Identification of ABC transporters acting in vitamin B12 metabolism in Caenorhabditis elegans.

Vitamin B12 (cobalamin, Cbl) is a micronutrient essential to human health. Cbl is not utilized as is but must go through complex subcellular and metabolic processing to generate two cofactor forms: methyl-Cbl for methionine synthase, a cytosolic enzyme; and adenosyl-Cbl for methylmalonyl-CoA mutase, a mitochondrial enzyme. Some 10-12 human genes have been identified responsible for the intracellular conversion of Cbl to cofactor forms, including genes that code for ATP-binding cassette (ABC) transporters acting at the lysosomal and plasma membranes. However, the gene for mitochondrial uptake is not known. We hypothesized that ABC transporters should be candidates for other uptake and efflux functions, including mitochondrial transport, and set out to screen ABC transporter mutants for blocks in Cbl utilization using the nematode roundworm Caenorhabditis elegans. Thirty-seven mutant ABC transporters were screened for the excretion of methylmalonic acid (MMA), which should result from loss of Cbl transport into the mitochondria. One mutant, wht-6, showed elevated MMA excretion and reduced [14C]-propionate incorporation, pointing to a functional block in methylmalonyl-CoA mutase. In contrast, the wht-6 mutant appeared to have a normal cytosolic pathway based on analysis of cystathionine excretion, suggesting that cytosolic methionine synthase was functioning properly. Further, the MMA excretion in wht-6 could be partially reversed by including vitamin B12 in the assay medium. The human ortholog of wht-6 is a member of the G family of ABC transporters. We propose wht-6 as a candidate for the transport of Cbl into mitochondria and suggest that a member of the corresponding ABCG family of ABC transporters has this role in humans. Our ABC transporter screen also revealed that mrp-1 and mrp-2 mutants excreted lower MMA than wild type, suggesting they were concentrating intracellular Cbl, consistent with the cellular efflux defect proposed for the mammalian MRP1 ABC transporter.

Holocarboxylase Synthetase: A Moonlighting Transcriptional Coregulator of Gene Expression and a Cytosolic Regulator of Biotin Utilization.

The vitamin biotin is an essential nutrient for the metabolism and survival of all organisms owing to its function as a cofactor of enzymes collectively known as biotin-dependent carboxylases. These enzymes use covalently attached biotin as a vector to transfer a carboxyl group between donor and acceptor molecules during carboxylation reactions. In human cells, biotin-dependent carboxylases catalyze key reactions in gluconeogenesis, fatty acid synthesis, and amino acid catabolism. Biotin is attached to apocarboxylases by a biotin ligase: holocarboxylase synthetase (HCS) in mammalian cells and BirA in microbes. Despite their evolutionary distance, these proteins share structural and sequence similarities, underscoring their importance across all life forms. However, beyond its role in metabolism, HCS participates in the regulation of biotin utilization and acts as a nuclear transcriptional coregulator of gene expression. In this review, we discuss the function of HCS and biotin in metabolism and human disease, a putative role for the enzyme in histone biotinylation, and its participation as a nuclear factor in chromatin dynamics. We suggest that HCS be classified as a moonlighting protein, with two biotin-dependent cytosolic metabolic roles and a distinct biotin-independent nuclear coregulatory function.

Mutation in folate metabolism causes epigenetic instability and transgenerational effects on development.

The importance of maternal folate consumption for normal development is well established, yet the molecular mechanism linking folate metabolism to development remains poorly understood. The enzyme methionine synthase reductase (Mtrr) is necessary for utilization of methyl groups from the folate cycle. We found that a hypomorphic mutation of the mouse Mtrr gene results in intrauterine growth restriction, developmental delay, and congenital malformations, including neural tube, heart, and placental defects. Importantly, these defects were dependent upon the Mtrr genotypes of the maternal grandparents. Furthermore, we observed widespread epigenetic instability associated with altered gene expression in the placentas of wild-type grandprogeny of Mtrr-deficient maternal grandparents. Embryo transfer experiments revealed that Mtrr deficiency in mice lead to two distinct, separable phenotypes: adverse effects on their wild-type daughters' uterine environment, leading to growth defects in wild-type grandprogeny, and the appearance of congenital malformations independent of maternal environment that persist for five generations, likely through transgenerational epigenetic inheritance.

