生物化学词典

编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第215页 共215页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 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.Back to Topactive 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.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 activating 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.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.adenylate 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 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.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 of 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 with the concomitant production of ATP.aldol condensation The combination of two carbonyl compounds (e.g., an aldehyde and a ketone) to form a -hydroxycarbonyl compound, or aldol.aldose A monosaccharide whose C-1 carbon atom contains an aldehyde group.alleles Alternative forms of a gene at a particular site on a chromosome.allosteric interaction An interaction between a small molecule (a ligand) and a site on a protein that may be some distance away from the active site; the interaction causes a conformational change and consequent alteration in the catalytic activity of the protein. helix A common structural motif in proteins, in which a polypeptide main chain forms the inner part of a right-handed helix, with the side chains extending outward; the helix is stabilized by intrachain hydrogen bonds between NH and CO groups of the main chain.-ketoglutarate dehydrogenase A citric acid cycle enzyme that catalyzes the oxidative decarboxylation of -ketoglutarate to yield succinyl CoA. This enzyme, which helps to regulate the rate of the citric acid cycle, is structurally and mechanistically similar to the pyruvate dehydrogenase complex.alternative splicing The generation of unique but related mRNA molecules by the differential splicing of the pre-mRNA transcript. By allowing the synthesis of more than one mRNA molecule from a premRNA transcript, alternative splicing increases the encoding potential of the genome.Amanitin A cyclic octapeptide from the mushroom Amanita phalloides (the destroying angel) that is a potent inhibitor of the elongation phase of RNA synthesis catalyzed by RNA polymerase II.Ames test A simple, rapid means of detecting carcinogens by measuring a chemicals ability to induce mutations in Salmonella bacteria.amiloride-sensitive sodium channel A sodium channel important in the detection of the taste of salt. Such channels are inhibited by amiloride, which also mutes the taste of salt.Back to Topamino acid An organic acid with an -carbon atom linked to a carboxylic acid, an amino group, a hydrogen atom, and a side chain (the R group). Twenty different amino acids are the building blocks of proteins.amino sugar A sugar that contains an amino group rather than a hydroxyl group at the C-2 position; the most common amino sugars are glucosamine and galactosamine.aminoacyl-tRNA An amino acid ester of transfer RNA.aminoacyl-tRNA synthetase An enzyme that activates an amino acid and then links it to transfer RNA. Also known as an activating enzyme, each aminoacyl-tRNA synthetase is specific for a particular amino acid.aminotransferase A class of enzymes that transfer an -amino group from an -amino acid to an - keto acid. Also called transaminases.ammoniotelic Characteristic of organisms in which excess ammonia is directly secreted; many aquatic animals are ammoniotelic.AMP-dependent protein kinase A protein kinase, conserved among eukaryotes, that is activated on binding of AMP and inhibited by ATP; consequently, it functions as a cellular fuel gauge, inhibiting certain process by phosphorylating key enzymes when the energy supply is low.ampere A measure of electrical current: the flow of 6.24 1018 charges per second.amphibolic reactions Metabolic reactions that can be anabolic or catabolic, depending on the energy conditions in the cell.amphipathic molecule A molecule, such as a membrane lipid, that contains both a hydrophobic and a hydrophilic moiety.amylopectin The branched form of starch, containing glucose residues in about one -1,6 linkage per thirty -1,4 linkages.amylose The unbranched form of starch, containing glucose residues in -1,4 linkage.amytal A barbiturate that blocks the respiratory chain by inhibiting electron transfer in the NADH-Q reductase complex.anabolic steroid A steroid, such as testosterone, that acts through the androgen receptor to stimulate genes that enhance the development of lean muscle mass.Back to Topanabolism The set of metabolic reactions that require energy to syntheize molecules from simpler precursors.anaerobic In the absence of oxygen.anapleurotic reaction From the Greek for fill up, referring to a reaction that replenishes intermediates removed from a metabolic pathway. The most common example is the reaction catalyzed by pyruvate carboxylase, in which the carboxylation of pyruvate produces oxaloacetate, a key component of the citric acid cycle.Andersens disease A disease due to an inability to introduce -1,6-glycosidic bonds