Converts methylene to carbonyl with cyclisation to give the D ring in bacteriochlorophyll (B12 biding)
Booker SJ
Anaerobic functionalization of unactivated C-H bonds
▸ Abstract
The functionalization of alkanes was once thought to lie strictly within the domain of enzymes that activate dioxygen in order to generate an oxidant with suitable potency to cleave inert C-H bonds. The emergence of the radical SAM superfamily of enzymes-those which use S-adenosyl-l-methionine as a precursor to a 5'-deoxyadenosyl 5'-radical-has kindled a renaissance in the study of radical-dependent enzymatic reactions, and is ushering in a wealth of new and intriguing chemistry that remains to be elucidated. This review will focus on a special subclass of radical SAM enzymes that functionalize inert C-H bonds, highlighting the functional groups and the chemistry that leads to their insertion. Within this class are first, enzymes that catalyze sulfur insertion, the prototype of which is biotin synthase; second, enzymes that catalyze P-methylation or C-methylation, such as P-methylase or Fom3; third, enzymes that catalyze oxygen insertion, such as the anaerobic magnesium protoporphyrin-IX oxidative cyclase (BchE); and fourth, enzymes that functionalize n-hexane or other alkanes as the first step in the metabolism of these inert compounds by certain bacteria. In addition to surveying reactions that have been studied at various levels of detail, this review will speculate on the mechanisms of other types of reactions that this chemistry lends itself to.
The importance of chlorophyll (Chl) to the process of photosynthesis is obvious, and there is clear evidence that the regulation of Chl biosynthesis has a significant role in the regulation of assembly of the photosynthetic apparatus. The understanding of Chl biosynthesis has rapidly advanced in recent years. The identification of genetic loci associated with each of the biochemical steps has been accompanied by a greater appreciation of the role of Chl biosynthesis intermediates in intracellular signaling. The purpose of this review is to provide a source of information for all the steps in Chl and bacteriochlorophyll a biosynthesis, with an emphasis on steps that are believed to be key regulation points.
Photosynth Res
2006;90(2):173-194
| PubMed ID:
17370354
Ouchane S, Steunou AS, Picaud M, Astier C
Aerobic and anaerobic Mg-protoporphyrin monomethyl ester cyclases in purple bacteria: a strategy adopted to bypass the repressive oxygen control system
▸ Abstract
Two different mechanisms for Mg-protoporphyrin monomethyl ester (MgPMe) cyclization are shown to coexist in Rubrivivax gelatinosus and are proposed to be conserved in all facultative aerobic phototrophs: an anaerobic mechanism active under photosynthesis or low oxygenation, and an aerobic mechanism active only under high oxygenation conditions. This was confirmed by analyzing the bacteriochlorophyll accumulation in the wild type and in three mutant strains grown under low or high aeration. A mutant lacking the acsF gene is photosynthetic, exhibits normal bacteriochlorophyll accumulation under low oxygenation and anaerobiosis, and accumulates MgPMe under high oxygenation. The photosynthesis-deficient bchE mutant produces bacteriochlorophyll only under high oxygenation and accumulates MgPMe under low oxygenation and anaerobiosis. The double knockout mutant is devoid of photosystem and accumulates MgPMe under both conditions indicating the involvement of the two enzymes at the same step of the biosynthesis pathway. Oxygen-mediated expression of bchE was studied in the wild type and in a regulatory mutant. The reverse transcriptase-PCR and the bchE promoter activity results demonstrate that the expression of the bchE gene is oxygen-independent and suggest that it is rather the enzyme activity that should be oxygen-sensitive. No obvious sequence similarities were found between oxygen-dependent AcsF and the oxygen-independent anaerobic Mg-protoporphyrin monomethylester cyclase (BchE) enzymes. However, common to all BchE proteins is the conserved CXXX-CXXC sequence. This motif is essential for 4Fe-4S cluster formation in many anaerobic enzymes. Expression and purification of BchE were achieved, and the UV-visible spectral analyses confirmed the presence of an active 4Fe-4S cluster in this protein. The use of different classes of enzymes catalyzing the same reaction under different oxygen growth conditions appears to be a common feature of different biosynthetic pathways, and the benefit of possessing both aerobic and anaerobic systems is discussed.
Anaerobic chlorophyll isocyclic ring formation in Rhodobacter capsulatus requires a cobalamin cofactor
▸ Abstract
The isocyclic ring of bacteriochlorophyll (BChl) is formed by the conversion of Mg-protoporphyrin monomethyl ester (MPE) to protochlorophyllide (PChlide). Similarities revealed by blast searches with the putative anaerobic MPE-cyclase BchE suggested to us that this protein also uses a cobalamin cofactor. We found that vitamin B(12) (B(12))-requiring mutants of the bluE and bluB genes of Rhodobacter capsulatus, grown without B(12), accumulated Mg-porphyrins. Laser desorption/ionization time-of-flight (LDI-TOF) MS and NMR spectroscopy identified them as MPE and its 3-vinyl-8-ethyl (mvMPE) derivative. An in vivo assay was devised for the cyclase converting MPE to PChlide. Cyclase activity in the B(12)-dependent mutants required B(12) but not protein synthesis. The following reaction mechanism is proposed for this MPE-cyclase reaction. Adenosylcobalamin forms the adenosyl radical, which leads to withdrawal of a hydrogen atom and formation of the benzylic-type 13(1)-radical of MPE. Withdrawal of an electron gives the 13(1)-cation of MPE. Hydroxyl ion attack on the cation gives 13(1)-hydroxy-MPE. Withdrawal of three hydrogen atoms leads successively to 13(1)-keto-MPE, its 13(2)-radical, and cyclization to PChlide.
Proc Natl Acad Sci U S A
2000;97(12):6908-6913
| PubMed ID:
10841582
