CoachAmmonium enters organic matter almost entirely through glutamate: glutamate dehydrogenase adds it to -ketoglutarate, and glutamine synthetase adds a second to glutamate’s side chain (one ATP), making glutamine. From these two donors, transaminases (aminotransferases, with a vitamin B coenzyme) move the amino group onto any -keto acid, building amino acids from the carbon skeletons of the central pathways: the glutamate family from -ketoglutarate, the aspartate family from oxaloacetate, alanine and serine from pyruvate and 3-phosphoglycerate. Plants and bacteria make all twenty; animals have lost the long pathways to nine of them, the essential amino acids, and take them from food. The same transaminases, run backward, strip nitrogen from surplus amino acids for excretion as ammonia (fish), urea (mammals) or uric acid (birds, insects).
উদাহরণ
Example 16.10 (Nucleotides)
A purine ring is assembled on ribose-5-phosphate from glycine, aspartate, two glutamines, two one-carbon units and , at the cost of six ATP; a pyrimidine from aspartate and carbamoyl phosphate. Deoxynucleotides are made from ribonucleotides by reducing the sugar, using NADPH. A cell about to divide makes some nucleotides in an hour; the drugs that block these syntheses — methotrexate, 5-fluorouracil — stop dividing cells first, which is why they treat cancer.