Structure of MMACHC reveals an arginine-rich pocket and a domain-swapped dimer for its B12 processing function.

Defects in the MMACHC gene represent the most common disorder of cobalamin (Cbl) metabolism, affecting synthesis of the enzyme cofactors adenosyl-Cbl and methyl-Cbl. The encoded MMACHC protein binds intracellular Cbl derivatives with different upper axial ligands and exhibits flavin mononucleotide (FMN)-dependent decyanase activity toward cyano-Cbl as well as glutathione (GSH)-dependent dealkylase activity toward alkyl-Cbls. We determined the structure of human MMACHC·adenosyl-Cbl complex, revealing a tailor-made nitroreductase scaffold which binds adenosyl-Cbl in a "base-off, five-coordinate" configuration for catalysis. We further identified an arginine-rich pocket close to the Cbl binding site responsible for GSH binding and dealkylation activity. Mutation of these highly conserved arginines, including a replication of the prevalent MMACHC missense mutation, Arg161Gln, disrupts GSH binding and dealkylation. We further showed that two Cbl-binding monomers dimerize to mediate the reciprocal exchange of a conserved "PNRRP" loop from both subunits, serving as a protein cap for the upper axial ligand in trans and required for proper dealkylation activity. Our dimeric structure is supported by solution studies, where dimerization is triggered upon binding its substrate adenosyl-Cbl or cofactor FMN. Together our data provide a structural framework to understanding catalytic function and disease mechanism for this multifunctional enzyme.

Genetic disorders of vitamin B12 metabolism: eight complementation groups--eight genes.

Vitamin B12 (cobalamin, Cbl) is an essential nutrient in human metabolism. Genetic diseases of vitamin B12 utilisation constitute an important fraction of inherited newborn disease. Functionally, B12 is the cofactor for methionine synthase and methylmalonyl CoA mutase. To function as a cofactor, B12 must be metabolised through a complex pathway that modifies its structure and takes it through subcellular compartments of the cell. Through the study of inherited disorders of vitamin B12 utilisation, the genes for eight complementation groups have been identified, leading to the determination of the general structure of vitamin B12 processing and providing methods for carrier testing, prenatal diagnosis and approaches to treatment.

Mice doubly-deficient in lysosomal hexosaminidase A and neuraminidase 4 show epileptic crises and rapid neuronal loss.

Tay-Sachs disease is a severe lysosomal disorder caused by mutations in the HexA gene coding for the α-subunit of lysosomal ß-hexosaminidase A, which converts G(M2) to G(M3) ganglioside. Hexa(-/-) mice, depleted of ß-hexosaminidase A, remain asymptomatic to 1 year of age, because they catabolise G(M2) ganglioside via a lysosomal sialidase into glycolipid G(A2), which is further processed by ß-hexosaminidase B to lactosyl-ceramide, thereby bypassing the ß-hexosaminidase A defect. Since this bypass is not effective in humans, infantile Tay-Sachs disease is fatal in the first years of life. Previously, we identified a novel ganglioside metabolizing sialidase, Neu4, abundantly expressed in mouse brain neurons. Now we demonstrate that mice with targeted disruption of both Neu4 and Hexa genes (Neu4(-/-);Hexa(-/-)) show epileptic seizures with 40% penetrance correlating with polyspike discharges on the cortical electrodes of the electroencephalogram. Single knockout Hexa(-/-) or Neu4(-/-) siblings do not show such symptoms. Further, double-knockout but not single-knockout mice have multiple degenerating neurons in the cortex and hippocampus and multiple layers of cortical neurons accumulating G(M2) ganglioside. Together, our data suggest that the Neu4 block exacerbates the disease in Hexa(-/-) mice, indicating that Neu4 is a modifier gene in the mouse model of Tay-Sachs disease, reducing the disease severity through the metabolic bypass. However, while disease severity in the double mutant is increased, it is not profound suggesting that Neu4 is not the only sialidase contributing to the metabolic bypass in Hexa(-/-) mice.

Structures of the human GTPase MMAA and vitamin B12-dependent methylmalonyl-CoA mutase and insight into their complex formation.

