Tyrosyl groups

Tetranitromethane reacts slowly with tyrosyl groups to form 3-nitrotyrosyl groups (Eq. 3-45). The by-product nitroform is intensely yellow with e350 = 14,400. The reagent also oxidizes SH groups and reacts with other anionic groups. 

At first it appeared that PQQ had a broad distribution in enzymes, including eukaryotic amine oxidases. However, it was discovered, after considerable effort, that there are additional quinone cofactors that function in oxidation of amines. These are derivatives of tyrosyl groups of specific enzyme proteins. Together with enzymes containing bound PQQ they are often called quinoproteins.11, 1... 

All four tyrosyl groups are titrated normally in solvents which denature chymotrypsin. 

The spectrum of the phenoUc-ionized tyrosyl group (Fig. 9) is similar to that of un-ionized tyrosine, except that both its peaks are somewhat intensified and shifted about 200 A to longer wavelengths. The spectral change of tyrosine on ionization is discussed in many texts and references [for example, Edsall and Wyman (1958), and Tanford (1961), and in detail by Beaven and Holiday (1952)], and is assumed to be generally familiar. 

The great specificity of tyrosinase, the simplicity of monitoring its action spectrally, and in some cases, the discrimination shown among the several tyrosyl groups of a protein [it oxidizes only one of the six tyrosyl groups of ribonuclease (Yasuiiobu and Dandliker, 1957)] appear to recommend its serious consideration as a tool in protein structure studies. 

The acid transformation of serum albumin has been studied further with the solvent-perturbation technique (Section VI,G ) by Leonard and Foster (1961), and by Herskovits and Laskowski (1962). The results of these studies support the idea that the tyrosyl groups of serum albumin do become progressively more exposed to solvent, both in the N-F transformation and in the molecular expansion at slightly lower pH. However, it is not yet possible to make unique assignments for the causes of the spectral perturbations observed by Williams and Foster (1959). 

It appears from these model studies on spectra of phenols that we cannot, by spectral means alone, discriminate between a hydrogen bonding, a hydrophobic bonding, or an electrostatic mechanism for perturbations of protein tyrosyl groups. 

Another important feature of Fig. 19 is that it shows a twofold increase in the solvent-accessibility of the tyrosyl groups of oxidized ribonuclease (and acid-denatured) compared to the native protein. This result fits nicely with the findings of Shugar (1952) and of Tanford et al. (1955), who showed by spectrophotometric titration that only three out of a total of six tyrosyl groups of ribonuclease can be titrated reversibly. It appears... 

---