L-Arabinose isomerase

Arabinose isomerase [EC 5.3.1.3] catalyzes the interconversion of D-arabinose and D-ribulose. The enzyme will also utilize L-fucose and more slowly utilize L-galactose and D-altrose as substrates. L-Arabinose isomerase [EC 5.3.1.4] catalyzes the interconversion of L-arabinose and L-ribulose. 

These enzymes vary widely in secondary and tertiary structure.1273 Mannose-6-phosphate isomerase is a 45 kDa Zn2+-containing monomer. The larger 65 kDa L-fucose isomerase, which also acts on D-arabinose, is a hexameric Mn2+-dependent enzyme.1273 L-Arabinose isomerase of E. coli, which interconverts arabinose and L-ribulose, is a hexamer of 60-kDa subunits128 while the D-xylose isomerase of Streptomyces is a tetramer of 43-kDa subunits.129 The nonenzymatic counterpart of the isomerization catalyzed by the enzyme is the base-catalyzed Lobry deBruyn-Alberda van Ekenstein transformation (Eq. 13-25).130... 

L-arabinose isomerase, which interconverts L-arabinose and L-ribulose araB, L-ribulokinase, which uses ATP to phosphorylate L-ribulose at C-5 araD, L-ribulose 5-phosphate epimerase, which interconverts L-ribulose 5-phosphate and L-xylulose 5-phosphate talB, transaldolase and tktA, transketolase. 

L-Arabinose isomerase has been purified from Escherichia coli. The enzyme catalyzes the isomerization of L-arabinose to L-ribulose by a proton transfer mechanism in contrast to xylose isomerase which uses a hydride transfer mechanism to perform a similar isomerization [20], This enzyme has high substrate specificity for L-arabinose, and its catalytic activity is dependent on manganese(II). It has been shown, however, that the enzyme could catalyze the exchange of the proton attached to C2 of arabinose with the solvent even in the absence of metal ion [20],... 

In Escherichia coli, L-arabinose isomerase (ECAI) catalyzes the isomerization of L-arabinose to L-ribulose. This enzyme is also of commercial interest as it catalyzes the conversion of D-galactose to D-tagatose in vitro. The crystal stmcture of ECAI was solved and refined at 2.6 angstrom resolution [152]. 

P. Kim, S. H. Yoon, H. J. Roh, and J. H. Choi, High production of D-tagatose, a potential sugar substitute, using immobilized L-arabinose isomerase, Biotechnol. Prog., 17 (2001) 208-210. 

Kim, R, Current studies on biological tagatose production using L-arabinose isomerase A review and future perspective, Appl. Microbiol. Biotechnol., 65, 243, 2004. 

Fig. 1) [1-4]. Genes araA, araB, and araD are the structural genes for L-arabinose isomerase [5], L-ribulokinase [5], and L-ribulose 5-phosphate 4-epimerase, respectively, the first three enzymes involved in L-arabinose metabolism. These genes, together with their controlling sites,... 

Gene araA has been shown to be the structural gene for the enzyme L-arabinose isomerase. This has been demonstrated by the finding that among the L-arabinose isomerase-deficient mutants in the araA cistron... 

The L-arabinose isomerase has been purified from the homogenotic, hyperinducible merodiploid F araB24jaraB24, which produces L-ara-binose isomerase as 10% of its total soluble protein. The purified... 

In merodiploids of the type VA B C IA2B C 67, where C" is a nonsense mutation or a C deletion, L-arabinose isomerase and kinase are produced constitutively, and there is an apparent gene dosage effect with the kinase [7,8,52]. Thus, the C allele produces an activator in the absence of L-arabinose which is able to turn on the operon both cis and tmns to the C allele. Thus, dominance of both C and C alleles to C" is consistent. 

Polarity is in the direction araB, ara A, araP. Nonsense mutations in gene araB lead to reduced levels of L-arabinose isomerase and L-ribulose 5-phosphate 4-epimerase [5,6]. Nonsense mutations in gene araA result in redueed levels of L-ribulose 5-phosphate 4-epimerase, but do not effect a reduction of kinase levels [40]. Thus the controlling elements for this operon must be the proximal end of the araB gene—that is, the end farthest from gene araA. 

The effect of mutations in one structural gene on the activity of an adjacent structural gene in an operon was discovered in the L-arabinose system [2,73]. Certain mutations in gene araB were found to produce a coordinate decrease or increase in inducible levels of L-arabinose isomerase and L-ribulose 5-phosphate 4-epimerase as compared to corresponding levels found in the wild type [2,5,37]. In addition, mutations in the araA gene were found to produce increased inducible levels of L-ribulokinase [2], while some mutations effected a decrease in inducible levels of the epimerase [40]. Moreover, the increases or... 

Effect of Nonsense and Missense Mutations in Gene araB on l-Arabinose Isomerase Activity in Haploids and Ara+ Merodiploids°... 

Induced L-arabinose isomerase Strains (jx moles ribulose/hour/mg protein)... 

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