Multienzyme complex assembly

One of the most fascinating recent developments in biology has been the discovery of numerous highly complex biopolymer assemblies (see also section C2.14.2.3) such as the ribosome or the bacterial flagellum [93, 94 and 95], the envy of nanoteclmologists seeking to miniaturize man-made mechanical devices (note that the word machinery is also sometimes used to refer to multienzyme complexes such as the proteasome [96]), and an entire... 

In biological systems molecular assemblies connected by non-covalent interactions are as common as biopolymers. Examples arc protein and DNA helices, enzyme-substrate and multienzyme complexes, bilayer lipid membranes (BLMs), and aggregates of biopolymers forming various aqueous gels, e.g, the eye lens. About 50% of the organic substances in humans are accounted for by the membrane structures of cells, which constitute the medium for the vast majority of biochemical reactions. Evidently organic synthesis should also develop tools to mimic the Structure and propertiesof biopolymer, biomembrane, and gel structures in aqueous media. 

Acetyl-CoA carboxylase is required to convert acetyl-CoA to malonyl-CoA. In turn, fatty acid synthase, a multienzyme complex of one polypeptide chain with seven separate enzymatic activities, catalyzes the assembly of palmitate from one acetyl-CoA and seven malonyl-CoA molecules. 

E Guenzi, G Galli, I Grgurina, E Pace, P Ferranti, G Grandi. Coordinate transcription and physical linkage of domains in surfactin synthetase are not essential for proper assembly and activity of the multienzyme complex. J Biol Chem 273 14403-14410, 1998. 

There are multienzyme complexes that efficiently catalyze sequential reactions in some metabolic pathways. The overall rate of a sequential reaction is greatly enhanced by the assembly of subunits possessing sequential metabolic activities, because the metabolites are transferred directly from one active site to another without diffusing in the solution.9 Typical examples are the mammalian pyruvate dehydrogenase complex (three catalytic subunits)10 and the bacterial trytophan synthase (two catalytic subunits).11 ... 

It can be advantageous for a series of enzymes catalyzing a sequence of reactions in a metabolic pathway to assemble into a multienzyme complex. This assembly increases the efficiency of the pathway in that the product of one enzymatic reaction is in place to be the substrate of the next... 

But in other thiamin-responsive cases, the mutation is in the E2 subunit, suggesting that assembly of the active multienzyme complex affects the affinity of the thiamin diphosphate binding site of the Ela subunit. 

The oxidase is a cell membrane-multienzyme complex. It has a cell surface receptor linked to a G-protein that activates a phosphatidyl inositol cascade leading to assembly and activation of the oxidase complex. The receptor is activated by the following ... 

The limit imposed by the rate of diffusion in solution can also be partly overcome by confining substrates and products in the limited volume of a multienzyme complex. Indeed, some series of enzymes are associated into organized assemblies (Section 17.1.9) so that the product of one enzyme is very rapidly found by the next enzyme. In effect, products are channeled from one enzyme to the next, much as in an assembly line. 

GJ. Domingo, H.J. Chauhan, LA. Lessard, C. Fuller, and R.N. Perham. 1999. Self-assembly and catalytic activity of the pyruvate dehydrogenase multienzyme complex from Bacillus stearothermophilus Eur. J. Biochem. 266 1136-1146. (PubMed)... 

Several techniques have established the association of enzymes in peptidoglycan assembly and degradation. In vivo crosslinking experiments in bacteria including E. coli, B. subtilis, and H. influenzae have shown that the PBPs are organized into several multienzyme complexes [99,100,101]. Data from co-immunoprecipitation studies support this conclusion [102], and affinity chromatography has additionally shown that the PBPs interact with peptidoglycan hydrolases [103,104,105]. 

The multienzyme complexes are self-assembling emd will reassemble to an active complex after resolution of the individual enzymes. The core enzyme of the complex is the dihydrolipoyl acyltransferase (E2) the oxo-acid dehydrogenase (El) and dihydrolipoyl dehydrogenase (E3) subunits form noncovalent bonds to this central catalytic unit. 

