As described in this chapter, the most common post-translational modifications of proteins are proteolysis, phosphorylation, methylation, acetylation, and linkage with ubiquitin and SUMO proteins. Carry out a Web search to identify at least eight other post translational modifications and the amino acid residues involved in these modifications.

Proteolysis is one of the key post-translational modifications (PTMs) where proteins are cleaved into smaller polypeptides or amino acids. This process is critical as it helps regulate protein activity, life-span, and function inside the cell. Proteolysis can activate certain proteins or deactivate others, and it is involved in numerous cellular processes, including the cell cycle, apoptosis, and digestion.

One well-known example of proteolytic activation is the conversion of the inactive zymogen proinsulin into its active form, insulin, by the removal of specific peptide segments. Proteases, the enzymes that carry out proteolysis, target specific sequences within a protein and cleave the peptide bond between amino acid residues. The specificity is determined by the structure of both the enzyme and the protein substrate. It's vital to note that unregulated proteolysis can lead to diseases like cancer and Alzheimer's, reinforcing the importance of precise control over this modification.

Phosphorylation is a ubiquitous PTM involving the addition of a phosphate group to amino acid residues, such as serine, threonine, or tyrosine. This modification is reversible and is facilitated by enzymes called kinases, while the removal of the phosphate group is achieved by phosphatases. Phosphorylation alters the structural conformation of proteins, thus regulating their activity, localization, and interaction with other molecules.

Due to these changes, phosphorylation is a critical mechanism for controlling signal transduction pathways and cellular processes such as growth, differentiation, and metabolism. Its dysregulation can result in severe medical conditions like diabetes, heart disease, and neurological disorders. Therefore, understanding phosphorylation's role at a molecular level is fundamental for therapeutic developments in medicine.

Amino acid residues are the building blocks that create proteins when they're linked together by peptide bonds during protein biosynthesis. Each residue in a polypeptide chain contributes to the protein's final structure and function. The uniqueness of these residues comes from their side chains, which vary in size, charge, and polarity, and can influence how a protein interacts with other molecules.

PTMs often take place on these side chains, which can dramatically affect the protein's properties. The chemical nature of the side chains also determines which modification a residue can undergo; not all residues can be phosphorylated or glycosylated, for example. Amino acid residues thus play a vital role in determining the 3D structure and biological activity of the proteins they comprise.

Protein biosynthesis is the process by which cells construct proteins based on genetic information coded in DNA. This complex operation begins with transcription, where RNA polymerase generates a messenger RNA (mRNA) copy of a DNA sequence. The mRNA then travels to the ribosome, where translation occurs: ribosomes read the mRNA and use transfer RNA (tRNA) to add the appropriate amino acid residues in the correct sequence to form a polypeptide chain.

After initial synthesis, proteins often undergo folding and several PTMs before they become fully functional. These modifications, which include proteolysis, phosphorylation, and others, can alter a protein's activity, life span, localization, and interaction with other molecules. Protein biosynthesis and its associated PTMs are tightly regulated; errors in this process can lead to various diseases, underscoring its importance in cellular homeostasis and organismal health.