Molecular Biology

Actin-Binding Proteins Part 3 (Molecular Biology)

4.1. Arp The first reports describing a family of actin-related proteins, the Arps, appeared a few years ago (2, 209). These are highly conserved proteins that share a 30-60% homology with actin, but are functionally distinct from actin. Two of these actin-related proteins, Arp2 (44 kD) and Arp 3 (47 kD) have attracted a great […]

Actin Polymerization Toxins (Molecular Biology)

Several bacteria release toxins that modify actin or proteins controlling its polymerization (1). A group of Clostridia spp. secrete binary toxins, whose most studied member is the C2 toxin released by Cl. botulinum. Botulinum C2 toxin is not neurospecific, as are botulinum neurotoxins, but it affects many nonneuronal cells (2). In the intestinal loop model, […]

Actinomycin D (Molecular Biology)

Actinomycins are chromopeptide antibiotics produced by various species of streptomycetes (1). They differ only in their content of certain amino acids in their peptide rings (Fig. 1). Actinomycin D, which is active against several forms of cancer, has the distinction of being the first antibiotic discovered (in 1943) to possess useful antitumor activity. It is […]

Activation Energy (Molecular Biology)

The kinetic rates of chemical and enzyme-catalyzed reactions depend on temperature. The relationship for this dependence is known as the Arrhenius law: where A and E are constants, R is the gas constant, and T is the absolute temperature in Kelvin units. The activation energy E is the height of the energy barrier that the […]

Active Site (Molecular Biology)

The folding of a polypeptide chain that produces the final protein structure of an enzyme also leads to formation of the active site. From X-ray crystallography studies, it is apparent that the active site of an enzyme is a groove, cleft, or pocket that has access to the solvent and forms only a small part […]

Active Site-Directed Irreversible Inhibitors (Molecular Biology)

Active site-directed irreversible inhibitors of enzymes are also known as active site-direcetd inactivating reagents, affinity labels, andphotoaffinity labels. They combine the features of a substrate, or substrate analogue, with those of a group-specific reagent, as in affinity labeling, and have been used to determine the amino acid residues that are present in the active site […]

Active-Site Titrants (Molecular Biology)

For determination of values for the kinetic rate constants and ligand-binding stoichiometries associated with an enzyme-catalyzed reaction, it is necessary to know the concentration of functional active sites of the enzyme. The latter value cannot be calculated simply from the total concentration of protein and molecular weight of the enzyme; even when the enzyme preparation […]

Adenovirus (Molecular Biology)

The adenoviruses constitute a large group of DNA viruses, the Adenoviridae family. Genera include Mastadenovirus (human, simian, bovine, equine, porcine, ovine, canine, and opossum) and Aviadenovirus (birds). Adenoviruses are nonenveloped icosahedral particles of 70 to 100 nm in diameter, with 252 capsomeres, of which 240 are hexons and 12 are penton bases. A projecting fiber […]

Adenylate Charge (Molecular Biology)

The adenylate charge, or adenylate energy charge, is a linear measure of the energy stored in the adenylate system) in a living cell. It is analogous to the charge of a storage battery. Its value is defined by the expression: The status of the adenylate system is best described by the adenylate charge and appears […]

Adenylate Cyclases (Molecular Biology)

The ubiquity of cyclic AMP (cAMP) in regulating enzymatic activity and/or genetic expression in all kingdoms of life, except for the archaea (but see later), accounts for the interest displayed in its mode of synthesis and the vast amount of literature devoted to the enzymes that produce it, the adenylate cyclases. These enzymes, which catalyze […]

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