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[Protein phosphorylation]http://www.creative-proteomics.com/services/phosphorylation.htm is the most commonly studied area of post-translational modification since it plays a vital role in intracellular signal transduction and is involved in regulating cell cycle progression, differentiation, transformation, development, peptide hormone response, and adaptation. It has been estimated that one third of mammalian proteins may be phosphorylated and this modification often plays a key role in modulating protein function. Reversible protein phosphorylation, principally on serine, threonine or tyrosine residues, is one of the most important and well-studied post-translational modifications.

[protein n-acetylation]http://www.creative-proteomics.com/services/n-acetylation.htm, or the transfer of an acetyl group to nitrogen, occurs in almost all eukaryotic proteins through both irreversible and reversible mechanisms. N-terminal acetylation requires the cleavage of the N-terminal methionine by methionine aminopeptidase (MAP) before replacing the amino acid with an acetyl group from acetyl-CoA by N-acetyltransferase (NAT) enzymes. This type of acetylation is co-translational, in that N-terminus is acetylated on growing polypeptide chains that are still attached to the ribosome. While 80-90% of eukaryotic proteins are acetylated in this manner, the exact biological significance is still unclear.

[host cell protein hcp analysis]http://www.creative-proteomics.com/application/host-cell-protein-analysis.htm Because most of recombinant proteins are synthesized by cell-based systems in biochemical researches, the host cells derived from bacteria, yeast, mammalian cells, insect cells and plants (such as rice and tobacco) can be used for protein therapeutics manufacturing. But limited by current purification techniques, low levels (1 to 100 ppm) of host cell proteins (HCPs) may still remain in the purified biotherapeutics, even after a series of purifications. The ppm-level contaminants in biotherapeutics may trigger an unpredictable immune response in patients after dosing, and are required to be identified and quantified as part of drug safety evaluation, by the regulatory agencies.

The transfer of one-carbon methyl groups to nitrogen or oxygen (N- and O-methylation, respectively) to amino acid side chains increases the hydrophobicity of the protein and can neutralize a negative amino acid charge when bound to carboxylic acids. Methylation is mediated by [post translational modification methylation]http://www.creative-proteomics.com/services/methylation.htm, and S-adenosyl methionine (SAM) is the primary methyl group donor. Methylation occurs so often that SAM has been suggested to be the most-used substrate in enzymatic reactions afterATP. Additionally, while N-methylation is irreversible, O-methylation is potentially reversible.