Tryptophan residues molecular properties

One of the major chemical properties of the phenolic acids is the ease with which they oxidize. In the presence of oxygen, chlorogenic acid, caffeic acid and other related o-diphenols can oxidize by the action of polyphenol oxidase or in alkaline solution. These polyphenol oxidases are copper-containing proteins and are widespread in nature. Some of them oxidize only selected o-dihydroxyphenols while others, such as tyrosinase, not only oxidize dihydroxyphenols but convert monohydroxyphenols, such as tyrosine, to oxidizable dihydroxyphenols. The quinones formed are highly reactive substances which normally react further with other quinones to produce coloured compounds of high molecular weight. They may also react however with lysine, methionine, cysteine and tryptophan residues in the protein chain (Fig. 1). 

Considerable information is available on the molecular properties of this enzyme, on the catalytic mechanism, and on the stereochemistry of the enzymic reaction. The isomerase is composed of identical associating subunits. The primary structure is known and comprises 125 residues (MW 13,394) including all the common amino acids except cysteine and tryptophan. The enzyme, which exhibits exceptionally high catalytic activity, has a molecular activity at saturating concentrations of A -androstene-3,17-dione of 4.38 X 10 min per monomer at pH 7.0 and 25°. Mechanistic studies have disclosed that the isomerase catalyzes the reaction of A < -3-ketostcroids by a direct, stercospccific, and intramolecular transfer of the 4 8-proton to the 6)8 position. There is considerable evidence for the involvement of an enolic intermediate in the... 

Varying the side groups X in 27b affects both the stability and selectivity of the complexes (lateral discrimination), and allows the receptor-substrate interactions in biological systems to be modelled, for instance, the interaction between nicotinamide and tryptophan [2.109b]. One may attach to 27b amino acid residues (leading to parallel peptides [2.109] as in 27c), nucleic acid bases or nucleosides, saccharides, etc. The structural features of 27 and its remarkable binding properties make it an attractive unit for the construction of macropolycyclic multisite receptors, molecular catalysts, and carriers for membrane transport. Such extensions require sepa-... 

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