POST-TRANSLATIONAL MODIFICATION When a protein gets translated, this isn’t the end of the process. There are many different things that happen to a peptide chain so that a different functional protein can result from this. These changes involve the addition of functional groups, partial protein degradation, or proteolytic cleavage of parts of the protein. As mentioned, the genome in humans involves about 20,000 genes, while the proteome (the numbers of different protein molecules) is about a million. This means that a single gene can encode for multiple proteins. It is believed that about 5 percent of the human proteome is made from enzymes that participate in more than 200 different types of post-translational modifications. There are phosphatases, kinases, ligases, and transferases that add and subtract things from the pre-proteins. Many proteins also have the capability of acting on themselves to create a unique protein under specific circumstances. The post-translational modification of proteins can be permanent or reversible, depending on the type of modification that occurs. For example, phosphate groups can be added or subtracted as needed, while degradation is usually irreversible. In the next few paragraphs, we will look at some changes that can happen to a protein as part of these processes. Proteins and peptides can be phosphorylated, which is the addition of a phosphate group on some of the amino acids that make up part of the peptide. This is a reversible process. Glycosylation is also a common modification—adding sugars to the peptide in certain places. This is also somewhat reversible. There is a process too called ubiquitination, which is the addition of 76 amino acids to the lysine amino acid on the peptide. Nitric oxide can be used in a reversible S-nitrosylation of the peptide chain. The process is used to stabilize proteins and to provide nitric oxide donors in the cell. S-adenosyl methionine or SAM is commonly used to at a methyl or CH3 group to proteins. In the same way, an acetyl group can be added to nitrogen in reversible and irreversible reactions. It is used to add acetyl groups to histone proteins, which are the proteins used to condense DNA that is not being used for transcription. When lipids are
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