Enzymes homologous

The class I FruA isolated from rabbit muscle aldolase (RAMA) is the aldolase employed for preparative synthesis in the widest sense, owing to its commercial availability and useful specific activity of 20 U mg . Its operative stability in solution is limiting, but the more robust homologous enzyme from Staphylococcus carnosus has been cloned for overexpression [87], which offers unusual stability for synthetic purposes. Recently, it was shown that less polar substrates may be converted as highly concentrated water-in-oil emulsions [88]. 

Homologation, of boronic esters, 23 671 Homologous enzyme structures, 10 337 Homologous promoters, 22 453 Homologous recombination, 22 459-460 Homologous temperature, 23 487 Homology, of proteins, 20 833-834... 

Enzymes that perform the same catalytic function are known as homologous enzymes and fall into two classes. Heteroenzymes are derived from different sources and although they catalyse the same reaction they show different physical and kinetic characteristics. The hydrolytic enzyme a-amylase (EC 3.2.1.1) is found in the pancreatic secretion in man and is different from the enzymes of the same name which are derived from bacteria or malt. Iso-enzymes, sometimes referred to as isozymes, are different molecular forms of the same enzyme and are found in the same animal or organism although they often show a pattern of distribution between tissues. 

Fig. 9. Pathway duplication the methyl citrate cycle and the glyoxylate shunt. A pathway for acetate metabolism in E. coli that uses the glyoxylate shunt is depicted on the right. Part of the methyl citrate cycle, a pathway for propionate metabolism, is depicted on the left. The pathways are analogous furthermore, three of the four steps are catalyzed by homologous enzymes. PrpE (propionyl-CoA synthase) is homologous to AcsA (acetyl-CoA synthase). PrpC (2-methyl-citrate synthase) is homologous to GltA (citrate synthase). PrpB (2-methyl-isocitrate lyase) is homologous to AceA (isocitrate lyase). The third step in the methyl citrate cycle has been suggested to be catalyzed by PrpD the second half of the reaction (the hydration) can be catalyzed by aconitase.

Hrdy I, Muller M (1995b) Primary structure of the hydrogenosomal malic enzyme of Trichomonas vaginalis and its relationship to homologous enzymes. J Eukaryot Microbiol 42 593-603... 

Schultz, C.P. Ylisastigui-Pons, L. Serina, L. Sakamoto, H. Mantsch, H.H. Neuhard, J. Barzu, 0. Gilles, A.M. Structural and catalytic properties of CMP kinase from Bacillus subtilis a comparative analysis with the homologous enzyme from Escherichia coli. Arch. Biochem. Biophys., 340, 144-153 (1997)... 

Families a group of homologous enzymes catalyzing the same reaction (mechanism and substrate specificity) there is often > 30% sequence identity. 

Superfamilies a group of homologous enzymes that catalyze either (i) the same reaction with differing substrate specificity, or (ii) different reactions with a shared mechanistic attribute (transition state, intermediate) there is often < 20% sequence identity. 

Suprafamilies a group of homologous enzymes that catalyze different overall reactions which do not share common mechanistic attributes and do not share a high level of sequence identity (< 20%) but are metabolically linked such as in successive reactions. Active-site residues may be conserved but perform different functions in family members.Tables 16.2 and 16.3 list known superfamilies and suprafamilies with some of their family members. 

PAH, TH and TPH are highly homologous enzymes. These enzymes catalyze a hydroxylation reaction of aromatic amino acids that requires reduced pterin cofactor 43, molecular oxygen, and iron (Scheme 28). Iron is present at the active sites of the enzymes. Ferrous iron (Fe(II)) is essential for the catalysis, although, the iron was found to be in the ferric form (Fe(III)) when the enzymes were purified from tissues or cells. The ferric iron at the active site of the enzymes was found to be reduced to the ferrous form by BH4 [125]. Thus, BH4 serves a bi-functional role for aromatic amino acid hydroxylases one is the reduction of iron at the active sites from the ferric form to the ferrous form and the other is an electron donor for the hydroxylation reaction. 

The positions of the amino acid substitutions identified in the various pNB esterases are illustrated in Fig. 7, on a model of the pNB esterase developed from the X-ray crystal structures of homologous enzymes [2]. Known beneficial mutations are indicated also shown are the mutations believed to be neutral or to affect expression. Positions of the additional translated mutations found in the variants produced by DNA shuffling are indicated by the white arrows. None of the effective amino acid substitutions lie in segments of the esterase predicted to interact directly with the bound substrate. It may be that the amino acid substitutions sampled at positions adjacent to the substrate were all... 

In the short term, the most productive method for identifying yeast reductases will involve testing deletion strains for an inability to reduce ketones that are accepted by the parent strain. This search can be focused on the most likely candidates if a homologous enzyme from another species with similar stereoselectivity has been identified. Ideally, a single knockout mutation would abrogate... 

Further modifications using the same strain of ODC S. cerevisiae reconstituted a bacterial/plant polyamine synthesis pathway in yeast [41], The ODC strain was transformed with plasmids encoding arginine decarboxylase and ag-matine ureohydrolase, which conferred polyamine-independent growth on the recombinant microbe. A similar construction could be used to screen for inhibitors of the homologous enzymes from Apicomplexan protozoa, which synthesize poly amines through this pathway [42]. 

Comparison of EDso Values of Thyroxine (Tj) in Various Types of Heterologous and Homologous Enzyme Immunoassays of T4° fc... 

The argument previously outlined provides an appealing physiochemi-cal explanation for the stability and activity behavior of homologous enzymes adapted to different temperatures. However, one cannot interpret the behavior of a biological system solely in physiochemical terms. All these enzymes are the products of evolution. While they are certainly subject to the laws of physics and chemistry, the evolutionary process imposes its own, additional constraints. We will see that the stability-activity trade-off is not a necessary characteristic of enzymes, especially not those evolved in the laboratory. 

Fig. 3. Homologous enzymes adapted to different temperatures show a trade-off between catalytic activity at low temperatures (high for enzymes from psychrophilic organisms, but generally low for enzymes from thermophiles) and thermostability (high for thermophilic enzymes, but low for enzymes from psychrophiles). These natural enzymes lie in the darker shaded area, which is bounded on one side by the minimal stability and activity required for biological function. Enzymes that are both highly thermostable and highly active at low temperature (lighter shaded area) are generally not found in nature.

The carbon backbones of homologous proteins (especially closely related ones, such as orthologs) are often nearly superimposable in three-dimensional space.3 This type of observation led to the widespread belief that the observed differences in function between homologous enzymes are due primarily to replacements of amino acid side chains, rather than to rearrangements of the carbon backbone. For example, the backbones of the substrate binding pockets of cow CPA1 and CPB are nearly superimposable in three-dimensional space (not shown), suggesting that the differ-... 

F. Labeyrie and M. Somlo, Homologous Enzymes and Biochemical Evolution Colloquium (Nguyen van Thoai and J. Roche, eds.), p. 93. Gordon Breach, New York, 1968. 

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