Superoxide dismutase catalyzes the conversion of the single electron reduced species of molecular oxygen to hydrogen peroxide and oxygen. The widely distributed copper enzyme has been purified from two fungi. Identical chromatographic and electrophoretic behavior of the enzyme isolated from different sources indicates great similarity in the molecular properties of the enzyme from eucaryotic organisms. A photosensitized recording assay procedure for the enzyme was developed which elminates the use of a second enzyme system for generating the substrate, superoxide anion. Kinetic data indicate that the reaction between enzyme and superoxide anion shows a logarithmic dependence on concentration under the conditions of the method.The fungal enzyme contains 2 mol of zinc and 2 mol of copper per mole of holoenzyme. The reaction between the enzyme-bound copper and several catechol derivatives has been examined through the use of electron spin resonance spectroscopy. It was concluded from these studies that these compounds reduce a portion of the copper to the cuprous form and also may form a complex with enzyme copper. The substituted catechols are oxidized noncatalytically by the enzyme to the semiquinone forms in the presence of oxygen. Much higher concentrations of the semiquinones are formed in the presence of the enzyme than are possible with oxygen alone at pH 8. Catechols also react readily with superoxide anion. Because of the complex relationships between catechols, superoxide anion, and superoxide dismutase, it is difficult to assess the effect of catechols on the catalytic activity of the enzyme.

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5-Aminolevulinate synthase (ALAS) and 8-amino-7-oxononanoate synthase (AONS) are homodimeric members of the α-oxoamine synthase family of pyridoxal 5′-phosphate (PLP)-dependent enzymes. Previously, linking two ALAS subunits into a single polypeptide chain dimer yielded an enzyme (ALAS/ALAS) with a significantly greater turnover number than that of wild-type ALAS. To examine the contribution of each active site to the enzymatic activity of ALAS/ALAS, the catalytic lysine, which also covalently binds the PLP cofactor, was substituted with alanine in one of the active sites. Albeit the chemical rate for the pre-steady-state burst of ALA formation was identical in both active sites of ALAS/ALAS, the kcat values of the variants differed significantly (4.4 ± 0.2 min−1 vs. 21.6 ± 0.7 min−1) depending on which of the two active sites harbored the mutation. We propose that the functional asymmetry for the active sites of ALAS/ALAS stems from linking the enzyme subunits and the introduced intermolecular strain alters the protein conformational flexibility and rates of product release. Moreover, active site functional asymmetry extends to chimeric ALAS/AONS proteins, which while having a different oligomeric state, exhibit different rates of product release from the two ALAS and two AONS active sites due to the created intermolecular strain.

Desaturation of oleoyl CoA by the microsomal fraction of Fusarium oxysporum hyphal cells required O2, NADPH, MgCl2, and the addition of either bovine serum albumin or the 105 000g supernatant fraction. In the absence of reduced nucleotide, [14C]oleoyl CoA was rapidly incorporated into phospholipid and triacylglycerol and hydrolyzed to free fatty acids. After addition of NADPH, oleate was desaturated at the normal rate. Analysis of the distribution of [14C]oleate and [14C]linoleate between different lipid classes revealed that phosphatidylcholine and phosphatidylethanolamine were labeled with [14C]linoleate before any other lipid class. These results are consistent with oleoyl phospholipid being a direct intermediate in the desaturation of oleoyl CoA. The preference of the oleoyl-desaturase for NADPH, the relatively high pH optimum of 8.2, and the sensitivity to thenoyltrifluoroacetone inhibition suggest that some components of the microsomal electron-transport chain are common to both the oleoyl desaturase and stearoyl CoA desaturase systems in this fungus.