Arginine enzymes

Differential repressor effectiveness would imply a nonuniformity of repressive conditions in the formation of the arginine enzymes, although the same arginine repressor appears to be involved in each case. Presumably, this nonuniformity reflects varying degrees of effectiveness of the repressive complex [67, 68] which must be assumed to exist at the site of repression. Among the elements in the repressive complex, the arginine repressor is, of course, a key component. 

Fairly recently, evidence has been accumulating for the presence of another kind of component in the repressive complex, namely, finished arginine enzymes. In some sense, the enzymes would thus have a self-regulatory role. 

Also in strain K12, an arginine auxotroph evidently producing a nonfunctional A-acetylglutamic y-semialdehyde dehydrogenase was found to show diminished repressibility of other arginine enzymes. 

In order to explore translational aspects of repression, it is desirable to separate the translation of mRNA from its formation. This was accomplished in experiments of the following design (McLellan and Vogel [68]) mRNA for arginine enzymes was accumulated by arginine starvation of suitable auxotrophs further transcription was inhibited by rifampicin or miracil D f and enzyme formation was measured as... 

Trypsin (Section 27 10) A digestive enzyme that catalyzes the hydrolysis of proteins Trypsin selectively catalyzes the cleavage of the peptide bond between the carboxyl group of lysine or arginine and some other amino acid... 

ENZYMATIC ANALYSIS WITH CARBOXYPEPTIDASES. Carboxypeptidases are enzymes that cleave amino acid residues from the C-termini of polypeptides in a successive fashion. Four carboxypeptidases are in general use A, B, C, and Y. Carboxypeptidase A (from bovine pancreas) works well in hydrolyzing the C-terminal peptide bond of all residues except proline, arginine, and lysine. The analogous enzyme from hog pancreas, carboxypeptidase B, is effective only when Arg or Lys are the C-terminal residues. Thus, a mixture of carboxypeptidases A and B liberates any C-terminal amino acid except proline. Carboxypeptidase C from citrus leaves and carboxypeptidase Y from yeast act on any C-terminal residue. Because the nature of the amino acid residue at the end often determines the rate at which it is cleaved and because these enzymes remove residues successively, care must be taken in interpreting results. Carboxypeptidase Y cleavage has been adapted to an automated protocol analogous to that used in Edman sequenators. 

FIGURE 5.20 Trypsin is a proteolytic enzyme, or protease, that specifically cleaves only those peptide bonds in which arginine or lysine contributes the carbonyl function. The products of the reaction are a mixture of peptide fragments with C-terminal Arg or Lys residues and a single peptide derived from the polypeptide s C-terminal end. 

Partial hydrolysis of a peptide can be carried out either chemically with aqueous acid or enzymatically. Acidic hydrolysis is unselective and leads to a more or less random mixture of small fragments, but enzymatic hydrolysis is quite specific. The enzyme trypsin, for instance, catalyzes hydrolysis of peptides only at the carboxyl side of the basic amino acids arginine and lysine chymotrypsin cleaves only at the carboxyl side of the aryl-substituted amino acids phenylalanine, tyrosine, and tryptophan. 

Task (3) is more difficult. It uses a series of reagents each of which is capable of breaking only certain amide bonds. One of these reagents is the enzyme trypsin, which breaks only those bonds formed by the carboxyl groups in arginine and lysine. It would break the polypeptide... 

NO synthases (NOS, L-arginine, NADPH oxygen oxi-doreductases, nitric oxide forming EC 1.14.13.39) represent a family of enzymes that catalyze the formation of nitric oxide (NO) from the amino acid L-arginine. In mammals, three isoforms of NOS have been identified. They are termed neuronal NOS (nNOS, NOS I, NOS1), inducible NOS (iNOS, NOS H, NOS2), and endothelial... 

All NOS isoforms utilize L-arginine as the substrate, and molecular oxygen and reduced nicotinamide adenine dinucleotide phosphate (NADPH) as cosubstrates. Flavin adenine dinucleotide (FMN), flavin mononucleotide (FAD), and (6R)-5,6,7,8-tetrahydro-L-biopterin (BH4) are cofactors of the enzyme. All NOS isoforms contain heme and bind calmodulin. In nNOS and eNOS,... 

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