The irreversible thermal inactivation of Bacillus licheniformis alpha-amylase was studied. A two-step behaviour in the irreversible denaturation process was found. Our experimental results are consistent only with the two-step model and rule out the two-isoenzyme one. They suggest that the deactivation mechanism involves the existence of a temperature-dependent intermediate form. Therefore the enzyme could exist in a great number of active conformational states. We have shown that Ca2+ is necessary for the structural integrity of alpha-amylase. Indeed, dialysis against chelating agents leads to a reversible enzyme inactivation, though molecular sieving has no effect. Further, the key role of Ca2+ in the alpha-amylase thermostability is reported. The stabilizing effect of Ca2+ is reflected by the decrease of the denaturation constants of both the native and the intermediate forms. Below 75 degrees C, in the presence of 5 mM-CaCl2, alpha-amylase is completely thermostable. Neither other metal ions nor substrate have a positive effect on enzyme thermostability. The effect of temperature on the native enzyme and on one intermediate form was studied. Both forms exhibit the same optimum temperature. Identical activation parameters for the hydrolytic reaction catalysed by these two forms were found.

The chlorate-resistant (chlR) mutants are pleiotropically defective in molybdoenzyme activity. The inactive derivative of the molybdoenzyme, respiratory nitrate reductase, present in the cell-free extract of a chlB mutant, can be activated by the addition of protein FA, the probable active product of the chlB locus. Protein FA addition, however, cannot bring about the activation if 10 mM sodium tungstate is included in the culture medium for the chlB strain. The inclusion of a heat-treated preparation of a wild-type or chlB strain prepared after growth in the absence of tungstate, restores the protein-FA-dependent activation of nitrate reductase. All attempts to activate nitrate reductase in extracts prepared from tungstate-grown wild-type Escherichia coli strains failed. It appears that during growth with tungstate, the possession of the active chlB gene product leads to the synthesis of a nitrate reductase derivative which is distinct from that present in the tungstate-grown chlB mutant. Heat-treated preparations from chlA and chlE mutants which do not possess molybdenum cofactor activity fail to restore the activation. Fractionation by gel filtration of the heat-treated preparation from a wild-type strain produced two active peaks in the eluate of approximate M, 12000 and 11500. The active material in the heat-treated extract was resistant to exposure to proteinases, but after such treatment the active component, previously of approximate M , 12000, eluted from the gel filtration column with the material of M , I 1500. The active material is therefore of low molecular mass and can exist either in a protein-bound form or in an apparently free state. Molybdenum cofactor activity, assayed by the complementation of the apoprotein of NADPH : nitrate oxidoreductase in an extract of the nit-1 mutant of Neurospora crassa, gave a profile following gel filtration similar to that of the ability to restore respiratory nitrate reductase activity to the tungstate-grown chlB mutant soluble fraction. This was the case even after proteinase treatment of the heat-stable fraction. Analysis of the chlC (narC) mutant, defective in the structural gene for nitrate reductase, revealed that heat treatment is not necessary for the expression of the active component. Furthermore both the active component and molybdenum cofactor activity are present in corresponding bound and free fractions in the non-heat-treated soluble subcellular fraction. However, a heattreated extract from a chlG mutant, while possessing molybdenum cofactor as assayed by N. crassa nit-1 complementation, was unable to function in the activation of nitrate reductase in the tungstate-grown chlB mutant extract. We conclude that the active component in the heat-stable fraction is either the molybdenum cofactor or a closely related compound. This is consistent with the lack of molybdenum cofactor activity in extracts from tungtstate-grown cells.Nitrate reductase of Escherichia coli is a molybdoenzyme whose synthesis is specifically induced by anaerobic growth in the pre...

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The TMAO reductase activity of Escherichia coli grown anaerobically in the presence or absence of TMAO was analysed on linear sucrose gradients and on non‐denaturing polyacrylamide gels. The results, together with those obtained by analysis of some other properties of TMAO reductase, showed that there are significant differences between the enzyme synthesized in the absence of TMAO (“constitutive” enzyme) and that synthesized in its presence (“inducible” enzyme). A similar study of a tor mutant specifically altered in the structural gene for TMAO reductase, showed that the enzymes synthesized under the 2 growth conditions are probably 2 distinct enzymes encoded by different genes.