Protein isoform

A protein isoform is any of several different forms of the same protein. Different forms of a protein may be produced from related genes, or may arise from the same gene by alternative splicing. A large number of isoforms are caused by single-nucleotide polymorphisms or SNPs, small genetic differences between alleles of the same gene. These occur at specific individual nucleotide positions within a gene.

The discovery of isoforms partly explains the apparently small number of coding regions (genes) revealed by the human genome project: the ability to create categorically different proteins from the same gene increases the diversity of the proteome. Isoforms are readily described and discovered by microarray studies and cDNA libraries.

Glycoforms
A glycoform is an isoform of a protein that differs only with respect to the number or type of attached glycan. Glycoproteins often consist of a number of different glycoforms, with alterations in the attached saccharide or oligosaccharide. These modifications may result from differences in biosynthesis during the process of glycosylation, or due to the action of glycosidases or glycosyltransferases. Glycoforms may be detected through detailed chemical analysis of separated glycoforms, but more conveniently detected through differential reaction with lectins, as in lectin affinity chromatography and lectin affinity electrophoresis. Typical examples of glycoproteins consisting of glycoforms are the blood proteins as orosomucoid, antitrypsin, and haptoglobin. An unusual glycoform variation is seen in neuronal cell adhesion molecule, NCAM involving polysialic acids, PSA.

Examples

 * G-actin, despite its conserved nature, has varying number of isoforms, with a single isoform in yeasts and multiple (at least six in mammals) in eukaryotes.
 * Creatine kinase, the presence of which in the blood can be used as an aid in the diagnosis of myocardial infarction, exists in 3 isoforms.
 * Hyaluronan synthase, the enzyme responsible for the production of hyaluronan, has three isoforms in mammalian cells.
 * UDP-glucuronosyltransferase, an enzyme superfamily responsible for the detoxification pathway of many drugs, environmental pollutants, and toxic endogenous compounds has 16 known isoforms encoded in the human genome.