The crystal structure of MT0146/CbiT suggests that the putative precorrin-8w decarboxylase is a methyltransferase

The CbiT and CbiE enzymes participate in the biosynthesis of vitamin B12. They are fused together in some organisms to form a protein called CobL, which catalyzes two methylations and one decarboxylation on a precorrin intermediate. Because CbiE has sequence homology to canonical precorrin methyltransferases, CbiT was hypothesized to catalyze the decarboxylation. We herein present the crystal structure of MT0146, the CbiT homolog from Methanobacterium thermoautotrophicum. The protein shows structural similarity to Rossmann-like S-adenosyl-methionine-dependent methyltransferases, and our 1.9 A cocrystal structure shows that it binds S-adenosyl-methionine in standard geometry near a binding pocket that could accommodate a precorrin substrate. Therefore, MT0146/CbiT probably functions as a precorrin methyltransferase and represents the first enzyme identified with this activity that does not have the canonical precorrin methyltransferase fold.     

Role of the precorrin 6-X reductase gene in cobamide biosynthesis in Methanococcus maripaludis

In Methanococcus maripaludis strain JJ, deletion of the homolog to cbiJ, which encodes the corrin biosynthetic enzyme precorrin 6-X reductase, yielded an auxotroph that required either cobamide or acetate for good growth. This phenotype closely resembled that of JJ117, a mutant in which tandem repeats were introduced into the region immediately downstream of the homolog of cbiJ. Mutant JJ117 also produced low quantities of cobamides, about 15 nmol g(-1) protein or 1-2% of the amount found in wild-type cells. These results confirm the role of the cbiJ homolog in cobamide biosynthesis in the Archaea and suggest the presence of low amounts of a bypass activity in these organisms.     

Cobalt proteins.

In the form of vitamin B12, cobalt plays a number of crucial roles in many biological functions. However, recent studies have provided information on the biochemistry and bioinorganic chemistry of several proteins containing cobalt in a form other than that in the corrin ring of vitamin B12. To date, eight noncorrin-cobalt-containing enzymes (methionine aminopeptidase, prolidase, nitrile hydratase, glucose isomerase, methylmalonyl-CoA carboxytransferase, aldehyde decarbonylase, lysine-2,3-aminomutase, and bromoperoxidase) have been isolated and characterized. A cobalt transporter is involved in the metallocenter biosynthesis of the host cobalt-containing enzyme, nitrile hydratase. Understanding the differences between cobalt and nickel transporters might lead to drug development for gastritis and peptic ulceration.     

Alternative route for biosynthesis of amino sugars in Escherichia coli K-12 mutants by means of a catabolic isomerase.

By inserting a lambda placMu bacteriophage into gene glmS encoding glucosamine 6-phosphate synthetase (GlmS), the key enzyme of amino sugar biosynthesis, a nonreverting mutant of Escherichia coli K-12 that was strictly dependent on exogenous N-acetyl-D-glucosamine or D-glucosamine was generated. Analysis of suppressor mutations rendering the mutant independent of amino sugar supply revealed that the catabolic enzyme D-glucosamine-6-phosphate isomerase (deaminase), encoded by gene nagB of the nag operon, was able to fulfill anabolic functions in amino sugar biosynthesis. The suppressor mutants invariably expressed the isomerase constitutively as a result of mutations in nagR, the locus for the repressor of the nag regulon. Suppression was also possible by transformation of glmS mutants with high-copy-number plasmids expressing the gene nagB. Efficient suppression of the glmS lesion, however, required mutations in a second locus, termed glmX, which has been localized to 26.8 min on the standard E. coli K-12 map. Its possible function in nitrogen or cell wall metabolism is discussed.     

The ethylene response factor AtERF98 enhances tolerance to salt through the transcriptional activation of ascorbic acid synthesis in Arabidopsis

