STRUCTURAL ASPECTS OF HUMAN PBG DEAMINASE IN RELATION TO ACUTE INTERMITTENT PORPHYRIA

Peter Shoolingin-Jordan, Luke McNeill, Abeer Al Dbass, Raj Gill, Steve Wood, Fiyaz Mohammed, Julie Mosley and Danica Butler

University of Southampton, UK

X-Ray structures of both erythroid and ubiquitous human PBGD will be presented and the differences between the human and E. coli enzyme, previously used as a human enzyme model, will be discussed.  Several newly described human deaminase mutants have been expressed in vitro, all of which lead to acute intermittent porphyria (AIP).  One mutant, Asp 99 Gly, involves the essential cataytic group Asp 99 and, although the enzyme is completely inactive, it is correctly folded and has self-assembled the dipyrromethane cofactor at the active site.  Other mutations result in deaminases that cannot assemble the cofactor and exist as highly unstable, inactive, CRIM -ve apo-proteins.   Studies on the mechanism of dipyrromethane cofactor assembly have shown that porphobilinogen strongly inhibits the formation of holo-deaminase from apo-deaminase and that the elevated level of porphobilinogen that accompanies an acute attack of AIP is high enough to inhibit the formation of holo-deaminase and dramatically reduce the level of hepatic deaminase in vivo.  This is consistent with the observations on liver biopsy samples from the laboratory of Marver in the 1970s.  A lowered level of hepatic porphobilinogen deaminase during an acute attack of AIP would contribute to maintaining the vicious cycle of low haem synthesis and raised 5-aminolaevulinate synthase in the liver and would also explain why erythrocyte levels of deaminase are not substantially affected during an acute attack.