Biochemical reactions within cells define cellular function and activity. These reactions include a complex interplay between receptor ligand interactions and enzymatic reactions, which orchestrate the signaling and activities of a cell. Physiological conditions influence the activity levels of enzymes and proteins mediating these reactions.
For example, modifications of amino acid residues on peptide chains play an important role in the regulation of protein function in cells. Transfer of phosphate groups, methyl groups or carbohydrates induces conformational changes in protein substrates, which in turn results in changes in protein function. These reactions are usually reversible. Protein modification involves particular classes of enzymatic activities. For example, methylation is generally catalyzed by a family of enzymes, known as methyltransferases. Protein methylation regulates membrane attachment of cytosolic proteins and contributes to prevention of C-terminal proteolytic degradation of peptides. For example, most G proteins are methylated. The methylated cysteine residue is located at or near the carboxyl terminus of G proteins may facilitate attachment of the peptides to the membrane for signal transduction (Rando, Biochim BiophysActa 1300(1):5–12 (1996), Hrycyna, Pharmacol. Ther. 59(3):281–300 (1993)). Histidine at position 73 on several kinds of actin is also methylated. It has been shown that methylation is required for maintaining proper conformation of actin molecules (Yao, J. Biol. Chem. 274(52):37443–9 (1999)). Methylation of lysine residue in the S-1 region of myosin results in decreased ATP binding in myofibrine contraction (Bivin, Proc. Nat'l Acad. Sci USA 91(18):8665–9 (1994)). It has also been shown that methylation of membrane proteins may contribute to the development of cardiovascular disease in diabetic patients (Schaffer, Mol. Cell Biochem 107(1):1–20 (1991)). In addition, methylation reduces protein-RNA interaction in nuclear proteins (Kim, Amino Acids 15(4):291–306 (1998)), and selectively modulates SH3 domain-mediated protein-protein interactions (Bedford, J. Biol. Chem, 275(21):16030–6 (2000)).
Similarly, carbohydrates can be transferred to side chains of specific asparagine, serine, or threonine residues in many secreted proteins or proteins displayed on cell-surfaces. The transfer of a carbohydrate to asparagine at position 2181 on human coagulation factor V results in impaired interaction between factor V and phospholipid membranes (Nicolaes, Biochemistry 38(41):13584–91 (1999)). Large carbohydrates, known as polysaccharides, are formed by linking many sugar monomers.