Ascorbic acid (AsA) is an important antioxidant in plants, and its biosynthesis is finely regulated through developmental and environmental cues; however, the regulatory mechanism remains unclear. In this report, the knockout and knockdown mutants of Arabidopsis AtERF98 decreased the AsA level, whereas the overexpression of AtERF98 increased it, which suggests that AtERF98 plays an important role in regulating AsA biosynthesis. AtERF98-overexpressing plants showed enhanced expression of AsA synthesis genes in the d-mannose/l-galactose (d-Man/l-Gal) pathway and the myo-inositol pathway gene MIOX4, as well as of AsA turnover genes. In contrast, AtERF98 mutants showed decreased expression of AsA synthesis genes in the d-Man/l-Gal pathway but not of the myo-inositol pathway gene or AsA turnover genes. In addition, the role of AtERF98 in regulating AsA production was significantly impaired in the d-Man/l-Gal pathway mutant vtc1-1, but the expression of the myo-inositol pathway gene or AsA turnover genes was not affected, which indicates that the regulation of AtERF98 in AsA synthesis is primarily mediated by the d-Man/l-Gal pathway. Transient expression and chromatin immunoprecipitation assays further showed that AtERF98 binds to the promoter of VTC1, which indicates that AtERF98 modulates AsA biosynthesis by directly regulating the expression of the AsA synthesis genes. Moreover, the knockout mutant aterf98-1 displayed decreased salt-induced AsA synthesis and reduced tolerance to salt. The supplementation of exogenous AsA increased the salt tolerance of aterf98-1; coincidently, the enhanced salt tolerance of AtERF98-overexpressing plants was impaired in vtc1-1. Thus, our data provide evidence that the regulation of AtERF98 in AsA biosynthesis contributes to enhanced salt tolerance in Arabidopsis.     

Triosephosphate isomerase: removal of a putatively electrophilic histidine residue results in a subtle change in catalytic mechanism

An important active-site residue in the glycolytic enzyme triosephosphate isomerase is His-95, which appears to act as an electrophilic component in catalyzing the enolization of the substrates. With the techniques of site-directed mutagenesis, His-95 has been replaced by Gln in the isomerase from Saccharomyces cerevisiae. The mutant isomerase has been expressed in Escherichia coli strain DF502 and purified to homogeneity. The specific catalytic activity of the mutant enzyme is less than that of wild type by a factor of nearly 400. The mutant enzyme can be resolved from the wild-type isomerase on nondenaturing isoelectric focusing gels, and an isomerase activity stain shows that the observed catalytic activity indeed derives from the mutant protein. The inhibition constants for arsenate and for glycerol phosphate with the mutant enzyme are similar to those with the wild-type isomerase, but the substrate analogues 2-phosphoglycolate and phosphoglycolohydroxamate bind 8- and 35-fold, respectively, more weakly to the mutant isomerase. The mutant enzyme shows the same stereospecificity of proton transfer as the wild type. Tritium exchange experiments similar to those used to define the free energy profile for the wild-type yeast isomerase, together with a new method of analysis involving 14C and 3H doubly labeled substrates, have been used to investigate the energetics of the mutant enzyme catalyzed reaction. When the enzymatic reaction is conducted in tritiated solvent, the mutant isomerase does not catalyze any appreciable exchange between protons of the remaining substrate and those of the solvent either in the forward reaction direction (using dihydroxyacetone phosphate as substrate) or in the reverse direction (using glyceraldehyde phosphate as substrate). However, the specific radioactivity of the product glyceraldehyde phosphate formed in the forward reaction is 31% that of the solvent, while that of the product dihydroxyacetone phosphate formed in the reverse reaction is 24% that of the solvent. The deuterium kinetic isotope effects observed with the mutant isomerase using [1(R)-2H]dihydroxyacetone phosphate and [2-2H]glyceraldehyde 3-phosphate are 2.15 +/- 0.04 and 2.4 +/- 0.1, respectively. These results lead to the conclusion that substitution of Gln for His-95 so impairs the ability of the enzyme to stabilize the reaction intermediate that there is a change in the pathways of proton transfer mediated by the mutant enzyme. The data allow us more closely to define the role of His-95 in the reaction catalyzed by the wild-type enzyme, while forcing us to be alert to subtle changes in mechanistic pathways when mutant enzymes are generated.     

Modulation of Allosteric Regulation by E38K and G101N Mutations in the Potato Tuber ADP-glucose Pyrophosphorylase

