Multifunctional polypeptides

Five of the first six enzyme activities of pyrimidine biosynthesis reside on multifunctional polypeptides. One such polypeptide catalyzes the first three reactions of Figure 34-2 and ensures efficient channeling of carbamoyl phosphate to pyrimidine biosynthesis. A second bifunctional enzyme catalyzes reactions 5 and 6. 

Wilson, W.R., Tuan, R.S., Shepley, K.J., Freedman, D.O., Greene, B.M., Awadzi, K. and Unnasch, T.R. (1994) The Onchocerca volvulus homologue of the multifunctional polypeptide protein disulfide isomerase. Molecular and Biochemical Parasitology 68, 103-117. 

The fatty acid synthases of yeast and of vertebrates are also multienzyme complexes, and their integration is even more complete than in E. coli and plants. In yeast, the seven distinct active sites reside in two large, multifunctional polypeptides, with three activities on the a subunit and four on the /3 subunit. In vertebrates, a single large polypeptide (Afr 240,000) contains all seven enzymatic activities as well as a hydrolytic activity that cleaves the finished fatty acid from the ACP-like part of the enzyme complex. The vertebrate enzyme functions as a dimer (Afr 480,000) in which the two identical subunits lie head-to-tail. The subunits appear to function independently. When all the active sites in one... 

Reactions catalyzed by acetyl-CoA carboxylase. In E. coli, BCCP and the two enzymatic activities (biotin carboxylase and carboxyltransferase) can be separated from each other. In contrast, in the liver all three components exist on a single multifunctional polypeptide. 

A distinctly different form of acetyl-CoA carboxylase is found in the cytosol of animal tissues. The rat liver enzyme is a dimer composed of two identical subunits (Mr of each = 265,000) with one biotin per subunit. In contrast to the multienzyme complex in E. coli, the three functional parts of acetyl-CoA carboxylase in rat liver occur in a single multifunctional polypeptide. The enzyme, as a dimer, has very low activity. However, in the presence of citrate the enzyme oligomerizes to an active form, a polymer with a... 

J Cortes, SH Haydock, GA Roberts, DJ Bevitt, PF Leadlay. An unusually large multifunctional polypeptide in the erythromycin-producing polyketide synthase of Saccharopolyspora erythraea. Nature 348 176-178, 1990. 

The activities involved in yeast fatty acid biosynthesis are covalently linked as separate domains of two multifunctional polypeptides, a and p, encoded by the fas2 and fasl genes, respectively (Fig. 2) [57,58]. The functionalities associated with the 220 kDa a subunit include -ketoacyl synthase activity, -ketoacyl reductase activity, and an AGP domain which bears a phosphopantetheinylated serine. The 208 kDa -subunit has acetyl and malonyl CoA transacylase, palmi-toyl transferase, -hydroxyacyl-enzyme dehydratase, and enoyl acyl-enzyme reductase activities. The two subunits can be readily dissociated, and the individual activities maybe measured [57]. 

Velkov T, Lawen A. Mapping and molecular modeling of S- 28. adenosyl-L-methionine binding sites in N-methyltransferase domains of the multifunctional polypeptide cyclosporin synthetase. 

Regulation of metabolic processes can be accomplished by other methods. One is the use of a multienzyme complex (e.g., pyruvate dehydrogenase complex or fatty acid synthase complex) in which various enzymes are organized such that the product of one becomes the substrate for an adjacent enzyme. A single polypeptide chain may contain multiple catalytic centers that carry out a sequence of transformations (e.g., the mammalian fatty acid synthase see Chapter 18). Such multifunctional polypeptides increase catalytic efficiency by abolishing the accumulation of free intermediates and by maintaining a stoichiometry of 1 1 between catalytic centers. 

Diagram of a fatty acid synthase dimer with its head-to-tail association of the two multifunctional polypeptides. 

The assembly of rat liver FAS involves three stages synthesis of the multifunctional polypeptide chains, formation of the dimer, and attachment of a 4 -phosphopantetheine group by an enzyme-catalyzed reaction. This assembly process is influenced by changes in developmental, hormonal, and nutritional states. The FAS complex provides considerable catalytic efficiency, since free intermediates do not accumulate and the individual activities are present in equal amounts. 

In mammalian tissue, the six enzymes are encoded by three genes. One gene codes for a multifunctional polypeptide (Pyr 1-3) that is located in the cytosol and has carbamoyl phosphate synthetase II... 

Figure 27-27), aspartate transcarbamoylase, and dihydroorotase activity. Each subunit of Pyr 1-3 has a molecular weight of 200,000-220,000, and the native enzyme exists as multiples of three subunits. The second gene codes for dihydroorotate dehydrogenase which is located on the outer side of the inner mitochondrial membrane. Dihydroorotate, the product of Pyr 1-3, passes freely through the outer mitochondrial membrane and converted to orotate. Orotate readily diffuses to the cytosol for conversion to UMP. The third gene codes for another multifunctional polypeptide known as UMP synthase (Pyr 5,6). Pyr 5,6 (M.W. 55,000) contains orotate phosphoribosyltransferase and orotidylate (orotidine-5 -monophosphate) decarboxylase activity. Use of multifunctional polypeptides is very efficient, since the intermediates neither accumulate nor become consumed in side reactions. They are... 

Animal and fungal ACCs are comprised of large multifunctional polypeptides containing the biotin carboxylase, biotinyl carboxyl carrier protein, and carboxyltransferase... 

Fatty acid synthase is the name of the complex of six enzymatic activities that performs biosynthesis of fatty acids in cells. The enzyme is composed of two multifunctional polypeptide chains, which contain the enzymatic activities below ... 

In eukaryotes, enzymes 1 -3 of Figure 22.10 are all part of a single multifunctional polypeptide chain called CAD. In mammals, reactions 5 and 6 are catalyzed by a single protein called UMP synthase. 

Possible advantages of multifunctional polypeptide chains, that is, polypeptide chains having more than one active site, include which of the following ... 

The biosynthesis of a variety of biologically active peptides proceeds nucleic acid-free on protein templates (IK Peptide synthetases generally activate an acceptor amino acid by formation of amino-acyl adenylates or phosphates, which will be stabilized in an enzyne-aminoacylation step, similar as in tRNA-aminoacylation. Reaction with a donor peptide, which may be covalently bound, leads to a specific chain elongation. While small peptides like glutathione are formed by "one-step"-synthetases, more complex structures like gramicidin S are produced by multienzvme systems, which may contain multifunctional polypeptides. Characteristic features of such systems are 1.)activation as aminoacyl adenylates, 2.) aminoacylation of enzyme thiol-groups, 3.) covalently bound peptide intermediates and 4.) a specific intrinsic transport mechanism similar to the biosynthesis of fatty acids. 

The >20 natural amino acids and their synthetic derivatives constitute an abundant monomer source for the construction of multifunctional polypeptides. Like the micelles, liposomes, and hydrogels described above, photoresponsive polypeptides can be successfully constructed by introducing some photoresponsive moieties such as coumarin, 2-nitrobenzyl, cinnamyl, and... 

Wu Y, Wang T, Ng DYW, Weil T (2012) Multifunctional polypeptide-PEO nanoreacfaus via the hydrophobic switch. Macromol Rapid Cranm 33 1474-1481. doi 10.1002/marc. 

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