Diiron desaturases

Dihydroxyproline 80 Diiron desaturases 863 Diiron oxygenases 863 Diiron proteins 862-864 Diiron-tyrosinate proteins 862-863 Diisopropylfluorophosphate (DFP, diisopropylphosphofluoridate) 610s Dimethylallyl diphosphate 712s Dimethylallyl pyrophosphate, condensation of 527... 

Figure 13.25 Three-dimensional structures of diiron proteins. The iron-binding subunits of (a) haemery-thrin, (b) bacterioferritin, (c) rubryerythrin (the FeS centre is on the top), (d) ribonucleotide reductase R2 subunit, (e) stearoyl-acyl carrier protein A9 desaturase, (f) methane monooxygenase hydroxylase a-subunit. (From Nordlund and Eklund, 1995. Copyright 1995, with permission from Elsevier.)...

Figure 13.26 Dioxygen-utilizing carboxylate-bridged diiron centres (a) Oxidized (top) and reduced (bottom) MMOH (b) oxidized (top) and Mnn-reconstituted ToMOH (bottom) (c) oxidized (top) and reduced (bottom) RNR-R2 (d) oxidized (top) and reduced (bottom) rubryerythrin (e) reduced stearoyl-acyl carrier protein A9 desaturase (f) reduced bacterioferritin (g) methaemerythrin. Fel is on the left and Fe2 on the right. (Reprinted with permission from Sazinsky and Lippard, 2006. Copyright (2006) American Chemical Society.)...

In the diiron active sites of RNR-R2, A9 desaturase, bacterioferritin and rubrerythrin the flanking carboxyl ligands on the opposite side of the diiron centre are all quite different,... 

Figure 16-20 (A) The active site of hemerythrin showing the two iron atoms (green) and their ligands which include the (X oxo bridge and two bridging car-boxylate groups. From Lukat et al.193 The active site is between four parallel helices as shown in Fig. 2-22. (B) Stereoscopic view of the backbone structure of a A9 stearoyl-acyl carrier protein desaturase which also contains a diiron center.

Notice the nine-antiparallel-helix bundle. The diiron center is between four helices as in hemerythrin. (C) Stereoscopic view of the diiron center of the desaturase.333 (B) and (C) courtesy of Ylva Lindqvist. 

In plants a similar enzyme catalyzes formation of the first double bond in a fatty acyl group converting stearoyl-ACP into oleoyl-ACP in the chloroplasts.72 753/105 108 The soluble A9 stearoyl-ACP desaturase has a diiron-oxo active site (Fig. 16-20, B, C).i°9 no Electrons are donated from light-generated reduced ferredoxin (see Chapter 23). In addition to the A9 desaturase both plants and cyanobacteria usually desaturate C18 acids also at the A12 and A15 positions and C16 acids at the A7, A20, and A13 (co3) positions.iii ii2 Desaturation of oleate occurs primari-... 

The origin of ricinoleic acid, an abundant constitu-tuent of castor beans, is also shown in Fig. 21-2. It is formed by an oleate hydroxylase that has an amino acid sequence similar to those of oleate desaturases.113 Both hydroxylation and desaturation are reactions catalyzed by diiron centers.114 Other fatty acid hydroxylases act on the alpha115 and the omega positions. The latter are members of the cytochrome P450 family.116 117... 

Conformational changes in diiron center of stearoyl-acyl carrier protein desaturase caused by substrate binding have been probed by EPR and proton ENDOR of cryoreduced diferric protein.80 EPR spectra of the one-electron reduced Fe(III)Fe(II)... 

Fatty acyl-CoA desaturases are terminal oxidases of a membrane-bound enzyme complex that also includes cytochrome b5 and cytochrome b5 reductase (Bloomfield and Bloch, 1960). They remove substrate hydrogen atoms at a position determined by the specificity of the enzyme. They play essential roles in regulating membrane fluidity and are also involved in insect lipid and pheromone metabolism. They share the presence of three highly conserved histidine-rich sequences (H-boxes) that coordinate the diiron-oxo structure at the active sites (Shanklin and Cahoon, 1998) and four hydrophobic a helices that appear to anchor the protein into the lipid bilayer and situate the H-boxes in their correct position in the active site. 

Fox BG, Lyle KS, Rogge CE. Reactions of the diiron enzyme stearoyl-acyl carrier protein desaturase. Acc Chem Res. 2004 37 421-9. 

The mid-chain dehydrogenation of saturated fatty acyl derivatives is carried out by a large family of 02-dependent, nonheme diiron-containing enzymes known as desaturases. Both soluble and membrane-bound desaturases have been characterized. The mechanism of desaturation is thought to involve the stepwise syn removal of vicinal hydrogen atoms via a short-lived carbon-centered radical intermediate. The most common desaturase inserts a (Z)-double bond between the C-9,10 carbons of a stearoyl thioester however, many variations of this prototypical reaction have been discovered. Accounting for this diversity in terms of subtle alterations in active-site architecture constitutes a new frontier for research in this area. 

Diiron oxygenases and desaturases Hydrogen Transfer and Dependent... 

The presentation in 1993 of the structure of the hydroxylase component of methane monooxygenase (MMOH) by Rosenzweig et al. (15) is the third published three-dimensional structure of a diiron-oxygen protein (Fig. 1). The previous two are from hemerythrin (Hr) (16,17) and protein R2 of E. coli ribonucleotide reductase (RNR-R2) (18, 19). Some other dinuclear iron proteins with known fi-oxo or p.-hydroxo bridges are purple acid phosphatases (PAP) [(e.g., uteroferrin (Uf)] (20, 21), ferritins (in early stages of nucleation) (22), rubrerythrin (Rr) (23-26), nigerythrin (26), and soluble stearoyl-acyl carrier protein A desaturase (A-AGP) (27, 28). 

---