Phosphoribosyl anthranilate

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.

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. 

Besides having a noncovalent association of subunits as in tryptophan synthase, some enzymes are double-headed, in that they contain two distinct activities in a single polypeptide chain. A good example of this is the indole 3-glycerol phosphate-synthase-phosphoribosyl anthranilate isomerase bifunc-tional enzyme from the tryptophan operon of E. coli. The crystal structure of the complex has been solved at 2.0 A resolution.39 The two enzymes have been separated by genetic manipulation.40 The activity of the two separate monomeric monofunctional constituents is the same as in the covalent complex so there is no catalytic advantage of having the proteins fused. 

BV monooxygenase Baeyer-Villiger monooxygenase CHMO cyclohexanone monooxygenase COMT catechol O-methyltransferase PDC pyruvate decarboxylase PRAI phosphoribosyl anthranilate isomerase. 

C N-( 5 -phosphoribosyl) anthranilate isomerase none Amadori rearrangement... 

The biosynthesis of L-Trp from CHA is outlined in Fig. 8.16. The complex transformation of CHA into phosphoribosyl anthranilate (PRAA) is, in E. coli and C. glutamicum, catalyzed by a protein aggregate that is inhibited by the l-Trp end product. The final two steps are performed by a single protein indole, which is toxic to the cell, is channeled directly into the active site where the fi-... 

Fig. 8.16 Biosynthesis of L-Trp. Compounds ANT, anthrani-late CDRP, l-(o-carboxyphenylamino)-l-deoxyribulose-5-phosphate I3GP, indole-3-glycerolphosphate IND, indole PRAA, phosphoribosyl anthranilate PRPP, 5-phosphoribosyl-a-pyrophosphate.

Anthranilate synthase (I2, II2) iV-(5 -Phosphoribosyl)-anthranilate isomerase Tryptophan synthase ( 2 2) ... 

Isomerization of phosphoribosyl-anthranilate (TrpF activity) No detectable promiscuous activity... 

Figure 7 The reactions catalyzed by HisA (isomerization of A/ -[(5 -phosphoribosyl)formimino]-5-aminoimidazole-4-carboxamide ribonucleotide) and TrpF (isomerization of phosphoribosyl-anthranilate).

The high stability of the ( a)g barrel is emphasized in the statement that you can do almost anything and still get an a/jS-barrel [186], For example, a protein expected to contain a ( a)io barrel has been prepared, but it really forms a (j9a)g barrel and the other two )Sa-portions of the molecule form an additional dimer [187]. If the amino and carboxy ends of iV-(5 -phosphoribosyl)anthranilate isomerase are moved to different loops between j3-strands and a helices, there is not a large effect on enzymic activity [183], Since this ( )8 folding pattern is so common, its evolution has been the subject of much discussion, but no firm conclusions can yet be made [186], It is found that (jSa)g barrel enzymes can be recruited for other purposes , such as the use of enolase in the x-crystallin in the duck lens [186]. It has been noted that enolase and pyruvate kinase, both (jSa)g barrels, are consecutive enzymes in glycolysis [186, 188]. 

U. Hommel, M. Eberhard, and K. Kirsehner, Phosphoribosyl anthranilate isomerase eafalyzes a reversible Amadori reaction. Biochemistry, 34 (1995) 5429-5439. 

Biosynthesis Like other aromatic amino acids, e.g., Phe and Tyr, Trp is formed on the shikimic acid pathway. There is a branching point at chorismic acid one branch leads to Phe and Tyr, the other to Trp choris-mic acid - anthranilic acid (anthranilic acid synthase, EC 4.1.3.27)- A-(5 -0-phosphoribosyl)-anthranilic acid (anthranilic acid phosphoribosyl transferase, EC 2.4.2.18)- 1 -o-carboxyphenylamino-1 -deoxyribu-lose 5-phosphate [A-(5 -phosphoribosyl)anthranilic acid isomerase]- indole-3-glycerol phosphate (in-dole-3-glycerol phosphate synthase, EC 4.1.1.48) - indole (tryptophan synthase, EC 4.2.1,20)+serine - Trp. Many biologically active indole compounds are derived from Trp, e. g., 5-hydroxytryptophan, 5-hydroxy-tryptamine ( serotonin), and melatonin as well as many indole alkaloids. 

Phosphoribosyl anthranilate isomerase Indole glycerol phosphate synthase (E.C. 4.1.2.8)... 

The second step in the tryptophan branch, the conversion of anthranilate toN-phosphoribosyl anthranilate [Fig. 4 (14)], involves the addition of phos-phoribosyl moiety of 5-phosphoribosyl- 1-pyrophosphate to the C-3 position of anthranilate, catalyzed by anthranilate phosphoribosyltransferase (PRT). Maximum activity of the transferase appears to require both 5-phosphoribosyl pyrophosphate and MgCl2 for maximum activity. Certain members of the enteric bacteria have the enzymes catalyzing the first and second steps of this sequence aggregated into a single complex (Largen and Belser, 1975). However, this situation does not appear to be true in any other organisms studied (Hankins et al., 1976). 

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