Heme A synthase

The synthesis of heme A (Fig. 2) involves the initial addition of the farnesyl moiety to the heme 2-vinyl group by heme O synthase, which generates heme O that only has this modification. In a second step, heme A synthase oxidizes the 8-methyl of heme O to an aldehyde, which generates heme A. An electron transfer mechanism (rather than double hydroxylation) has been proposed for this final biosynthetic step (28). 

Heme A is an obligatory cofactor in all eukaryotic and most bacterial cytochrome c oxidase enzymes (CcO). Because of its importance to CcO and aerobic metabolism, considerable effort has recently been invested in understanding the mechanism and regulation of heme A biosynthesis. The activity of heme A synthase is strictly dependent on O2, and yet there is no incorporation of O2 into the products. Heme A synthase is now known to utilize a unique electron-transfer mechanism when oxidizing heme O to heme A. Interestingly, the heme A biosynthetic pathway is regulated at least partly via a heme-dependent process in which heme A synthase is positively regulated by intracellular heme levels via Hapl. 

CoxlO/CoxlS (Heme O Synthase/Heme A Synthase) and Related Heme A Insertion... 

Heme A is derived from the more generally utilized heme B. Heme O synthase (Cox 10) catalyzes the first reaction by the addition of the famesyl moiety to heme B to generate heme O, while heme A synthase (Cox 15) converts heme O to heme A by oxidizing the C-8 methyl substituent to an aldehyde (18-21) (Figure 3). 

A synthase does not incorporate molecular oxygen into the formyl 47. group of heme A. Biochemistry 2004 43 8616-8624. 

Cytochrome c oxidase (COX) is the terminal enzyme in the respiratory system of most aerobic organisms and catalyzes the four electron transfer from c-type cytochromes to dioxygen (115, 116). The A-type COX enzyme has three different redox-active metal centers A mixed-valence copper pair forming the so-called Cua center, a low-spin heme-a site, and a binuclear center formed by heme-fl3 and Cub. The Cua functions as the primary electron acceptor, from which electrons are transferred via heme-a to the heme-fl3/CuB center, where O2 is reduced to water. In the B-type COX heme-u is replaced by a heme-fo center. The intramolecular electron-transfer reactions are coupled to proton translocation across the membrane in which the enzyme resides (117-123) by a mechanism that is under active investigation (119, 124—126). The resulting electrochemical proton gradient is used by ATP synthase to generate ATP. 

Figure 3. Transformation of heme B to heme A catalyzed by the enzymes heme O synthase (Cox 10)...

In order for the cyclooxygenase to function, a source of hydroperoxide (R—O—O—H) appears to be required. The hydroperoxide oxidizes a heme prosthetic group at the peroxidase active site of PGH synthase. This in turn leads to the oxidation of a tyrosine residue producing a tyrosine radical which is apparendy involved in the abstraction of the 13-pro-(5)-hydrogen of AA (25). The cyclooxygenase is inactivated during catalysis by the nonproductive breakdown of an active enzyme intermediate. This suicide inactivation occurs, on average, every 1400 catalytic turnovers. 

NO-sensitive GC represents the most important effector enzyme for the signalling molecule NO, which is synthesised by NO synthases in a Ca2+-dependent manner. NO-sensitive GC contains a prosthetic heme group, acting as the acceptor site for NO. Formation of the NO-heme complex leads to a conformational change, resulting in an increase of up to 200-fold in catalytic activity of the enzyme [1]. The organic nitrates (see below) commonly used in the therapy of coronary heart disease exert their effects via the stimulation of this enzyme. 

Heme (C34H3204N4Fe) represents an iron-porphyrin complex that has a protoporphyrin nucleus. Many important proteins contain heme as a prosthetic group. Hemoglobin is the quantitatively most important hemoprotein. Others are cytochromes (present in the mitochondria and the endoplasmic reticulum), catalase and peroxidase (that react with hydrogen peroxide), soluble guanylyl cyclase (that converts guanosine triphosphate, GTP, to the signaling molecule 3, 5 -cyclic GMP) and NO synthases. 

Fig. 6. Comparison of VTMCD spectra for biological [Fe3S4] clusters. (A) D. gigas Fdll (20) (B) P. furiosus 3Fe Fd (42) (C) A. vinelandii Fdl (70) (D) T. thermophilus 7Fe Fd (70) (E) E. coli nitrate reductase (24) (F) E. coli fumarate reductase (53) (G) spinach glutEimate synthase (25) (H) beef heart aconitase (27). Spectra were recorded at temperatures between 1.5 and 70 K with an apphed magnetic field of 4.5 T (sdl trEmsitions increase in intensity with decreasing temperature). BEmds originating from minor heme contaminEmts Eire indicated by an asterisk.

The final step in heme synthesis involves the incorporation of ferrous iron into protoporphyrin in a reaction catalyzed by ferrochelatase (heme synthase), another mitochondrial enzyme (Figure 32-4). 

A summary of the steps in the biosynthesis of the porphyrin derivatives from PBG is given in Figure 32-8. The last three enzymes in the pathway and ALA synthase are located in the mitochondrion, whereas the other enzymes are cytosolic. Both erythroid and non-erythroid ( housekeeping ) forms of the first four enzymes are found. Heme biosynthesis occurs in most mammalian cells with the exception of mature erythrocytes, which do not contain mitochondria. However,... 

Alterations in blood heme metabolism have been proposed as a possible indicator of the biological effects of hydrogen sulfide (Jappinen and Tenhunen 1990), but this does not relate to the mechanism of toxicity in humans. The activities of the enzymes of heme synthesis, i.e., delta-aminolevulinic acid synthase (ALA-S) and heme synthase (Haem-S), were examined in 21 cases of acute hydrogen sulfide toxicity in Finnish pulp mill and oil refinery workers. Subjects were exposed to hydrogen sulfide for periods ranging from approximately 1 minute to up to 3.5 hours. Hydrogen sulfide concentrations were considered to be in the range of 20-200 ppm. Several subjects lost consciousness for up to 3 minutes. 

Oxygen activation is a central theme in biochemistry and is performed by a wide range of different iron and copper enzymes. In addition to our studies of the dinuclear non-heme iron enzymes MMO and RNR, we also studied oxygen activation in the mononuclear non-heme iron enzyme isopenicillin N synthase (IPNS). This enzyme uses O2 to transform its substrate ACV to the penicillin precursor isopenicillin N [53], a key step in the synthesis of the important P-lactam antibiotics penicillins and cephalosporins [54, 55],... 

M. Meier, M. Janosik, V. Kery, J.P. Kraus, and P. Burkhard, Structure of human cystathione beta-synthase a unique pyridozal 5 -phosphate-dependent heme protein. EMBO J. 20, 3910-3916 (2001). 

Peroxynitrite reacts with heme proteins such as prostacycline synthase (PGI2), microperoxidase, and the heme thiolate protein P450 to form a ferryl nitrogen dioxide complex as an intermediate [120]. Peroxynitrite also reacts with acetaldehyde with the rate constant of 680 1 mol 1 s" 1 forming a hypothetical adduct, which is decomposed into acetate, formate, and methyl radicals [121]. The oxidation of NADH and NADPH by peroxynitrite most certainly occurs by free radical mechanism [122,123], Kirsch and de Groot [122] concluded that peroxynitrite oxidized NADH by a one-electron transfer mechanism to form NAD and superoxide ... 

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