Synthetase complexes

Biosynthesis of coen2yme A (CoA) ia mammalian cells incorporates pantothenic acid. Coen2yme A, an acyl group carrier, is a cofactor for various en2ymatic reactions and serves as either a hydrogen donor or an acceptor. Pantothenic acid is also a stmctural component of acyl carrier protein (AGP). AGP is an essential component of the fatty acid synthetase complex, and is therefore requited for fatty acid synthesis. Free pantothenic acid is isolated from hver, and is a pale yeUow, viscous, and hygroscopic oil. 

Cusack, S., Yaremchuk, A., and Tukalo, M. (1996). The crystal structures of T. thermophilus lysyl-tRNA synthetase complexed with E. coli tRNA(Lys) and a T. thermophilus tRNA (Lys) transcript anticodon recognition and conformational changes upon binding of a lysyl-adenylate analogue. EM BO J. 15, 6321—6334. 

ESR spectrum of, 22 293, 295, 320, 321 glutamine synthetase complex, 28 362, 363 hydroxyl replacement, 33 49-51 poisoning, 27 317 sandwich compound, 27 227 -silica catalyst... 

CoRTi, O., et al.. The p38 subunit of the aminoacyl-tRNA synthetase complex is a Parkin substrate linking protein biosynthesis and neurodegeneration. Hum Mol Genet, 2003, 12(12), 1427-37. 

By observing changes in nucleotides that alter substrate specificity, researchers have identified nucleotide positions that are involved in discrimination by the amino-acyl-tRNA synthetases. These nucleotide positions seem to be concentrated in the amino acid arm and the anticodon arm, including the nucleotides of the anticodon itself, but are also located in other parts of the tRNA molecule. Determination of the crystal structures of aminoacyl-tRNA synthetases complexed with their cognate tRNAs and ATP has added a great deal to our understanding of these interactions (Fig. 27-17). 

The synthesis of fatty acids for incorporation into milk fat within the mammary gland is similar to that seen in other tissues. There are two basic reactions the conversion of acetyl-coenzyme A (CoA) to malonyl-CoA, followed by incorporation of the latter into a growing acyl chain via the action of the fatty acid-synthetase complex. However, the product of these reactions in lactating mammary tissue from many species is short and medium chain fatty acids. In most other tissues the product is palmitate. For more complete details see Moore and Christie, (1978), Bauman and Davis (1974), and Patton and Jensen (1976). 

The fatty acid-synthetase complex is located in the cytosol of the... 

Grunnet, I. and Knudsen, J. 1978. Medium chain acyl-thioester hydrolase activity in goat and rabbit mammary fatty acid synthetase complexes. Biochem. Biophys. Res. Commun. 80, 745-749. 

As an example of an asymmetric membrane integrated protein, the ATP synthetase complex (ATPase from Rhodospirillum Rubrum) was incorporated in liposomes of the polymerizable sulfolipid (22)24). The protein consists of a hydrophobic membrane integrated part (F0) and a water soluble moiety (Ft) carrying the catalytic site of the enzyme. The isolated ATP synthetase complex is almost completely inactive. Activity is substantially increased in the presence of a variety of amphiphiles, such as natural phospholipids and detergents. The presence of a bilayer structure is not a necessary condition for enhanced activity. Using soybean lecithin or diacetylenic sulfolipid (22) the maximal enzymatic activity is obtained at 500 lipid molecules/enzyme molecule. With soybean lecithin, the ATPase activity is increased 8-fold compared to a 5-fold increase in the presence of (22). There is a remarkable difference in ATPase activity depending on the liposome preparation technique (Fig. 41). If ATPase is incorporated in-... 

Cell-surface BI cellulase is envisaged as the form which is active against cellulose in peas in vivo, with a function that may be constructive in that it can act synergistically with plasma membrane-bound / -glucan synthetase complexes to enhance the rate of cellulose deposition (7,8,9). BS cellulase never appears to reach the wall in vivo in a form recognized by a BS antiserum (II). BS cellulase does not even bind readily to wall material in homogenates (Table III) despite its ability to bind to cellulose (3) and hydrolyze it (Table I). It is possible that BS cellulase functions intracellularly to hydrolyze a noncellulosic organelle-bound polysac-... 

Figure 11.7 Electron (e ) transport along the inner mitochondrial membrane resulting in the pumping of protons (P) out of the mitochondrial matrix. Protons are shuttled back into the matrix through the ATP synthetase complex where ATP is generated. Sites of toxicant action are indicated.

The rate limiting step in fatty acid synthesis is catalyzed by acetyl-CoA carboxylase to produce malonyl-CoA at the expense of one ATP.31 Malonate and acetate are transferred from CoA to acyl carrier protein in the cytosolic fatty acid synthetase complex, where chain extension leads to the production of palmitate. Palmitate can then be transferred back to CoA, and the chain can be extended two carbons at a time through the action of a fatty acid elongase system located in the endoplasmic reticulum. The >-hydroxylation that produces the >-hydroxyacids of the acylceramides is thought to be mediated by a cytochrome p450 just when the fatty acid is long enough to span the endoplasmic reticular membrane. 

Biou V et al (1994) The 2.9 A crystal structure of T. thermophilus seryl-tRNA synthetase complexed with tRNA(Ser). Science 263 1404-1410 PDBID 1SER... 

From the preceding discussions, it is evident that, in all systems studied, and, in particular, in higher plants, attempts to synthesize cellulose in vitro have met with only limited success this therefore leads to the conclusion that, for poorly understood reasons, the cellulose synthetase complex is a highly labile system. As a conclusion to this article, it may prove useful for future research to discuss possible reasons for this apparent lability. 

Several possible mechanisms can be envisaged to explain how Atjt may modulate the activity of the cellulose-synthetase complex. The effects of membrane fluidity on membrane-bound enzymes is well... 

Norcum MT, Warrington JA. Structural analysis of the multienzyme aminoacyl-tRNA synthetase complex A three domain model based on reversible chemical cross-linking. Protein Sci 1998 7 79-87. 

Figure 12.1 Formation of aminoacyl-tRNA involving the intermediate formation of an intermediate aminoacyl-adenylate-synthetase complex.

The pathway from acetate to palmitic acid (actually a palmitic acid-acyl carrier protein complex) involves at least nine enzymes acetyl CoA synthetase, acetyl CoA carboxylase, and the seven enzyme fatty acid synthetase complex. We chose first to test the effect of these compounds on acetyl CoA carboxylase (ACCase) activity. There were several reasons to select ACCase as the... 

Three levels of protein-nucleic acid recognition have been observed. Nature provides three examples of protein-nucleic acid interactions which we shall consider. The nucleic acid component can be (1) a single nucleotide, e.g., a coenzyme or a substrate, (2) a single-stranded DNA or RNA as in ribonucleases A and T, or (3) a double-stranded DNA or RNA as in the highly specific complexes with repressors in the tRNA-synthetase complex, or in the unspecific nuclease DNase I. 

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