Terminology

5-adenosylcobalamin - An enzymatically active form of vitamin B 12 responsible for isomerization of glutamate to methylaspartate. Figure 20.19
Adenylylation/deadenylylation - Covalent modification mechanism that regulates glutamine synthetase. In it, a specific tyrosine residue (close to the active site) reacts with ATP to form an ester between the phenolic hydroxyl group and the phosphate of the resultant AMP. Twleve sites in glutamine synthetase can be adenylylated. The effect is cumulative. Inhibition is correlated with the amount of adenylylation. The enzyme catalyzing the reaction is a complex of adenylyl transferase and a regulatory protein called P II. Figure 20.9
Anaerobic - Refers to the absence of oxygen or the absence of a need for it.
Antimetabolite - A synthetic compound, usually a structural analog of a normal metabolite, that interferes with the utilization of the metabolite to which it is structurally related.
Autolysis - The digestion of the cell by lysosomes as a normal morphogenetic process in development.
Biological Nitrogen Fixation - A term used to describe reduction of N 2 to NH 3. Figure 20.2
Calpains - Two calcium activated proteases in the cytosol.
Cathepsins - A group of lysosomal proteases designed to work under acidic conditions.
Cobamides - Term for a compound containing B 12. This arises from the fact that such molecules have many amide nitrogens and cobalt.
Corrin Ring - Tetrapyrrole ring that coordinates cobalt in vitamin B 12. Figure 20.18
Cumulative feedback Inhibition - A regulatory mechanism that controls the activity of glutamine synthetase. It involves the action of eight specific feedback inhibitors. Six of the inhibitors are metabolic endproducts of glutamine metabolism (tryptophan, histidine, glucosamine-6-phosphate, carbamoylphosphate, CTP and AMP). Two of the inhibitors are indicators of the general status of amino acid metabolism (alanine, glycine). Figure 20.9
Cytochrome 557 - Name of the cytochrome bound by nitrate reductase. It accepts electrons from the FAD cofactor and passes them on to the molybdenum cofactor.
Denitrifying Bacteria - Bacteria which catabolize NH 3 to N 2. Figure 20.2
Dihydrofolate (Dhf) - A reduced form of folic acid produced by dihydrofolate reductase-catalyzed reduction of folate using NADPH.
Essential Amino Acids - Amino acids that must be provided in the diet to meet an animal's metabolic needs.
Ferredoxin - A low potential electron carrier in Rhizobium, which donates electrons for the reduction of atmospheric nitrogen. Figure 20.4
Flavodoxin - A low potential electron carrier in Klebsiella, which donates electrons for the reduction of atmospheric nitrogen. Figure 20.4
Glucogenic - Substances (such as some amino acids) that can be used as substrates for glucose synthesis.
Glucose-Alanine Cycle - Muscle cycle for handling ammonia in which pyruvate from glycolysis is converted to alanine, which carries the amine nitrogen to the liver, where it is converted back to pyruvate for gluconeogenesis. Figure 20.14
Glycine Cleavage System - A multienzyme complex in mitochondria catalyzing the reaction: Glycine + Tetrahydrofolate + NAD+ <=> 5,10-methylenetetrahydrofolate + CO 2 + NH 3 + NADH + H+
Intrinsic Factor - A glycoprotein secreted by gastric tissue that complexes with ingested B 12 in the digestive tract and promotes its absorption through the small intestine into the blood.
Ketogenic - Substances (such as some amino acids) that can be used as substrates for ketone body synthesis.
Lysosomes - Eukaryotic cell organelle which forms by budding of the Golgi complex. It contains many digestive enzymes, including proteases, nucleases, lipases, and enzymes that break down carbohydrates.
Methionine Synthase - Homocysteine + Methylcobalamin <=> Methionine + Vitamin B 12.
Methylcobalamin - An enzymatically active form of vitamin B 12 that transfers a methyl group to homocysteine, converting it to methionine.
Negative Nitrogen Balance - A condition of nitrogen metabolism that exists in senescence, starvation, and certain disease states in which the daily intake of nitrogen is less than daily loss of nitrogen.
nif - A gene cluster of about 24,000 bases in Klebsiella involved in nitrogen fixation.
Nitrobacter - A genus of bacteria that oxidizes NO 2- to NO 3- for energy. Figure 20.2
Nitrogen Equilibrium (Normal Nitrogen Balance) - A condition of nitrogen metabolism that exists in a well-nourished adult in which the daily intake of nitrogen equals daily loss of nitrogen.
Nitrogenase Complex - The enzyme system responsible for N 2 reduction.
Nitrosomonas - A genus of bacteria that oxidizes NH 3 to NO 2- for energy. Figure 20.2
Nonessential Amino Acids - Amino acids that need not be provided in an animal's diet because they can be biosynthesized in adequate amounts.
Pernicious Anemia - Stomach disease arising from insufficient secretion of intrinsic factor.
Positive Nitrogen Balance - A condition of nitrogen metabolism that exists during pregnancy, in juveniles, or in recovery from starvation, in which the daily intake of nitrogen equals exceeds daily loss of nitrogen.
Proteasome - A large ATP-dependent protease found in the eukaryotic cytosol.
Pyridoxal Phosphate - A coenzyme in aminotransferases derived from vitamin B 6. Catalysis begins with condenstaion of the aldehyde of this group with the amino group of an amino acid, to give a Schiff base or aldimine, intermediate.
Siroheme - A partially reduced iron porphyrin found in nitrite reductase.
Transamination - A reaction involving transfer of the amino group (usually of glutamate) to an -keto acid, with formation of the corresponding amino acid plus the -keto derivative of glutamate (- ketoglutarate). Catalyzed by aminotransferases (transaminases).
Ubiquitin - A 76-residue, heat-stable protein found in all eukaryotic cells that is covalently attached to another protein to target it for degradation. Figure 20.11
Ubiquitination - An ATP-dependent reaction which attaches the C-terminal glycine residue of ubiquitin to lysine amino acid groups on a target protein. Such modified proteins are degraded soon afterward by a large protease complex that requires ATP and recognizes the ubiquitin marker. Figure 20.11
Urea - Chemical form in which most mammals excrete the bulk of their nitrogen.
Uric Acid - Chemical form in which birds, terrestrial reptiles, and insects convert most of their excess ammonia to for excretion.