Dismutations amino acids

The alkali-soluble protein of the peel of lemons treated with hydrogen sulfide, sulfur dioxide, and sulfuric acid contained radioactive sulfur, but the fruit treated with hydrogen sulfide had a significantly lower per cent specific activity in the alkali-soluble protein fraction than did the sulfur dioxide or sulfuric acid treated fruits (Table VII). These results suggest that sulfur dioxide and sulfuric acid react with protein more directly, while hydrogen sulfide perhaps must be oxidized first, as indicated in Table III. It also appears (from Table VII) that the alkali-soluble protein may have been dismuted as the amounts isolated were less in both the hydrogen sulfide and sulfur dioxide treated fruit than in the incubated or nonincubated controls. Other evidence of dismutation has been obtained in experiments where incubation at 60° C. was accompanied by the production of free ammonia (18), and the recovery of free ammonia and six amino acids in the exudates of incubated and sulfur-dusted fruits (18). 

Mn superoxide dismutases are found in both eubacteria and archaebacteria as well as in eukaryotes, where they are frequently found in mitochondria. They (Figure 16.1) have considerable structural homology to Fe SODs both are monomers of 200 amino acid and occur as dimers or tetramers, and their catalytic sites are also very similar. They both catalyse the two-step dismutation of superoxide anion and, like the Cu-Zn SODs, avoid the difficulty of overcoming electrostatic repulsion between two negatively charged superoxide anions by reacting with only one molecule at a time. As in the case of Cu-Zn SOD, a first molecule of superoxide reduces the oxidized (Mn3+) form of the enzyme, releasing... 

It is believed that MnSOD plays a pivotal role in many diseases. There have been many reviews of the biochemistry of MnSOD " and focusing on the structural aspects of the enzyme. Four different types of SOD are known, a Cu/Zn-containing SOD, a FeSOD, a NiSOD, and MnSOD. MnSODs, which are structurally related to the FeSODs, have a of 23,000 ( 200 amino acids) and function as a dimer or as a tetramer. MnSOD catalyzes the dismutation reaction by cycling between the - -2 and +3 oxidation states. One proton is taken up by the system in each step (Equation (2)) ... 

Dismutation—the Stickland reaction between pairs of amino acids (Section 6.7.1)... 

SODs are differentiated mainly by the redox-active metal in the active site copper, manganese, or iron. The iron and manganese SODs are structurally similar (5-11) and are structurally distinct from the Cu,Zn SOD (12). The dramatic features of these enzymes are that they catalytically dismutate superoxide at rates that are not only diffusion controlled but have been shown to be electrostatically facilitated (13). In these systems, modifications of amino acid residues near the active site have been shown to alter the enzymatic activity, indicating that superoxide is electrostatically drawn into the active site channel (14). In addition, in contrast to the spontaneous dismutation rate of 02 and the dismutation rates of 02 by many metal complexes, all of which are pH dependent, the enzymatic dismutation rate is largely pH independent over the pH range (5-10). 

Several species of Streptomyces possess a Ni-containing enzyme that has been purified on the basis of SOD activity. This enzyme catalyzes dismutation of 02 with rates comparable to those observed for the canonical SODs (1.3 x 10 M s on a per-Ni basis) and relatively independent of pH. The enzyme appears to be cytoplasmic and as-isolated lacks the N terminal 14 amino acids. ... 

---