Medical Definition of isoform. : any of two or more functionally similar proteins that have a similar but not identical amino acid sequence and are either encoded by different genes or by RNA transcripts from the same gene which have had different exons removed.
Post-translational modifications
Post-translational modifications (PTMs) are chemical modifications that play a key role in the function of a protein, since they regulate activity, localization and interaction with other cellular molecules such as proteins, nucleic acids, lipids, and cofactors. They also have the possibility to regulate cellular activity. PTMs occur at distinct amino acid side chains or peptide linkages and are most often mediated by enzymatic activity. Post-translational modification can occur at any step in the "life cycle" of a protein.
Some common and important types of PTMs:
Glycosylation: addition of sugar chains, either at the amide nitrogen on the side-chain of asparagine (N-glycosylation) or on the hydroxyl oxygen on the side-chain of serine or threonine (O-glycosylation). The list of glycoproteins is long and they can serve a number of different functions, for example in the immune response (immunoglobulins family), as structural molecules (collagen family), hormones (HCG, TSH, EPO), transport molecules (transferrin), enzymes (alkaline phosphatase) and receptors.
Phosphorylation: addition of a phosphate group, usually to tyrosine, serine, threonine, histidine or aspartate. This modification is reversible and can for example activate/inactivate enzymes and receptors. A classical example where phosphorylation plays a very important role is in the regulation of the p53 tumor suppressor protein and proteins in various signal pathways, such as RAS pathway and STAT.
Ubiquitination: addition of ubiquitin will give a signal for degradation, alter the cellular location or affect the activity or interactions.
Other common post-translational modifications are S-nitrosylation, methylation, N-acetylation, lipidation, disulfide bond formation, sulfation, acylation, deamination etc.
Genes that encode proteins are composed of a series of three-nucleotide sequences called codons, which serve as the "words" in the genetic "language". The genetic code specifies the correspondence during protein translation between codons and amino acids. The genetic code is nearly the same for all known organisms.
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