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.

Ames test

A simple, rapid means of detecting carcinogens by measuring a chemical's ability to induce mutations in Salmonella bacteria.

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.

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.

α-ketoglutarate dehydrogenase

A citric acid cycle enzyme that catalyzes the oxidative decarboxylation of α-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

α helix

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.

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.

alleles

Alternative forms of a gene at a particular site on a chromosome.

aldose

A monosaccharide whose C-1 carbon atom contains an aldehyde group.

aldol condensation

The combination of two carbonyl compounds (e.g., an aldehyde and a ketone) to form a β-hydroxycarbonyl compound, or aldol.

alcoholic fermentation

The anaerobic conversion of glucose into ethanol with the concomitant production of ATP.

alcaptonuria

A relatively harmless hereditary disorder resulting from the aberrant breakdown of tyrosine and phenylalanine.

agonists

Molecules that bind to receptor proteins and trigger signaling pathways.

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.

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.

aerobic

In the presence of oxygen.

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.

adipocytes

Mammalian cells that are the major storage site for triacylglycerols.

adenylate cyclase cascade

A signal-transduction pathway that employs cAMP and a series of enzymes to convert an extracellular signal into an intracellular signal.

adenylate cyclase

An enzyme that generates cAMP, a second messenger, from ATP.

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.

adaptation

The resetting of the sensitivity level of receptors due to the continued presence of ligands. Also called desensitization.

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.

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.

actomyosin

A complex formed in vitro between myosin and actin that displays certain properties of muscles, such as contracting in the presence of ATP.

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.

active site

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

activation domain

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

activated methyl cycle

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.

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.

action potential

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.

actinomycin

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.

actin

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.

acetyllysine-binding domain

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

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.

acetylcholine receptor

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.

acetyl CoA carboxylase

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.

Biochemistry

Biochemistry is the study of the chemical processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules.

Although there are a vast number of different biomolecules many are complex and large molecules (called polymers) that are composed of similar repeating subunits (called monomers). Each class of polymeric biomolecule has a different set of subunit types.[1] For example, a protein is a polymer whose subunits are selected from a set of 20 or more amino acids. Biochemistry studies the chemical properties of important biological molecules, like proteins, in particular the chemistry of enzyme-catalyzed reactions.

The biochemistry of cell metabolism and the endocrine system has been extensively described. Other areas of biochemistry include the genetic code (DNA, RNA), protein synthesis, cell membrane transport, and signal transduction.

Since all known life forms that are still alive today are descended from the same common ancestor, they have generally similar biochemistries. It is unknown whether alternative biochemistries are possible or practical.