, a monomer ( ; '' mono-'', "one" + ''-mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization.
Classification
...
References
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