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a Related Biological Terms:

active glycolaldehyde

The a-hydroxyethylidene group when attached to the thiamin pyrophosphate prosthetic group of transketolase.

active site

The binding and catalytic sites of an enzyme; more loosely, those residues of an enzyme that interact with a substrate or participate in any way in binding or catalysis.

active transport

An energy-requiring transport mechanism; one that works against a concentration gradient. (see also facilitated diffusion; passive diffusion)

acyl-enzyme

An intermediate in the hydrolysis of substrates by some peptidases and esterases, e.g. by serine proteinases, in which the acyl moiety of the substrate is transiently attached to a serine hydroxy group of the enzyme.

acylium ion

(see a-type ion)

adaptation

The evolution of a feature or function through natural selection of incremental improvements, as contrasted with exaptation. (see also junk DNA)

adaptive immunity

(see innate immunity)

adaptor hypothesis

The proposal, made before the roles of mRNA and tRNA in protein synthesis were established, that a low-molecular-mass form of RNA provides the interface between each amino acid and the template. The hypothesis assumed that the interaction of the adaptor and a polynucleotide template is much more specific than that between the amino acid and the template. Learn more about amino acid chart.

adaptor protein

adaptor protein (see scaffold protein)

additivity

The principle of thermodynamics that free energy (or enthalpy of entropy) is a sum of contributions of independent components, e.g. the free energy of a protein transition is the sum of free energy changes owing to hydrogen bonding, electrostatic interactions, van der Waals packing effects, restriction of rotation about bonds, interactions with solvents, etc. The utility of this approach is undermined by the lack of a true independence of these factors, resulting in an inability to accurately evaluate their individual contributions to the overall change in free energy.Dill, K. (1997) J. Biol. Chem. 272, 701-704 Check our biology tool: qpcr primer design

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