Bifunctional enzymes

Figure 4.6 The bifunctional enzyme PRA-isomerase (PRAI) IGP-synthase (IGPS) catalyzes two sequential reactions in the biosynthesis of tryptophan. In the first reaction (top half), which is catalyzed by the C-terminal PRAI domain of the enzyme, the substrate N-(5 -phosphoribosyl) anthranilate (PRA) is converted to l-(o-carboxyphenylamino)-l-deoxyribulose 5-phosphate (CdRP) by a rearrangement reaction. The succeeding step (bottom half), a ring closure reaction from CdRP to indole-3-glycerol phosphate (IGP), is catalyzed by the N-terminal IGPS domain.

PRA-isomerase lGP-synthase, a bifunctional enzyme from E. coli that catalyzes two reactions in the synthesis of tryptophan (Figure 4.6), has a polypeptide chain that forms two a/p barrels. The stmcture of this enzyme, solved at 2.8 A in the laboratory of Hans Jansonius in Basel, Switzerland, showed that residues 48-254 form one barrel with IGP-synthase activity, while residues 255-450 form the second barrel with PRA-isomerase activity (Figure 4.7). 

In Bacillus snbtilis these two reactions are catalyzed by two separate enzymes that have amino acid sequences homologous to the corresponding regions of the bifunctional enzyme from E. coli, and thus each forms a barrel... 

Figure 4.7 Two of the enzymatic activities involved in the biosynthesis of tryptophan in E. coli, phosphoribosyl anthranilate (PRA) isomerase and indoleglycerol phosphate (IGP) synthase, are performed by two separate domains in the polypeptide chain of a bifunctional enzyme. Both these domains are a/p-barrel structures, oriented such that their active sites are on opposite sides of the molecule. The two catalytic reactions are therefore independent of each other. The diagram shows the IGP-synthase domain (residues 48-254) with dark colors and the PRA-isomerase domain with light colors. The a helices are sequentially labeled a-h in both barrel domains. Residue 255 (arrow) is the first residue of the second domain. (Adapted from J.P. Priestle et al., Proc.

Priestle, J.P, et al. Three-dimensional structure of the bifunctional enzyme N-(5 -phosphoribosyl) anthranilate isomerase-indole-3-glycerol-phosphate synthase from Escheriehia eoli. Proc. Natl. Aead. 

Figure 19-3. Control of glycolysis and gluconeoge-nesis in the liver by fructose 2,6-bisphosphate and the bifunctional enzyme PFK-2/F-2,6-Pase (6-phospho-fructo-2-kinase/fructose-2,6-bisphosphatase). (PFK-1, phosphofructokinase-1 [6-phosphofructo-1 -kinase] ...

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. 

Dogbo, O. et al., Carotenoid biosynthesis isolation and characterization of a bifunctional enzyme catalyzing the synthesis of phytoene, Proc. Natl. Acad. Sci. USA 85, 7054, 1988. 

PAM is a bifunctional enzyme found in nearly all LDCVs (Fig. 18-6) [12]. PAM acts on peptide substrates... 

Bifunctional protein deficiency. The enzyme defect involves the D-bifunctional protein. This enzyme contains two catalytic sites, one with enoyl-CoA hydratase activity, the other with 3-hydroxyacyl-CoA activity [13]. Defects may involve both catalytic sites or each separately. The severity of clinical manifestations varies from that of a very severe disorder that resembles Zellweger s syndrome clinically and pathologically, to somewhat milder forms. Table 41-6 shows that biochemical abnormalities involve straight chain, branched chain fatty acids and bile acids. Bifunctional deficiency is often misdiagnosed as Zellweger s syndrome. Approximately 15% of patients initially thought to have a PBD have D-bifunctional enzyme deficiency. Differential diagnosis is achieved by the biochemical studies listed in Table 41-7 and by mutation analysis. 

Alkyl PAT, alkyl-dihydroxy phosphate synthase Bif, bifunctional enzyme DHAPAT, dihydroxyphosphate acyltransferase deficiency DHCA, dihydroxycholestanoic acid N, normal nd, not determined Ox, acyl-CoA oxidase Rac, 2-methylacyl-CoA racemase RCDP, rhizomelic chondrodysplasia punctata Ref, Refsum s disease THCA, trihydroxycholestanoic acid VLCFA, very-long-chain fatty acid. 

Figure lO.lOA shows the reaction scheme of the system outlining the electron transfer pathway among the different components, namely the electron donor Tris, the semiconductor Ti02 and the sulfhydrogenase of the hyperthermophilic archaeon P. furtosus, a bifunctional enzyme catalysing either proton or sulfur species reduction. 

This pyridoxal-phosphate-dependent enzyme [EC 2.1.2.5], also known as glutamate formyltransferase, catalyzes the reaction of 5-formiminotetrahydrofolate with L-glutamate to produce tetrahydrofolate and A-formim-ino-L-glutamate. The enzyme will additionally catalyze the transfer of the formyl moiety from 5-formyltetrahy-drofolate to L-glutamate. This protein occurs in eukaryotes as a bifunctional enzyme also having a formiminote-trahydrofolate cyclodeaminase activity [EC 4.3.1.4]. 

BIFUNCTIONAL CATALYSIS BIFUNCTIONAL ENZYME BIFURCATION THEORY BILIVERDIN REDUCTASE BIMOLECULAR... 

BIFUNCTIONAL ENZYME GLUTAMINYL-tRNA SYNTHETASE AMINOACYL-tRNA SYNTHETASES y-Glutamyl carboxylase,... 

T. Kawakami, R. Ohshima, T. ADP-dependent glucokinase/phosphofructo-kinase, a novel bifunctional enzyme from the hyperthermophilic archaeon Methanococcus jannaschii. J. Biol. Chem., Til, 12495-12498 (2002)... 

Meijer, P.-J. Lilius, G, Holmberg, N. Bulow, L. An artificial bifunctional enzyme, y-glutamyl kinase/y-glutamyl phosphate reductase, improves NaCl tolerance when expressed in Escherichia coli. Biotechnol. Lett, 18, 1133-1138 (1996)... 

Glucagon or epinephrine decreases [fructose 2,6-bisphosphate]. The hormones do this by raising [cAMP] and bringing about phosphorylation of the bifunctional enzyme that makes and breaks down fructose 2,6-bisphosphate. Phosphorylation inactivates PFK-2 and activates FBPase-2, leading to breakdown of fructose 2,6-bisphosphate. Insulin increases [fructose 2,6-bisphosphate] by activating a phosphoprotein phosphatase that dephosphorylates (activates) PFK-2. 

The a/ 3 barrel shown in Fig. 2-28 consists of 8 consecutive (3-a units in a symmetric array.265 266 By 1995 over 40 of these barrels had been identified in a diverse group of enzymes. One bifunctional enzyme contains two a/(3 barrels. Although the nature of the reaction catalyzed varies, the active site is always found in the center of the barrel at the C-terminal ends of the 8 parallel (3 strands and therefore between the N termini of the surrounding helices. The enzyme sequences show no homology and frequent occurrence of the 8-stranded barrel may reflect the fact that it is a natural packing arrangement of (3-a units. However, a 10-stranded barrel of this type has also been found.267... 

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