Simplified Chemistry

active-enzyme centrifugation a technique for determining the sedimentation and diffusion coefficients of an enzyme– substrate complex. A thin lamella of an enzyme solution is layered onto a substrate solution in an ultracentrifuge cell and, on rotation, as the enzyme molecules sediment in a band through the substrate solution they catalyse the enzymic reaction, the progress of which is observed optically …

activation 1 the action or process of rendering an atom, molecule, or other substance reactive or more reactive, whether physicochemically, chemically, or biochemically. 2 the process of rendering material artificially radioactive: radioactivation. See radioactivation analysis. 3 the initial changes in an ovum during fertilization, covering the periodfrom first contact with a sperm to dissolution of the nuclear membranes. 4 the …

acyclovir or acycloguanosine 9-(2-hydroxyethoxymethyl)guanine; an antiviral agent widely used in the treatment of human herpes infections. It is selectively phosphorylated by herpesvirus-induced thymidine kinase and the phosphorylated compound is a potent inhibitor of herpesvirus-induced DNA polymerase. One proprietary name is Zovirax.

activin one of two gonadal glycoproteins related to transforming growth factor-b (the other is inhibin); present in two forms in human gonads, it exists as a dimer of inhibin bA or bB chains, linked by disulfide bonds. Activin A is a dimer of bA chains; activin AB is a dimer of bA and bB chains. A potent selective stimulator of …

active site 1 the general region of an enzyme molecule containing the catalytic residues identified with the binding and reaction of substrate( s). It includes those amino-acid residues that are, in the enzyme– substrate complex, either contact amino acids, i.e. those that at some point are only one bond distance removed from some point of the substrate molecule, or auxiliary …

actinoid or actinide any member of the series of 15 metallic elements with proton numbers 89 (actinium) to 103 (lawrencium) inclusive that occur together in group 3 and period 7 of the IUPAC periodic table; sometimes the term is restricted to the 14 elements following actinium. Actinoid is now the preferred name. All actinoids are radioactive, and those of proton …

actinin a minor protein constituent of muscle, found to be concentrated in both the Z line and the I band. Two components of actinin have been identified: a-actinin, F-actin cross-linking protein, a dimer of ∼200 kDa with an action similar to that of actinogelin.; and b-actinin, a dimer of ∼70 kDa, similar in action to gelsolin.

actin-binding protein any of several proteins that associate with either actin monomers or actin filaments in cells and modify their properties. Many of these proteins are found in the cell cortex, an actin-rich layer just below the plasma membrane. Examples include dystropin, profilin, spectrin and ankyrin, fimbrin and a-actinin (see actinin), filamin, gelsolin, and the myosins. The term is sometimes …

actin a major protein constituent of the thin filaments of muscle and of the microfilaments found in practically all eukaryotic cells – it comprises 5–10% of the protein of such cells (see actin filament). In solutions of low ionic strength, actin is a globular 42 kDa monomer, termed G-actin. At physiological ionic strengths, G-actin polymerizes into a fibrous form, termed …

acridinium esters: esters of acridine-9-carboxylic acid. They have a quaternary nitrogen centre and are derivatized at the 9 position to give a labile phenyl ester moiety. Acridinium esters such as 2′,6′-dimethyl-4′-[N-succinimidyloxy carbonyl]phenyl-10-methylacridinium- 9-carboxylate can be used as chemiluminescent labels for antibodies. Light emission is activated by adding hydrogen peroxide under alkaline conditions. In hybridization protection assays, single-stranded DNA probes labelled …