Biochemical differences between mutant propionyl-CoA carboxylases from two complementation groups

The authors examined several biochemical parameters of propionyl-CoA carboxylase (PCC) activity in fibroblast extracts from PCC deficient patients belonging to the two major genetic complementation groups (A and C). Three representative fibroblast lines from the A complementation group and four from the C complementation group were compared with each other and with four control lines. Mutant enzymes from both groups contain biotin, possess a sulfhydryl group in their active sites, and exhibit Hill coefficients and K(m) values for all major substrates within the control range. The following chemical differences were observed between the complementation groups: (1) at 45°C PCC from the A complementation group decayed distinctly more rapidly than did the group C enzyme, and both mutant PCCs were significantly more thermolabile than control enzyme; (2) potassium activated PCC from both mutant classes and from controls, but group A PCC had a fivefold lower affinity for this monovalent cation; (3) α-aminoisobutyrate (AIB) enhanced mutant but not control PCC activity, this effect being greater in enzyme from group A than group C lines; (4) following total inhibition of PCC activity by avidin, excess biotin restored 100% of activity in group A lines and 30%-50% in group C lines, compared to only 10%-20% in controls. Based on the known structures of prokaryotic and eukaryotic carboxylases, the authors propose that mutant PCC from complementation group A is structurally altered in the region of the enzyme containing the bicarbonate and adenosine triphosphate (ATP) binding sites. They propose further that mutant PCC from complementation group C is modified in a region distinct from that affected in group A mutants, perhaps in that portion containing the propionyl-CoA binding site. It is not yet possible to determine if the mutations responsible for these two classes of structurally altered PCCs are allelic or nonallelic.