Trifunctional enzyme

In bacteria, such as E. coli, the first six reactions of the denovo pyrimidine pathway (Fig. 15-15) are catalyzed by six distinct and separable enzymes, but in higher animals reactions 1, 2 and 3 are catalyzed by a trifunctional enzyme, and reactions 5 and 6 by a bifunctional enzyme.- What advantages might there be in associations of these active sites ... 

There are four multifunctional enzymes in the pathway A trifunctional enzyme comprising GAR synthetase, GAR transformylase and AIR synthetase catalyzes reactions 2, 3 and 5 (PRA—> GAR— FGAR, FGAM > AIR Fig. 15-16), respectively. The GAR synthetase and GAR transformylase domains may be separated by digestion of the trifunctional enzyme with the protease, chymotrypsin. 

Dihydroorotase catalyzes the intramolecular cyclization of 7V-carbamyl-L-aspartic acid to L-dihydroorotic acid. In mammals, the activity is present in a trifunctional enzyme that catalyzes the first three steps in the de novo synthesis of pyrimidine nucleotides. 

Methylene-, methenyl-, and 10-formyl-tetrahydrofolates are freely interconvertible. The two activities involved - methylene-tetrahydrofolate dehydrogenase and methenyl-tetrahydrofolate cyclohydrolase - form a trifunctional enzyme with 10-formyl-tetrahydrofolate synthetase (Paukert et al., 1976). This means that single-carbon fragments entering the folate pool in any form other than as methyl-tetrahydrofolate can be readily available for any of the biosynthetic reactions shown in Figure 10.4. 

Human IMP synthase has a subunit molecular weight of 62.1 kDa and associates as a dimer. A trifunctional enzyme, Cj-THF synthase, containing (V °-methenyl tetrahydrofolate (5,10-CH-THF) cyclohydrolase and A °-formyl tetrahydrofolate (lO-CHO-THF) synthetase, catalyzes the reactions 5,10-CH2-THF 5,10-CH-THF and THF —> lO-CHO-THF. The A °-CH-THF produced is a substrate for GAR and AICAR transformylases catalyzing reactions 3 and 9 of the pathway. In higher eukaryotes, the dehydrogenase and cyclohydrolase activities are located in one domain of the protein, which is fused to a larger synthetase domain, forming a trifunctional enzyme. 

Figure 1. Comparison of the amino acid sequence in the N-terminal domain of the . coli multifunctional protein (MP) with those of homologous regions of rat peroxisomal trifunctional enzyme (TE), plant gly-oxysomal tetrafunctional protein (PT), pig mitochondrial long-chain-specific bifunctional enzyme (LT)," and rat mitochondrial enoyl-CoA hydratase (EH). Gly" , Glu , and Glu of MP, which are conserved in all known enoyl-CoA hydratases, are indicated by asterisks.

Palosaari, P.M. Hiltunen, J.K. (1990) J. Biol. Chem. 265,2446-2449. Peroxisomal bifunctional protein from rat liver is a trifunctional enzyme possessing 2-enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, and A3,A2-enoyl-CoA isomerase activities. 

Nuttley, W.M., Aitchison, J.D. Rachubinski, R.A. (1988) Gene 69, 171-180. cDNA cloning and primary structure determination of the peroxisomal trifunctional enzyme hydratase-dehydrogenase-epimerase from the yeast Candida tropic is pK233. 

Facilitated substrate channeling in a self-assembled trifunctional enzyme complex. Angew. Chem. Int Ed., 51, 8787-8790. 

Salim HMW, Negritto MC, Cavalcanti ARO. 1+1=3 A fusion of 2 enzymes in the methionine salvage pathway of Tettahymena thermophila creates a trifunctional enzyme that catalyzes 3 steps in the pathway. PLoS One. 2009 5(10) el000701. doi 10.1371/ joumal.pgen. 1000701. 

The conversion of FGAM to aminoimidazole ribonucleotide (AIR) is catalyzed by the aminoimidazole ribotide synthase, PurM. In this reaction the five-membered ring of the purine base is formed the formyl oxygen is activated by the y-phosphate of the ATP for the nucleophilic attack by N-1 of FGAR, resulting in ring closure [240]. In bacteria, PurM is encoded separately, and in mammals it is part of a trifunctional enzyme, which includes PurD and PurN [241]. 

InGP synthase (EC 4.1.1.48) catalyzes the ring closure of an N-alkylated anthranilate to a 3-aIkyl indole derivative releasing carbon dioxide and water. In some bacteria, InGP synthase is a single-chain enzyme. In others, such as E. colL, it is the N-terminal domain of a bifunctional enzyme that also catalyzes PRA isomerase activity, the third step of tryptophan biosynthesis [16]. In fungi, InGP synthase is the central domain of a trifunctional enzyme that contains... 

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