Phenylalanine residues ribonuclease

Padros E, Morrows A, Manosa J, Dunach M 1982 The state of tyrosine and phenylalanine residues in proteins analyzed by fourth-derivative spectrophotometry Histone HI and Ribonuclease A. Eur J Biochem 127 117-122... 

Luse and M(iLaren (1963) have reviewed published research on the photolysis products and quantum yields tor the destruction of amino acids and have attributed the photochemical inactivation of the enzymes chymo-trypsin, lysozyme, ribonuclease, and trypsin by UV light at 254 m i primarily to destruction of the cystyl and tryptophyl residues. The destruction of these residues in proteins was suggested to be a function of the product of the number of residues present, the molecular extinction coefficient, and the quantum yield for destruction of each residue. Cysteine and tryptamine were identified among the irradiation products from cystine and tryptophan, respectively. Tyrosine, histidine, and phenylalanine were also shown to be degraded by UV, histidine yielding histamine, urocanic acid, and other imidazole derivatives, and phenylalanine yielding tyrosine and dihydroxyphenylalanine. Destruction of these three amino acids was not considered to contribute appreciably to the enzyme inactivation. 

In contrast to the case of tryptophan the photoreactions with tyrosine and histidine probably involve hydrogen atom transfer as the primary step. There are several indications for this. First, 0-methylated tyrosine (p-methoxy phenylalanine) did not show any photo-CIDNP effect and its reactivity as a photo-reductant towards flavins is strongly reduced (19). Similarly, 1-N-methyl histidine is not polarized at high pH (> 7.5), when no abstractable hydrogen is present. Secondly, in the protein ribonuclease A, which has a well known 3-dimensional structure, the residues Tyr 92 and His 105 have exposed rings, but their OH and NH protons are hydrogen bonded to backbone carbonyl groups. 

Tryptophan (Trp), tyrosine (Tyr), cystine (Cys), and phenylalanine (Phe) moieties play a determinant role regarding UV light-induced chemical alterations in many proteins. After the absorption of light by these moieties, in most cases mainly by Trp and Tyr, they undergo photoionization and participate in energy-and electron-transfer processes. This not only holds for structural proteins such as keratin and fibroin [11], but also for enzymes in aqueous media such as lysozyme, trypsin, papain, ribonuclease A, and insulin [7]. The photoionization of Trp and/or Tyr residues is the major initial photochemical event, which results in inactivation in the case of enzymes. A typical mechanism pertaining to Trp residues (see Scheme 8.3) commences with the absorption of a photon and the subsequent release of an electron. In aqueous media, the latter is rapidly solvated. By the release of a proton, the tryptophan cation radical Trp is converted to the tryptophan radical Trp. ... 

Signals in the aromatic region of the n.m.r. spectrum of colipase have been assigned to L-histidine residues. L-Histidine C-2 proton signals have been used in studies of the binding to ribonuclease A of H, edta and of 2 -deoxy-2 -fluorouridilyl-(3, 5 )-adenosine. L-Phenylalanine, L-tyrosine, and methionine side-chain proton resonances were used as monitors for the unfolding and stabilization of ribonuclease." ... 

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