Chapter 24. The Biosynthesis of Amino Acids Flashcards
A series of reactions in which S-methyl groups from methionine are converted into a biochemically reactive form through insertion into an adenosyl group; an active S-methyl group can be transferred from S-adenosylmethionine to acceptor molecules such as norepinephrine. The remaining part of the cycle includes the regeneration of methionine from homocysteine and N5-methyltetrahydrofolate.
Activated methyl cycle
the conversion of diatomic nitrogen to ammonia; the first step in the flow of nitrogen into amino acids, nucleotides, and other nitrogen-containing compounds in organisms.
Nitrogen fixation
The first irreversible step in a metabolic pathway under physiologic conditions; this step is catalyzed by an allosteric enzyme and commits the product to a particular chemical fate.
Committed step
amino acids that cannot be synthesized de novo and therefore must be acquired from the diet; in adult mammals, at least nine amino acids are considered essential.
Essential amino acids
a regulatory strategy in which the committed step in common to several pathways is catalyzed by different enzymes with the same catalytic properties, but different regulatory properties. Each enzyme thus responds to the final product of one of the pathways sharing the committed step.
Enzyme multiplicity
prosthetic group derived from vitamin B2(pyridoxine) that plays a key role in transamination reactions.
Pyridoxal phosphate
an activated methyl donor that consists of an adenosyl group linked to the sulfur atom of methionine.
S-adenosylmethionine (SAM)
an enzyme complex that catalyzes the reduction of diatomic nitrogen to ammonia; found in bacteria and the blue-green algae.
Nitrogenase complex
amino acids that can be synthesized by an organism, and thus are not a dietary requirement.
Nonessential amino acids
a regulatory strategy in which the enzyme catalyzing the committed step common to several pathways is incrementally inhibited by the products of each of the pathways. Thus, each inhibitor can reduce the activity of the enzyme even if other inhibitors are bound at saturating levels.
Cumulative feedback inhibition
A tripeptide playing a role in combating oxidative stress by maintaining the reduced state of the cell. Glutathione cycles between the reduced (GSH) and oxidized (GSSG) state.
Glutathione
A free-radical gas, synthesized from arginine, that is an important messenger in many vertebrate signal transduction processes.
Nitric oxide (NO)
Inherited or acquired metabolic disorders caused by an enzyme deficiency in the biosynthetic pathway for heme; often characterized by the accumulation of one or more pathway intermediates in blood or other tissues, as well as their excretion in urine.
porphyria
a property of multienzyme complexes in which the product of one reaction is routed directly to the active site of the next enzyme in the complex, for which it is a substrate. Substrate channeling enhances catalytic rate by preventing loss of intermediates of overall reaction.
Substrate channeling
a highly versatile carrier of activated one carbon units.
Tetrahydrofolate
