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Coproporphyrinogen oxidase (CPO) is the sixth enzyme in the heme biosynthetic pathway, catalyzing two sequential oxidative decarboxylations of propionate moieties on coproporphyrinogen-III forming protoporphyrinogen-IX through a monovinyl intermediate, harderoporphyrinogen. Site-directed mutagenesis studies were carried out on three invariant amino acids, aspartate 400, arginine 262, and arginine 401, to determine residue contribution to substrate binding and/or catalysis by human recombinant CPO. Kinetic analyses were performed on mutant enzymes incubated with three substrates, coproporphyrinogen-III, harderoporphyrinogen, or mesoporphyrinogen-VI, in order to determine catalytic ability to perform the first and/or second oxidative decarboxylation. When Asp400 was mutated to alanine no divinyl product was detected, but the production of a small amount of monovinyl product suggested the K m value for coproporphyrinogen-III did not change significantly compared to the wild-type enzyme. Upon mutation of Arg262 to alanine, CPO was again a poor catalyst for the production of a divinyl product, with a catalytic efficiency <0.01% compared to wild-type, including a 15-fold higher K m for coproporphyrinogen-III. The efficiency of divinyl product formation for mutant enzyme Arg401Ala was ;3% compared to wild-type CPO, with a threefold increase in the K m value for coproporphyrinogen-III. These data suggest Asp400, Arg262, and Arg401 are active site amino acids critical for substrate binding and/or catalysis. Possible roles for arginine 262 and 401 include coordination of carboxylate groups of coproporphyrinogen-III, while aspartate 400 may initiate deprotonation of substrate, resulting in an oxidative decarboxylation.

A rapid, inexpensive and sensitive alternative to the traditional Western Blot is presented here. This method is very useful for detection of recombinant 6-his tagged proteins.

The effects of covalent cross-linkers on the enzyme, coproporphyrinogen oxidase, had been previously studied but their role in protecting the enzyme from protease cleavage has not been evaluated. Therefore, we examined how the cross-linker bis (sulfosuccinimidyl) suberate (BS³) affects the ability of trypsin to digest purified, wild type recombinant human coproporphyrinogen oxidase and selected mutants. Following incubation, the apparent molecular weights of peptides were evaluated by SDS-PAGE and enzymatic activity was assessed by spectroscopy following HPLC. For both wild type and mutants, the results indicated that the cross-linker was indeed able to protect against trypsin digestion relative to the enzyme incubated with trypsin in the absence of the cross-linker. These data have implications for the episodic nature of porphyria

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