The higher plant ADP-glucose (ADPG) pyrophosphorylase (AGPase), composed of two small subunits and two large subunits (LSs), produces ADPG, the sole substrate for starch biosynthesis from α-D-glucose 1-phosphate and ATP. This enzyme controls a key step in starch synthesis as its catalytic activity is activated by 3-phosphoglycerate (3-PGA) and inhibited by orthophosphate (Pi). Previously, two mutations in the LS of potato AGPase (PLS), PLS-E38K and PLS-G101N, were found to increase sensitivity to 3-PGA activation and tolerance to Pi inhibition. In the present study, the double mutated enzyme (PLS-E38K/G101N) was evaluated. In a complementation assay of ADPG synthesis in an Escherichia coli mutant defective in the synthesis of ADPG, expression of PLS-E38K/G101N mediated higher glycogen production than wild-type potato AGPase (PLS-WT) and the single mutant enzymes, PLS-E38K and PLS-G101N, individually. Purified PLS-E38K/G101N showed higher sensitivity to 3-PGA activation and tolerance to Pi inhibition than PLS-E38K or PLS-G101N. Moreover, the enzyme activities of PLS-E38K, PLS-G101N, and PLS-E38K/G101N were more readily stimulated by other major phosphate-ester metabolites, such as fructose 6-phosphate, fructose 2,6-bisphosphate, and ribose 5-phosphate, than was that of PLS-WT. Hence, although the specific enzyme activities of the LS mutants toward 3-PGA were impaired to some extent by the mutations, our results suggest that their enhanced allosteric regulatory properties and the broadened effector selectivity gained by the same mutations not only offset the lowered enzyme catalytic turnover rates but also increase the net performance of potato AGPase in vivo in view of increased glycogen production in bacterial cells.     

The shoot-specific expression of gamma-glutamylcysteine synthetase directs the long-distance transport of thiol-peptides to roots conferring tolerance to mercury and arsenic

Thiol-peptides synthesized as intermediates in phytochelatin (PC) biosynthesis confer cellular tolerance to toxic elements like arsenic, mercury, and cadmium, but little is known about their long-distance transport between plant organs. A modified bacterial gamma-glutamylcysteine synthetase (ECS) gene, S1ptECS, was expressed in the shoots of the ECS-deficient, heavy-metal-sensitive cad2-1 mutant of Arabidopsis (Arabidopsis thaliana). S1ptECS directed strong ECS protein expression in the shoots, but no ECS was detected in the roots of transgenic plant lines. The S1ptECS gene restored full mercury tolerance and partial cadmium tolerance to the mutant and enhanced arsenate tolerance significantly beyond wild-type levels. After arsenic treatment, the root concentrations of gamma-glutamylcysteine (EC), PC2, and PC3 peptides in a S1ptECS-complemented cad2-1 line increased 6- to 100-fold over the mutant levels and were equivalent to wild-type concentrations. The shoot and root levels of glutathione were 2- to 5-fold above those in wild-type plants, with or without treatment with toxicants. Thus, EC and perhaps glutathione are efficiently transported from shoots to roots. The possibility that EC or other PC pathway intermediates may act as carriers for the long-distance phloem transport and subsequent redistribution of thiol-reactive toxins and nutrients in plants is discussed.     

gid1, a gibberellin-insensitive dwarf mutant, shows altered regulation of probenazole-inducible protein (PBZ1) in response to cold stress and pathogen attack

A recessive gibberellin (GA)-insensitive dwarf mutant of rice, gibberellin-insensitive dwarf1 (gid1), has been identified, which shows a severe dwarf phenotype and contains high concentrations of endogenous GA. To elucidate the function of gid1, proteins regulated downstream of gid1 were analysed using a proteomic approach. Proteins extracted from suspension-cultured cells of gid1 and its wild type were separated by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). Of a total of 962 proteins identified from the suspension-cultured cells, 16 were increased and 14 were decreased in gid1 compared with its wild type. Among the proteins hyper-accumulated in gid1 were osmotin, triosephosphate isomerase, probenazole inducible protein (PBZ1) and pathogenesis-related protein 10. Of these four genes, only the expression of PBZ1 was increased by exogenous GA3 application. Expression of this gene was also enhanced in shoots of the wild type by cold stress or by rice blast fungus infection. Under normal growth conditions, there was more PBZ1 protein in gid1 than in the wild type. In addition, gid1 showed increased tolerance to cold stress and resistance to blast fungus infection. The entcopalyl diphosphate synthase (OsCPS) genes, which encode enzymes at the branch point between GA and phytoalexin biosynthesis, were expressed differentially in gid1 relative to the wild type. Specifically, OsCPS1, which encodes an enzyme in the GA biosynthesis pathway, was down-regulated and OsCPS2 and OsCPS4, which encode enzymes in phytoalexin biosynthesis, were up-regulated in gid1. These results suggest that the expression of PBZ1 is regulated by GA signalling and stress stimuli, and that gid1 is involved in tolerance to cold stress and resistance to blast fungus.     

