Stearoyl acyl carrier protein

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.)...

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.

In green plants a soluble A9 stearoyl-acyl carrier protein desaturase uses 02 and NADH or NADPH to introduce a double bond into fatty acids. The structure of this protein (Fig. 16-20B,C) is related to those of methane oxygenase and ribonucleotide reductase.333347 Tire desaturase mechanism is discussed in Chapter 21. 

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)... 

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

Broadwater, J. A., Ai, J., Loehr, T. M., Sanders-Loehr, J., and Fox, B. G., 1998, Peroxodiferric intermediate of stearoyl-acyl carrier protein A9 desaturase oxidase reactivity during single turnover and implications for the mechanism of desaturation. Biochemistry 37 14664nl4471. 

Lindqvist Y, Huang W, Schneider G, Shanklin J. Crystal Structure of h stearoyl-acyl carrier protein desaturase from castor seed and its relationship to other di-iron proteins. EMBO J. 199 15 4081 092. 

The iron ligands in the dinuclear sites of these proteins are very similar — the structures of the (p-carboxylato) di-iron cores found for MMOH, ToMOH, RNR-R2, rubreythrin, stearoyl-acyl carrier protein A desaturases, bacterioferritin, and methaemerythrin are presented in Figure 13.26. They all have the same three-amino-acid structural motif on one side of the di-iron site, made up of a bridging Glu and two His residues, coordinated in... 

VIII. Soluble Stearoyl-Acyl Carrier Protein A Desaturase... 

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). 

The soluble stearoyl-acyl carrier protein A desaturase from higher plants such as castor, cucumber, spinach, turnip, rape, and avocado catalyzes an important step in plant lipid desaturation (27, 28, 45). The reaction is important for regulation of membrane fluidity and is also an important factor in nutrition. 

Strater, Krebs, and Witzel, personal communication). The bacteriofer-ritin cluster is very similar to the MMOH cluster. In the near future the structures of the stearoyl-acyl carrier protein A desaturase (46), and rubrerythrin will be solved. 

A-ACPs in higher plants are homodimeric soluble proteins of about 70 kDa. They catalyze the first and most important desaturation of stearoyl-acyl carrier protein, resulting in an oleoyl-acyl carrier protein, which is a major precursor in fatty acid biosynthesis in plants. A-ACP is involved in controlling the ratio of saturated to unsaturated... 

Gibson, K.J. Palmitoleate formation by soybean stearoyl-acyl carrier protein desaturase. Biochim. Biophys. Acta 1993,1169, 231-235. 

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