Acetylcholine (ACh)

GLOSSARY

acetyl CoA

Small, water-soluble metabolite comprising an acetyl group linked to coenzyme A (CoA); formed during oxidation of pyruvate, fatty acids, and amino acids. Its acetyl group is transferred to citrate in the citric acid cycle. (Figure 16-10)

acetylcholine (ACh)

Neurotransmitter that functions at vertebrate neuromuscular junctions and at various neuron-neuron synapses in the brain and peripheral nervous system.

acid

A compound that can donate a proton (H+). The carboxyl and phosphate groups are the primary acidic groups in biological molecules.

actin

Abundant structural protein in eukaryotic cells that interacts with many other proteins. The monomeric globular form (G actin) polymerizes to form actin filaments (F actin). In muscle cells, F actin interacts with myosin during contraction. See also microfilaments.

action potential

Rapid, transient, all-or-none electrical activity that is propagated in the plasma membrane of excitable cells such as neurons and muscle cells. Action potentials, or nerve impulses, allow long-distance signaling in the nervous system. (Figure 21-14)

activation energy

The input of energy required to (overcome the barrier to) initiate a chemical reaction. By reducing the activation energy, an enzyme increases the rate of a reaction. (Figure 2-27)

active site

Region of an enzyme molecule where the substrate binds and undergoes a catalyzed reaction.

active transport

Energy-requiring movement of an ion or small molecule across a membrane against its concentration gradient or electrochemical gradient. Energy is provided by the coupled hydrolysis of ATP or the cotransport of another molecule down its electrochemical gradient.

adenosine triphosphate

See ATP.

adenylyl cyclase

Membrane-bound enzyme that catalyzes formation of cyclic AMP (cAMP) from ATP; also called adenylate cyclase. Binding of certain ligands to their cell-surface receptors leads to activation of adenylyl cyclase and a rise in intracellular cAMP. (Figure 20-15)

aerobic

Referring to a cell, organism, or metabolic process that utilizes O2 or that can grow in the presence of O2.

aerobic oxidation

Oxygen-requiring metabolism of sugars and fatty acids to CO2 and H2O coupled to the synthesis of ATP.

allele

One of two or more alternative forms of a gene located at the corresponding site (locus) on homologous chromosomes.

allosteric transition

Change in the tertiary and/or quaternary structure of a protein induced by binding of a small molecule to a specific regulatory site, causing a change in the protein’s activity. Allosteric regulation is particularly prevalent in multisubunit enzymes.

alpha (α) helix

Common secondary structure of proteins in which the linear sequence of amino acids is folded into a right-handed spiral stabilized by hydrogen bonds between carboxyl and amide groups in the backbone. (Figure 3-6)

amino acid

An organic compound containing at least one amino group and one carboxyl group. In the 20 different amino acids that compose proteins, an amino group and carboxyl group are linked to a central carbon atom, the α carbon, to which a variable side chain is bound. (Figure 3-2)

aminoacyl-tRNA

Activated form of an amino acid, used in protein synthesis, consisting of an amino acid linked via a highenergy ester bond to the 3′-hydroxyl group of a tRNA molecule. (Figure 4-29)

amphipathic

Referring to a molecule or structure that has both a hydrophobic and a hydrophilic part.

anabolism

Cellular processes whereby energy is used to synthesize complex molecules from simpler ones. See also catabolism.

anaerobic

Referring to a cell, organism, or metabolic process that functions in the absence of O2.

anaphase

Mitotic stage during which the sister chromatids (or paired homologs in meiosis I) separate and move apart (segregate) toward the spindle poles. (Figure 19-34)

antibody

A protein that interacts with a particular site (epitope) on an antigen and facilitates clearance of that antigen by various mechanisms. See also immunoglobulin. (Figure 3-21)

anticodon

Sequence of three nucleotides in a tRNA that is complementary to a codon in an mRNA. During protein synthesis, base pairing between a codon and anticodon aligns the tRNA carrying the corresponding amino acid for addition to the growing peptide chain.