Cloning,characterization and functional analysis of the role MhNCED3, a gene encoding 9-cis-epoxycarotenoid dioxygenase in Malus hupehensis Rehd., plays in plant tolerance to osmotic and Cd2+ stresses

Aims

Abscisic acid (ABA) plays an important role in the stress tolerance of seedlings and 9-cis-epoxycarotenoid dioxygenase (NCED) is considered to be the rate-limiting enzyme involved in ABA biosynthesis. However, the genes encoding NCED in M. hupehensis Rehd. have not been reported.

Methods

In this study, a gene encoding NCED, MhNCED3, was isolated from the roots of M. hupehensis Rehd. Its functions were investigated in M. hupehensis Rehd. seedlings and transgenic Arabidopsis lines under various abiotic stresses.

Results

The expression of MhNCED3 in M. hupehensis Rehd. roots was differentially induced by dehydration, chilling, salt and cadmium stresses and ABA biosynthesis was highly correlated with MhNCED3 expression. Ectopic expression of MhNCED3 successfully complemented the phenotypic defects of the 129B08/nced3 mutant. Furthermore, overexpression of MhNCED3, when it was transformed into the wild type (WT) seedling resulted in enhanced tolerance to osmotic and cadmium stresses compared to the normal WT seedling. The transgenic lines displayed higher rates of seed germination, improved growth and developmental status, reduced water loss/oxidative damage, lowered apoptosis rates and increased ABA accumulation. Furthermore, the higher antioxidant enzyme activities detected in the transgenic lines were probably responsible for the decrease in oxidative damage and apoptosis rates.

Conclusions

Overall, MhNCED3 played a significant role in enhancing plant tolerance to abiotic stresses through the regulation of endogenous ABA biosynthesis.     

Biosynthesis of cobalamin (vitamin B(12))

The biosynthesis of vitamin B(12) is summarized, emphasizing the differences observed between the aerobic and anaerobic pathways. The biosynthetic route to adenosylcobalamin from its five-carbon precursor, 5-aminolaevulinic acid, can be divided into three sections: (1) the biosynthesis of uroporphyrinogen III from 5-aminolaevulinic acid, which is common to both pathways; (2) the conversion of uroporphyrinogen III into the ring-contracted, deacylated intermediate precorrin 6 or cobalt-precorrin 6, which includes the primary differences between the two pathways; and (3) the transformation of this intermediate to form adenosylcobalamin.     

Hepatic Overexpression of a Constitutively Active Form of Liver Glycogen Synthase Improves Glucose Homeostasis

In this study, we tested the efficacy of increasing liver glycogen synthase to improve blood glucose homeostasis. The overexpression of wild-type liver glycogen synthase in rats had no effect on blood glucose homeostasis in either the fed or the fasted state. In contrast, the expression of a constitutively active mutant form of the enzyme caused a significant lowering of blood glucose in the former but not the latter state. Moreover, it markedly enhanced the clearance of blood glucose when fasted rats were challenged with a glucose load. Hepatic glycogen stores in rats overexpressing the activated mutant form of liver glycogen synthase were enhanced in the fed state and in response to an oral glucose load but showed a net decline during fasting. In order to test whether these effects were maintained during long term activation of liver glycogen synthase, we generated liver-specific transgenic mice expressing the constitutively active LGS form. These mice also showed an enhanced capacity to store glycogen in the fed state and an improved glucose tolerance when challenged with a glucose load. Thus, we conclude that the activation of liver glycogen synthase improves glucose tolerance in the fed state without compromising glycogenolysis in the postabsorptive state. On the basis of these findings, we propose that the activation of liver glycogen synthase may provide a potential strategy for improvement of glucose tolerance in the postprandial state.     

Modulation of innate and acquired immune responses by Escherichia coli heat-labile toxin: distinct pro- and anti-inflammatory effects of the nontoxic AB complex and the enzyme activity

We have examined the roles of enzyme activity and the nontoxic AB complex of heat-labile toxin (LT) from Escherichia coli on its adjuvant and immunomodulatory properties. LTK63, an LT mutant that is completely devoid of enzyme activity, enhanced Th1 responses to coinjected Ags at low adjuvant dose. In contrast, LTR72, a partially detoxified mutant, enhanced Th2 responses and when administered intranasally to mice before infection with Bordetella pertussis suppressed Th1 responses and delayed bacterial clearance from the lungs. LTR72 or wild-type LT inhibited Ag-induced IFN-gamma production by Th1 cells, and LT enhanced IL-5 production by Th2 cells in vitro. Each of the toxins enhanced B7-1 expression on macrophages, but enhancement of B7-2 expression was dependent on enzyme activity. We also observed distinct effects of the nontoxic AB complex and enzyme activity on inflammatory cytokine production. LT and LTR72 suppressed LPS and IFN-gamma induced TNF-alpha and IL-12 production, but enhanced IL-10 secretion by macrophages in vitro and suppressed IL-12 production in vivo in a murine model of LPS-induced shock. In contrast, LTK63 augmented the production of IL-12 and TNF-alpha. Furthermore, LTK63 enhanced NF-kappaB translocation, whereas low doses of LTR72 or LT failed to activate NF-kappaB, but stimulated cAMP production. Thus, E. coli LT appears to be capable of suppressing Th1 responses and enhancing Th2 responses through the modulatory effects of enzyme activity on NF-kappaB activation and IL-12 production. In contrast, the nontoxic AB complex can stimulate acquired immune responses by activating components of the innate immune system.     

