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1、Aacetyl 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 potentia
2、l; 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
3、 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 con
4、taining 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 R
5、NA 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
6、 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
7、-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 m
8、ethionine 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.Back to Topactive site A specific region of an enzyme that binds the substrate and carries out catal
9、ysis.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 f
10、ormed in vitro between myosin and actin that displays certain properties of muscles, such as contracting in the presence of ATP.acyl adenylate A mixed anhydride in which the carboxyl group of a molecule is linked to the phosphoryl group of AMP; the formation of acyl adenylates is a means of activati
11、ng carboxyl groups in biochemical reactions, such as the formation of fatty acyl CoA molecules from a free fatty acid and coenzyme A.acyl carrier protein A bacterial polypeptide that is linked to phosphopantetheine and acts as a carrier of the growing fatty acyl chain during fatty acid biosynthesis.
12、adaptation The resetting of the sensitivity level of receptors due to the continued presence of ligands. Also called desensitization.addition to or formation of double bond A reaction in which a functional group is added to a double bond or a group is removed from a molecule to form a double bond.ad
13、enylate cyclase An enzyme that generates cAMP, a second messenger, from ATP.adenylate cyclase cascade A signal-transduction pathway that employs cAMP and a series of enzymes to convert an extracellular signal into an intracellular signal.adipocytes Mammalian cells that are the major storage site for
14、 triacylglycerols.A-DNA helix A right-handed double helix made up of antiparallel strands held together by A-T and G- C base pairing; it is wider and shorter than B-DNA and is seen in dehydrated DNA as well as in doublestranded regions of RNA and in RNA-DNA helices.aerobic In the presence of oxygen.
15、affinity chromatography A protein-purification technique based on the high affinity of many proteins for specific chemical groups. Such groups are attached to an inert matrix, and the protein sample is applied; only those proteins with an affinity for the groups will bind.affinity labeling A means o
16、f mapping the active site of an enzyme by using a substrate analog that binds to the active site and forms a covalent bond with a nearby amino acid.Back to Topagonists Molecules that bind to receptor proteins and trigger signaling pathways.alcaptonuria A relatively harmless hereditary disorder resulting from the aberrant breakdown of tyrosine and phenylalanine.alcoholic fermentation The anaerobic conversion of glucose into ethanol