during the synthesis of glycogen. Glycogen is present in normal amounts but with long outer branches; clinically, the disease is characterized by progressive cirrhosis of the liver.androgens A class of steroid hormones, exemplified by testosterone, that are responsible for the development of male secondary sexual characteristics; synthesized by the testes.angstrom () A unit of length equal to 10-10 meter.anomers Isomers of cyclic hemiacetals or hemiketals, with different configurations only at the carbonyl carbon atom; that carbon is known as the anomeric carbon.antagonist A molecule that binds to a receptor protein but does not trigger the signaling pathway. Such molecules are like competitive inhibitors for enzymes.antibody A protein synthesized by an animal in response to the presence of a foreign substance, or antigen; often binds to the antigen, neutralizing it or marking it for destruction.anticodon Three-nucleotide sequence of tRNA that base-pairs with a codon in mRNA.antigen A foreign substance that elicits the synthesis of an antibody.antigenic determinant Site on an antigen to which an antibody binds. Also called an epitope.antimycin A An antibiotic from Streptomyces that inhibits the respiratory chain by blocking electron transfer in the cytochrome reductase complex.antiporter A transport system in which a molecule is carried across a membrane in the direction opposite that of an ion, which in turn is pumped back across the membrane through active transport linked to ATP hydrolysis.Back to Topantiserum Serum prepared from the blood of an immunized animal containing soluble antibodies specific for a particular antigen.apoptosis A cascade of proteolytic enzymes that results in controlled cell death in response to significant cell damage or specific developmental programs. Also called programmed cell death.arachidonate Derived from linoleate, a 20:4 fatty acid that is a major precursor to several classes of signal molecules, including prostaglandins.arrestin A protein that plays a role in the termination of the visual signal-transduction pathway by binding to phosphorylated rhodopsin and preventing further interaction with transducin. Arrestin may function similarly in other 7TM-dependent signal-transduction pathways.ascorbate (vitamin C) A water-soluble vitamin that functions as an antioxidant and is required for the hydroxylation of collagen; scurvy results if ascorbate is deficient.aspartyl proteases A class of protein-degrading enzymes whose activity is dependent on an aspartate residue at the active site. An aspartyl protease is required for HIV replication.ATP (adenosine 5-triphosphate) A nucleotide consisting of adenine, ribose, and triphosphate units that serves as the cellular energy currency.ATP-ADP translocase An adenine nucleotide carrying a transport protein that carries ADP into the mitochondria and ATP out in a coupled fashion.ATP-binding cassette (ABC) domain The ATP-binding domain characteristic of specific membranetransport proteins, called ABC transporters; these transporters also contain a membrane-spanning region.ATP-grasp fold A protein domain that surrounds ATP and orients it for nucleophilic attack at the phosphate. Enzymes with these domains catalyze the formation of carbon-nitrogen bonds through acylphosphate intermediates.ATP synthase Molecular assembly of the inner mitochondrial membrane responsible for the respiratory-chain-driven synthesis of ATP. Also called Complex V, mitochondrial ATPase, H+-ATPase, or F0F1-ATPase.atractyloside A plant glycoside that inhibits ATP-ADP translocase.attenuation In bacteria, a mechanism for transcriptional regulation in which a decrease in the rate of translation of an mRNA operon reduces the rate of transcription of that operon.autoimmune disease A disease, such as insulin-dependent diabetes and multiple sclerosis, that result from a failure to suppress the immune response to self-antigens.autoradiography A means of detecting radioactive molecules immobilized in a separation medium such as polyacrylamide; the radioactivity of the molecules will blacken x-ray film.autotrophs Photosynthetic organisms that synthesize glucose from carbon dioxide and water, by using sunlight as an energy source; the glucose is then used as a fuel for cellular metabolism.avidity The strength of an interaction comprising multiple independent binding interactions between partners, as would take place between an antigen and antibody.axoneme The fundamental design structure of cilia and flagella; it consists of a bundle of microtubules, enclosed in a membrane, in which nine microtubule doublets surround two microtubule singlets.Back to TopBblymphocyte Precursors to plasma cells, which are antibody-secreting cells.bacitracin An antibiotic that blocks the transfer of oligosaccharides from dolichol phosphate to proteins.bacteriorhodopsin A 26-kd integral membrane protein that absorbs light and converts its energy into proton-motive force, which is then used to synthesize ATP.base-excision repair A means of repairing DNA in which the damaged base is removed and replaced by a base