5t2k
Geobacillus stearothermophilus HemQ with Manganese-Coproporphyrin III
Structural highlights
FunctionCHDC_GEOS3 Involved in coproporphyrin-dependent heme b biosynthesis (PubMed:27936663). Catalyzes the decarboxylation of Fe-coproporphyrin III (coproheme) to heme b (protoheme IX), the last step of the pathway (PubMed:27936663). The reaction occurs in a stepwise manner with a three-propionate harderoheme intermediate (PubMed:27936663).[1] Publication Abstract from PubMedCoproheme decarboxylase catalyzes two sequential oxidative decarboxylations with H2O2 as the oxidant, coproheme III as substrate and cofactor, and heme b as the product. Each reaction breaks a C-C bond and results in net loss of hydride, via steps that are not clear. Solution and solid-state structural characterization of the protein in complex with a substrate analog revealed a highly unconventional H2O2-activating distal environment with the reactive propionic acids (2 and 4) on the opposite side of the porphyrin plane. This suggested that, in contrast to direct C-H bond cleavage catalyzed by a high-valent iron intermediate, the coproheme oxidations must occur through mediating amino acid residues. A tyrosine that hydrogen bonds to propionate 2 in a position analogous to the substrate in ascorbate peroxidase is essential for both decarboxylations, while a lysine that salt bridges to propionate 4 is required solely for the second. A mechanism is proposed in which propionate 2 relays an oxidizing equivalent from a coproheme compound I intermediate to the reactive deprotonated tyro-sine, forming Tyrblacksquare, square, filled. This residue then abstracts a net hydrogen atom (Hblacksquare, square, filled) from propionate 2, followed by migration of the unpaired propionyl electron to the coproheme iron to yield the ferric harderoheme and CO2 products. A similar pathway is proposed for decarboxylation of propionate 4, but with a lysine residue as an essential proton shuttle. The proposed reaction suggests an extended relay of heme-mediated e-/H+ transfers and a novel route for the conversion of carboxylic acids to alkenes. A Structure-Based Mechanism for Oxidative Decarboxylation Reactions Mediated by Amino Acids and Heme Propionates in Coproheme Decarboxylase (HemQ).,Celis AI, Gauss GH, Streit BR, Shisler K, Moraski GC, Rodgers KR, Lukat-Rodgers GS, Peters JW, DuBois JL J Am Chem Soc. 2016 Dec 12. PMID:27936663[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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