Nerve Growth Factor-Induced Down-Regulation of Calmodulin-Dependent Protein Kinase III in PC12 Cells Involves Cyclic AMP-Dependent Protein Kinase

Treatment of PC12 cells with nerve growth factor (NGF), epidermal growth factor (EGF), or agents that raise intracellular cyclic AMP (cAMP) levels (e.g., forskolin) reduces the activity of calmodulin-dependent protein kinase III (CaM-PK III) over a period of 8 h. The mechanism of this effect of NGF has now been examined in more detail, making use of a mutant PC12 cell line (A126-1B2) that is deficient in cAMP-dependent protein kinase activity. Control experiments showed that A126-1B2 cells retain other NGF-mediated responses (e.g., the induction of ornithine decarboxylase, a cAMP-independent event) and contain a complement of CaM-PK III and its substrate, elongation factor-2, comparable to that of wild-type cells. The ability of NGF or forskolin, but not of EGF, to down-regulate CaM-PK III was markedly attenuated in A126-1B2 compared to wild-type cells. Treatment of wild-type cells with the cAMP phosphodiesterase inhibitor, isobutylmethylxanthine, enhanced the effects of NGF, but not of EGF. The possibility that NGF led to a stimulation of cAMP-dependent protein kinase activity in wild-type cells was assessed by measurement of the "activation ratio" (-cAMP/+cAMP) of this enzyme before and at various times after NGF addition. A small, but significant, increase in the activation ratio from 0.3 to 0.48 was observed, reaching a peak 5 min after NGF treatment. EGF had no effect on the activation ratio in wild-type cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Segmental motion in catalysis: investigation of a hydrogen bond critical for loop closure in the reaction of triosephosphate isomerase.

A residue essential for proper closure of the active-site loop in the reaction catalyzed by triosephosphate isomerase is tyrosine-208, the hydroxyl group of which forms a hydrogen bond with the amide nitrogen of alanine-176, a component of the loop. Both residues are conserved, and mutagenesis of the tyrosine to phenylalanine results in a 2000-fold drop in the catalytic activity (kcat/Km) of the enzyme compared to the wild-type isomerase. The nature of the closure process has been elucidated from both viscosity dependence and primary isotope effects. The reaction catalyzed by the mutant enzyme shows a viscosity dependence using glycerol as the viscosogen. This dependence can be attributed to the rate-limiting motion of the active-site loop between the "open" and the "closed" conformations. Furthermore, a large primary isotope effect is observed with [1-2H]dihydroxyacetone phosphate as substrate [(kcat/Km)H/(kcat/Km)D = 6 +/- 1]. The range of isotopic experiments that were earlier used to delineate the energetics of the wild-type isomerase has provided the free energy profile of the mutant enzyme. Comparison of the energetics of the wild-type and mutant enzymes shows that only the transition states flanking the enediol intermediate have been substantially affected. The results suggest either that loop closure and deprotonation are coupled and occur in the same rate-limiting step or that these two processes happen sequentially but interdependently. This finding is consistent with structural information that indicates that the catalytic base glutamate-165 moves 2 A toward the substrate upon loop closure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Epidermal growth factor enhances a microsomal 12-lipoxygenase activity in A431 cells.

