Molecules Derived from Amino Acids:- Amino Acids Are Precursors of Creatine and Glutathione
Phosphocreatine, derived from creatine, is an important energy buffer in skeletal muscle . Creatine is synthesized from glycine and arginine (Fig. 1); methionine, in the form of S-adenosylmethionine, acts as methyl group donor. Glutathione (GSH), present in plants, animals, and some bacteria, often at high levels, can be thought of as a redox buffer. It is derived from glycine, glutamate, and cysteine (Fig. 2). The -carboxyl group of glutamate is activated by ATP to form an acyl phosphate intermediate, which is then attacked by the amino group of cysteine. A second condensation reaction follows, with the α-carboxyl group of cysteine activated to an acyl phosphate to permit reaction with glycine. The oxidized form of glutathione (GSSG), produced in the course of its redox activities, contains two glutathione molecules linked by a disulfide bond. Glutathione probably helps maintain the sulfhydryl groups of proteins in the reduced state and the iron of heme in the ferrous (Fe2+) state, and it serves as a reducing agent for glutaredoxin in deoxyribonucleotide synthesis. Its redox function is also used to remove toxic peroxides formed in the normal course of growth and metabolism under aerobic conditions:
2GSH+R-O-O-H → GSSG+H2O+R-OH
This reaction is catalyzed by glutathione peroxidase, a remarkable enzyme in that it contains a covalently bound selenium (Se) atom in the form of selenocysteine , which is essential for its activity.
FIGURE 1 Biosynthesis of creatine and phosphocreatine. Crea tine is made from three amino acids: glycine, arginine, and methionine. This pathway shows the versatility of amino acids as precursors of other nitrogenous biomolecules.
FIGURE 2 Glutathione metabolism. (a) Biosynthesis of glutathione. (b)Reduced form of glutathione.
