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acetyl CoA carboxylase An enzyme that catalyzes the ATP-dependent synthesis of malonyl CoA from acetyl CoA and carbon dioxide, the committed step in fatty acid synthesis.

acetylcholine receptor A ligand-gated channel that promotes a large inward current of sodium ions and triggers an action potential; composed of a pentamer of four kinds of polypeptide subunits, the channel opens to allow passage of sodium and potassium ions when two acetylcholine molecules promote transient opening of the channel. acetylcholinesterase An enzyme in the synaptic cleft that converts acetylcholine into choline and acetate; this enzyme promotes the closing of the acetylcholine-receptor membrane by rapidly reducing the concentration of acetylcholine in the region between the presynaptic and the postsynaptic membranes.

acetyllysine-binding domain A domain consisting of a four-helix bundle that binds peptides containing acetyllysine. Also called a bromodomain.

actin A highly conserved protein found in all eukaryotes; in striated muscle, it forms the thin filaments of the sarcomere and activates the ATPase of myosin.

actinomycin A polypeptide antibiotic from Streptomyces that inhibits the elongation phase of RNA synthesis by binding to double-helical DNA by intercalating with it, thereby preventing the DNA from serving as a template. action potential The increase in membrane potential and the changes in sodium and potassium conductances that result from alterations in the permeability of the axon membrane to those ions. Also called nerve impulse. activated carriers Small molecules carrying activated functional groups that can be donated to other molecules; for instance, ATP carries activated phosphate groups and CoA carries activated acyl groups.

activated methyl cycle 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.

activation domain The structural region of a transcription factor that facilitates transcription in some manner. See also DNA-binding domain.

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active site A specific region of an enzyme that binds the substrate and carries out catalysis.

active transport The transport of an ion or a molecule against a concentration gradient, where DG for the transported species is positive; the process must be coupled to an input of free energy from a source such as ATP, an electrochemical gradient of Na+ or K+, or light. actomyosin A complex formed in vitro between myosin and actin that displays certain properties of muscles, such as contracting in the presence of ATP.