12-Hydroxyeicosatetraenoic acid (12-HETE) is formed from arachidonic acid either by 12-lipoxygenase or by a cytochrome P450 monooxygenase. 12-Lipoxygenase is generally localized in the soluble cytosolic fraction, and the cytochrome P450 monooxygenase is a microsomal enzyme. In this study, 12-HETE biosynthesis and the regulation of 12-HETE biosynthesis by epidermal growth factor (EGF) in A431 cells were investigated. 12-HETE was biosynthesized from arachidonic acid by the microsomal fraction of A431 cells, but not by the cytosolic fraction. The formation of 12-HETE was inhibited by 5,8,11,14-eicosatetraynoic acid, nordihydroguaiaretic acid, and caffeic acid. Nordihydroguaiaretic acid at 10(-4) M and 5,8,11,14-eicosatetraynoic acid at 10(-5) M almost completely inhibited its formation. However, the formation of 12-HETE was not affected by the presence of an NADPH-generating system, carbon monoxide, or SKF 525A. The biosynthetic 12-HETE was analyzed by chiral stationary phase high performance liquid chromatography and was highly enriched in (12S)-HETE. We therefore concluded that the enzyme responsible for the formation of (12S)-HETE in the microsomes of A431 cells is a 12-lipoxygenase. The microsomal 12-lipoxygenase of A431 cells belongs to the "leukocyte-type" enzyme as determined by substrate specificity and enzyme kinetics studies. The microsomal 12-lipoxygenase oxygenated linoleic acid much faster than the cytosolic platelet 12-lipoxygenase and is a "self-catalyzed inactivation" enzyme. Treatment of cells with 50 ng/ml EGF significantly induced microsomal 12-lipoxygenase activity. The lag period for the expression of the stimulatory effect of EGF on 12-lipoxygenase activity was approximately 10 h. The stimulatory effect of EGF on 12-lipoxygenase activity was completely blocked by treatment with 35 microM cycloheximide, indicating a requirement for de novo protein biosynthesis. Furthermore, the presence of the endogenous inhibitor of 12-lipoxygenase (which masked (12S)-HETE biosynthesis in intact cells) was identified in the cytosolic fraction of A431 cells. The putative inhibitor was enzyme-selective. It inhibited the leukocyte-type 12-lipoxygenase, but not the "platelet-type" enzyme.     

Segmental movement: definition of the structural requirements for loop closure in catalysis by triosephosphate isomerase.

To determine what drives the closure of the active-site loop in the reaction catalyzed by triosephosphate isomerase, several residues involved in hydrogen bonding between the loop and the bulk of the protein have been altered. It was known from earlier work that the loop serves two functions: to stabilize the reaction intermediate (and the two transition states that flank it) and to prevent the loss of this unstable species into free solution. To discover what elements of the protein are necessary for proper closure of the loop, selective destabilization of the "open" and the "closed" forms of the enzyme with respect to one another has been attempted. The mutant Y164F isomerase has been prepared to evaluate the importance of the structure of the "open" form, and the mutant E129Q, Y208F, and S211A enzymes have allowed investigation of the "closed" form. The integrity of the loop itself has been destabilized by making the T172A isomerase. We have found that only those mutations that destabilize the "closed" form of the enzyme significantly perturb the catalytic properties of the isomerase. The second-order rate constants (kcat/Km) of the S211A and E129Q enzymes are reduced 30-fold, and that of the mutant Y208F enzyme is reduced 2000-fold, from the level of the wild-type enzyme. The dramatic drop in activity of the Y208F enzyme is accompanied by a 200-fold increase in the dissociation constant of the intermediate analogue phosphoglycolohydroxamate. The most important property of the mobile loop of triosephosphate isomerase lies, therefore, in the stability of the system when the active site contains ligand and the loop is closed.     

A Clostridium perfringens hem gene cluster contains a cysG(B) homologue that is involved in cobalamin biosynthesis

The hem gene cluster, which consists of hemA, cysG(B), hemC, hemD, hemB, and hemL genes, and encodes enzymes involved in the biosynthetic pathway from glutamyl-tRNA to uroporphyrinogen III, has been identified by the cloning and sequencing of two overlapping DNA fragments from Clostridium perfringens NCTC8237. The deduced amino acid sequence of the N-terminal region of C. perfringens HemD is homologous to those reported for the C-terminal region of Salmonella typhimurium CysG and Clostridium josui HemD. C. perfringens CysG(B) is a predicted 220-residue protein which shows homology to the N-terminal region of S. typhimurium CysG. Disruption of the cysG(B) gene in C. perfringens strain 13 by homologous recombination reduced cobalamin (vitamin B12) levels by a factor of 200. When grown in vitamin B12-deficient medium, the mutant strain showed a four-fold increase in its doubling time compared with that of the wild-type strain, and this effect was counteracted by supplementing the medium with vitamin B12. These results suggest that C. perfringens CysG(B) is involved in the chelation of cobalt to precorrin II as suggested for the CysG(B) domain of S. typhimurium CysG, enabling the synthesis of cobalamin.