Cysteine proximate

Figure 21-2. Fatty acid synthase multienzyme complex. The complex is a dimer of two identical polypeptide monomers, 1 and 2, each consisting of seven enzyme activities and the acyl carrier protein (ACP). (Cys— SH, cysteine thiol.) The— SH of the 4 -phosphopantetheine of one monomer is in close proximity to the— SH of the cysteine residue of the ketoacyl synthase of the other monomer, suggesting a "head-to-tail" arrangement of the two monomers. Though each monomer contains all the partial activities of the reaction sequence, the actual functional unit consists of one-half of one monomer interacting with the complementary half of the other. Thus, two acyl chains are produced simultaneously. The sequence of the enzymes in each monomer is based on Wakil.

There are two types of electron transport those involving flavoproteins and iron-sulfur proteins, and those requiring only flavoproteins. The X-ray crystal structure of the soluble cytochrome P450 from Pseudomonas putida grown on camphor (P-450-CAM) has been determined (Poulos et ah, 1985), as have several others. The haem group is deeply embedded in the hydrophobic interior of the protein, and the identity of the proximal haem iron ligand, based on earlier spectroscopic studies (Mason et ah, 1965) is confirmed as a specific cysteine residue. 

The central unit of these peptidomimetics imitates a /1-turn and brings the NH2-terminus of the cysteine analogue and the CO OH terminus of the methionine in spatial proximity these can then complex the Zn2+ ion which is essential for activity of the FTase [26]. The free acid 7 inhibits the enzyme with an IC50 value of 1 nmol/1, whilst in intact cells the methyl ester 8, despite its weaker in vitro activity, is significantly more potent because it can penetrate the plasma membrane better due to its lower polarity. This property can be used to convert the morphology of H-Ras-transformed cells back to the normal form and to inhibit growth of these cells, whereas the substance shows no effect on Src-transformed and untransformed rat fibroblasts. The inhibitor therefore acts selectively on transformed cells and does not influence growth of normal cells. This result is noteworthy because farnesylation of the wild type H-Ras protein... 

Brocke, L., Bendahan, A., Grunewald, M., and Kanner, B. I. (2002) Proximity of two oppositely oriented reentrant loops in the glutamate transporter GLT-1 identified by paired cysteine mutagenesis. J. Biol. Chem. 277, 3985-3992. 

A related fibril model for A/ o was proposed based on scanning proline mutagenesis (Williams et al, 2004) and molecular modeling (Guo et al., 2004). This model proposes that residues 15-21, 24-28, and 31-36 form 3 /-strands, with 2 intervening turns formed by residues 22-23 and 29-30 (Fig. 17G). Residues 17 and 34 are placed in close proximity, as double cysteine mutants at these positions form disulfide bonds on oxidation after fibrillization (Shivaprasad and Wetzel, 2004). Since fibrils with this triangular cross section would not be expected to show an H0-A... 

Isolated proximal tubules have been utilized to study the mechanisms of nephrotoxicity induced by antibiotics (Sina et al., 1985, 1986), radiocontrast dyes (Humes et al., 1987), metals (Rylander et al., 1985), anoxia (Weinberg, 1985 Weinberg et al., 1987), cellular oxidants (Messana et al., 1988), cysteine conjugates (Rylander et al., 1985 Schnellman et al., 1987 Zhang and Stevens, 1989), and a variety of nephrotoxic bromobenzene metabolites (Schnellman and Mandel, 1986 Schnellman et al., 1987). 

Schnellman, R.G., Lock, E.A. and Mandel, L.J. (1987). A mechanism of 5-(l,2,3,4,4-Pentachloro-l,3-butadienyl)-L-cysteine toxicity to rabbit proximal tubules. Toxicol. Appl. Pharmacol. 90 513-521. 

Zhang, G. and Stevens, J.L. (1989). Tansport and activation of 5-(l, 2-dichlorovinyl)-L-cysteine andA-acetyl-S-(l,2-dichlorovinyl)-L-cysteine in rat kidney proximal tubules. Toxicol. Appl. Pharmacol. 100 51-61. 

Figure 3.12 Active site of a reduced form of the Fe-only hydrogenase from Desulphovibrio desul-phuricans. The Fe atom on the right is defined as the proximal Fe (relative to the neighbouring [Fe-S] cluster), Fep the Fe atom on the left is defined as the distal Fe, FeD. The arrow indicates the potential hydron-binding site on FeD that is occupied by either HzO or an extrinsic CO in the structure of Cp I. Also shown is a close contact between the bridgehead atom X of the exogenous dithiolate ligand and the S atom of cysteine-178. (Reprinted with permission from Parkin et al., 2006. Copyright (2005) American Chemical Society.)...

It is noteworthy that there is another limiting factor in the choice of amino acid types at the junction sites which affect the enzymatic process of the intein. For example, in the case of SceVMA (also called PI-Seel) from the IMPACT system, proline, cysteine, asparagine, aspartic acid, and arginine cannot be at the C-terminus of the N-terminal target protein just before the intein sequence. The presence of these residues at this position would either slow down the N-S acyl shift dramatically or lead to immediate hydrolysis of the product from the N-S acyl shift [66]. The compatibility of amino acid types at the proximal sites depends on the specific inteins and needs to be carefully considered during the design of the required expression vectors. The specific amino acid requirements at a particular splicing site depends on the specific intein used and is thus a crucial point in this approach. 

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