CYP121, CYP51 and associated redox systems in Mycobacterium tuberculosis: towards deconvoluting enzymology of P450 systems in a human pathogen

An extraordinary array of P450 (cytochrome P450) enzymes are encoded on the genome of the human pathogen Mycobacterium tuberculosis (Mtb) and in related mycobacteria and actinobacteria. These include the first characterized sterol 14alpha-demethylase P450 (CYP51), a known target for azole and triazole drugs in yeasts and fungi. To date, only two Mtb P450s have been characterized in detail: CYP51 and CYP121. The CYP121 P450 shows structural relationships with P450 enzymes involved in synthesis of polyketide antibiotics. Both P450s exhibit tight binding to a range of azole drugs (e.g. clotrimazole and fluconazole) and the same drugs also have potent effects on growth of mycobacteria (but not of e.g. Escherichia coli). Atomic structures are available for both Mtb CYP51 and CYP121, revealing modes of azole binding and intriguing mechanistic and structural aspects. This paper reviews our current knowledge of these and the other P450 systems in Mtb including recent data relating to the reversible conversion of the CYP51 enzyme between P450 (thiolate-co-ordinated) and P420 (thiol-co-ordinated) species on reduction of the haem iron in the absence of a P450 substrate. The accessory flavoprotein and iron-sulfur proteins required to drive P450 catalysis are also discussed, providing an overview of the current state of knowledge of Mtb P450 redox systems.

Flavocytochrome P450 BM3: an update on structure and mechanism of a biotechnologically important enzyme

Since its discovery in the 1980s, the fatty acid hydroxylase flavocytochrome P450 (cytochrome P450) BM3 (CYP102A1) from Bacillus megaterium has been adopted as a paradigm for the understanding of structure and mechanism in the P450 superfamily of enzymes. P450 BM3 was the first P450 discovered as a fusion to its redox partner – a eukaryotic-like diflavin reductase. This fact fuelled the interest in soluble P450 BM3 as a model for the mammalian hepatic P450 enzymes, which operate a similar electron transport chain using separate, membrane-embedded P450 and reductase enzymes. Structures of each of the component domains of P450 BM3 have now been resolved and detailed protein engineering and molecular enzymology studies have established roles for several amino acids in, e.g. substrate binding, coenzyme selectivity and catalysis. The potential of P450 BM3 for biotechnological applications has also been recognized, with variants capable of industrially important transformations generated using rational mutagenesis and forced evolution techniques. This paper focuses on recent developments in our understanding of structure and mechanism of this important enzyme and highlights important problems still to be resolved.

Fast cytochrome bo from Escherichia coli binds two molecules of nitric oxide at CuB

The reaction of nitric oxide (NO) with fast cytochrome bo from Escherichia coli has been studied by electronic absorption, MCD, and EPR spectroscopy. Titration of the enzyme with NO showed the formation of two distinct species, consistent with NO binding stoichiometries of 1:1 and 2:1 with observed dissociation constants at pH 7.5 of approximately 2.3 x 10(-)6 and 3.3 x 10(-)5 M. Monitoring the titration by EPR spectroscopy revealed that the broad EPR signals at g approximately 7.3, 3.7, and 2.8 due to magnetic interaction between high-spin heme o (S = 5/2) and CuBII (S = 1/2) are lost. A high-spin heme o signal at g = 6.0 appears as the 1:1 complex is formed but is lost again on formation of the 2:1 complex, which is EPR silent. The absorption spectrum shows that heme o remains in the high-spin FeIII state throughout the titration. These results are consistent with the binding of up to two NO molecules at CuBII. This has been confirmed by studies with the Cl- adduct of fast cytochrome bo. MCD evidence shows that heme o remains ligated by histidine and water. Addition of excess NO to the Cl- adduct leads to the appearance of a high-spin FeIII heme EPR signal. Hence chloride ion binds to CuB, blocking the binding of a second NO molecule. These results suggest a mechanism for the reduction of NO to nitrous oxide by cytochrome bo and cytochrome c oxidase in which the binding of two cis NO molecules at CuB permits the formation of an N-N bond and the abstraction of oxygen by the heme group.