Many eukaryotic multienzyme complexes are multifunctional proteins in which different enzymes are linked covalently. An advantage of this arrangement is that the synthetic activity of different enzymes is coordi nated. In addition, intermediates can be efficiently handed from one active site to another without leaving the assembly. Furthermore, a complex of covalently joined enzymes is more stable than one formed by noncovalent attractions. Each of the component enzymes is recognizably homologous to its bacterial counterpart. It seems likely that multifunctional enzymes sudi as fatty acid synthase arose in eukaryotic evolution by fusion of the individual genes of evolutionary ancestors,... 

Bates, D. L., Danson, M. J., Hale, G., Hooper, E. A., Peeham, R. N. (1977), Self-assembly and catalytic activity of the pyruvate dehydrogenase multienzyme complex of Escherichia coli, Nature 268, 313-316. 

In the cell, compartmentation of enzymes into multienzyme complexes or organelles provides a means of regulation, either because the compartment provides unique conditions or because it limits or channels access of the enzymes to substrates. Enzymes or pathways with a common function are often assembled into organelles. For example, enzymes of the TCA cycle are all located within the mitochondrion. The enzymes catalyze sequential reactions, and the product of one reaction is the substrate for the next reaction. The concentration of the pathway intermediates remains much higher within the mitochondrion than in the surrounding cellular cytoplasm. 

Flg. 13. The chloroplast envelope plays a predominant role in the assembly of the three parts of the galactolipid molecule (galactose, glycerol, fatty acids). Saturated and monounsat-urated fatty acids are synthesized in the stroma by a multienzyme complex (fatty acid synthetase). Then, the different steps occur on the envelope, probably at the level of the inner membrane. Under these conditions, massive transport of galactolipids should occur very rapidly between the inner layer of the inner envelope membrane and the thylakoids (reproduced from Douce and Joyaid, 1979a, by permission). 

It is now known that the mode of biosynthesis of phenolic acids and related phenols strongly resembles that of fatty acids universally found in nature. Both classes of compound are assembled by multienzyme complexes catalyzing condensation reactions between acyl and malonyl residues. The formation of phenols usually entails few reductive steps, in contrast to fatty acid synthesis, and the oxygen functions are therefore retained. 

The initial assembly of seven acyl CoA precursors to build a polyketide carbon chain is carried out by a multienzyme complex called a polyketide synthase, or PKS. The 6-deoxyerythronolide B synthase (DEBS) is a massive structure of greater than 2 million molecular weight and containing more than 20,000 amino acids. Furthermore, it is a homodimer, meaning that it consists... 

Self-sorting is one of the most fundamental processes in living systems, which led complex mixtures of biomolecules (e.g., proteins, nucleic acids, oligosaccharides, lipids) to self-organize into larger biomacromolecular assemblies (e.g., DNA double-helix, multienzyme complexes, ribosomes) and finally into cellular compartments essential for the development of life on Earth. 

Finally, the enzyme organization in multienzyme complexes or polyenzyme systems also significantly affects the phenotypic expression of various enzymes. The assembling of such systems in the cell is apparently one of the basic mechanisms of intefgenic interactions since the enzymes comprising the multienzyme complex are products of different loci (De Moss et al., 1967 Chalmers and De Moss, 1970 Maletsky, 1972). 

There are numerous variants in the conditions of assembly of multienzyme complexes since loci which are responsible for enzyme syntheses could be heterozygous. If all of these loci are heterozygous, the number of variants in complexes is determined by the following product (Maletsky, 1972) ... 

Each variant of the enzyme assembly of the multienzyme complex is realized with a certain degree of probability which is dependent upon numerous conditions and differs in various tissues and cell types. 

The activity of either isoenzyme, naturally, depends, to a great degree, upon which variant of a given multienzyme system will be realized. Since in some cases some of the enzyme molecules cannot associate, this could occur, for instance, as a result of a lack of conformational accordance. Thus, the complex effect of differences in translation rate and also post-translational events (assembly with membranes or inhibitors, degradation, formation of multienzyme complexes, etc.) is an important factor in the realization of differential expression of biochemical traits in the phenotype. 

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