complementary to the undamaged DNA strand.B-DNA helix A right-handed double helix with the following characteristics: the two strands are antiparallel; the bases are inside the helix and the phosphates and deoxyribose sugars are on the outside; adenine forms hydrogen bonds with thymine, and guanine forms them with cytosine; the bases in each pair are coplanar; there are 10.4 residues per turn, with a pitch of 35 .Beers law The relations between the absorbance of light (A) by a compound, its extinction coefficient (), concentration (c) and the length (l) of the light path: A = clberiberi A neurologic and cardiovascular disorder caused by a dietary deficiency of thiamine (vitamin B1). sheet A common structural motif in proteins, in which two or more strands are associated as stacks of chains, stabilized by interchain hydrogen bonds; a number of strands running in the same direction form a pleated sheet, whereas such strands running in opposite directions form an antiparallel pleated sheet.-galactosidase An essential enzyme in lactose metabolism that hydrolyzes lactose into galactose and glucose.bifunctional enzyme An enzyme with two different, often opposing, catalytic activities on one polypeptide chain. For instance, phosphofructokinase 2 synthesizes fructose 2,6-bisphosphate and fructose 2,6-bisphosphatase hydrolyzes it, yet both active sites are on the same polypeptide chain.bile salts Polar derivatives of cholesterol that are made in the liver, stored in the gall bladder, and released into the small intestine, where they act as detergents to solubilize dietary lipids, facilitating their digestion and absorption.binding energy The free energy released in the formation of the weak interactions between enzyme and substrate.biopterin A cofactor from which the electron carrier tetrahydrobiopterin is derived.biotin A vitamin that plays a role in carboxylation and decarboxylation reactions.Bohr effect The observation made by Christian Bohr that H+ and CO2 promote the release of oxygen from oxyhemoglobin.bongkrekic acid An antibiotic that inhibits the action of ATP-ADP translocase.branch migration The ability of a DNA strand partly paired with its complementary strand to move, displacing its homologous resident strand and extending the pairing.bromodomain A domain consisting of a four-helix bundle that binds peptides containing acetyllysine. Also called an acetyllysine-binding domain.Back to TopCC3 plant A plants that lacks the C4 pathway.C4 pathway A means by which four-carbon compounds, such as oxaloacetate and malate, carry carbon dioxide from mesophyll cells in contact with the air to bundle-sheath cells, which are the major sites of photosynthesis. The pathway accelerates photosynthesis by concentrating carbon dioxide in photosynthetic cells.C4 plant A plant that utilizes the C4 pathway.CAAT box A component of many eukaryotic promoters with the consensus sequence 5-GNCAATCT- 3.Calcium ATPase (Ca2+ ATPase) An ATP-driven calcium pump that maintains the large electrochemical gradient of calcium ion across the plasma membrane.calmodulin In vertebrates, a ubiquitous protein in vertebrates that, when bound to calcium, stimulates many enzymes and transporters.calmodulin-dependent kinase A protein kinase that is activated by the binding of a Ca2+-calmodulin complex.calnexin Anchored in the endoplasmic reticulum membrane, a chaperone protein that prevents the export of immature or defective glycoproteins by binding glucose residues on the glycoproteins.caloric homeostasis Maintenance of a constant body weight by a complex network of hormonal interactions.calorie The amount of heat required to raise the temperature of 1 gram of water from 14.5C to 15.5C.Calvin cycle In plants, a cyclic metabolic pathway in which carbon dioxide is incorporated into ribulose 1,5-bisphosphate to give compounds that can be used for the synthesis of glucose.5 cap A structure at the 5 end of eukaryotic mRNA that stabilizes the mRNA and enhances its translation. The cap contains a 7-methyl guanylate residue attached by a triphosphate linkage to the sugar at the 5 end of the mRNA in a rare 5-5 linkage.capsaicin receptors Ion channels expressed in nociceptors that open in response to noxious stimuli such as heat, acidity or chemicals such as capsaicin, the chemical responsible for the hot taste of spicy food. Also called the vanilloid receptor 1 (VR1).capsid The protein coat surrounding viral DNA or RNA.Back to Topcarbamoyl phosphate synthetase An enzyme that begins the urea cycle by catalyzing the synthesis of carbamoyl phosphate from bicarbonate, ammonium ion, and ATP. The enzyme also catalyzes the initial reaction in pyrimidine biosynthesis.carbohydrates Saccharides, which are aldehyde or ketone compounds with multiple hydroxyl groups. Also defined as organic compounds with the empirical formula (CH2O)n.carbonium ion A carbon compound that contains a positively charged carbon atom; a carbonium atom is critical for catalysis by lysozymecarboxylase An enzyme that catalyzes a carboxyl transfer reaction; biotin is usually required as a coenzyme.carc