Vitamin B(12) (cobalamin, Cbl) is essential to the function of two human enzymes, methionine synthase (MS) and methylmalonyl-CoA mutase (MUT). The conversion of dietary Cbl to its cofactor forms, methyl-Cbl (MeCbl) for MS and adenosyl-Cbl (AdoCbl) for MUT, located in the cytosol and mitochondria, respectively, requires a complex pathway of intracellular processing and trafficking. One of the processing proteins, MMAA (methylmalonic aciduria type A), is implicated in the mitochondrial assembly of AdoCbl into MUT and is defective in children from the cblA complementation group of cobalamin disorders. To characterize the functional interplay between MMAA and MUT, we have crystallized human MMAA in the GDP-bound form and human MUT in the apo, holo, and substrate-bound ternary forms. Structures of both proteins reveal highly conserved domain architecture and catalytic machinery for ligand binding, yet they show substantially different dimeric assembly and interaction, compared with their bacterial counterparts. We show that MMAA exhibits GTPase activity that is modulated by MUT and that the two proteins interact in vitro and in vivo. Formation of a stable MMAA-MUT complex is nucleotide-selective for MMAA (GMPPNP over GDP) and apoenzyme-dependent for MUT. The physiological importance of this interaction is highlighted by a recently identified homoallelic patient mutation of MMAA, G188R, which, we show, retains basal GTPase activity but has abrogated interaction. Together, our data point to a gatekeeping role for MMAA by favoring complex formation with MUT apoenzyme for AdoCbl assembly and releasing the AdoCbl-loaded holoenzyme from the complex, in a GTP-dependent manner.

Structural impact of human and Escherichia coli biotin carboxyl carrier proteins on biotin attachment.

Holocarboxylase synthetase (HCS, human) and BirA (Escherichia coli) are biotin protein ligases that catalyze the ATP-dependent attachment of biotin to apocarboxylases. Biotin attachment occurs on a highly conserved lysine residue within a consensus sequence (Ala/Val-Met-Lys-Met) that is found in carboxylases in most organisms. Numerous studies have indicated that HCS and BirA, as well as biotin protein ligases from other organisms, can attach biotin to apocarboxylases from different organisms, indicating that the mechanism of biotin attachment is well conserved. In this study, we examined the cross-reactivity of biotin attachment between human and bacterial biotin ligases by comparing biotinylation of p-67 and BCCP87, the biotin-attachment domain fragments from human propionyl-CoA carboxylase and E. coli acetyl-CoA carboxylase, respectively. While BirA has similar biotinylation activity toward the two substrates, HCS has reduced activity toward bacterial BCCP87 relative to its native substrate, p-67. The crystal structure of a digested form of p-67, spanning a sequence that contains a seven-residue protruding thumb loop in BCCP87, revealed the absence of a similar structure in the human peptide. Significantly, an engineered "thumbless" bacterial BCCP87 could be biotinylated by HCS, with substrate affinity restored to near normal. This study suggests that the thumb loop found in bacterial carboxylases interferes with optimal interaction with the mammalian biotin protein ligase. While the function of the thumb loop remains unknown, these results indicate a constraint on specificity of the bacterial substrate for biotin attachment that is not itself a feature of BirA.

Biotin is not a natural histone modification.

In addition to its role as the cofactor of biotin-dependent carboxylases, biotin has been demonstrated to have a role in cellular processes including transcription and gene silencing. Histones have been proposed to be modified by biotin in a site-specific manner, providing a pathway by which biotin acts as a regulatory molecule for gene expression. However, there is uncertainty whether biotin attachment to histones in vitro can be extrapolated to biotin as a native histone modification. We critically examined a number of methods used to detect biotin attachment on histones, including [(3)H]-biotin uptake, Western blot analysis of histones, and mass spectrometry of affinity purified histone fragments with the objective of determining if the in vivo occurrence of histone biotinylation could be conclusively established. We found for each of these methods that, while biotin could be readily detected on native carboxylases or histones biotinylated in vitro, biotin attachment on native histones could not be detected in cell cultures from various sources. We conclude that biotin is absent in native histones to a sensitivity of at least one part per 100,000, suggesting that the regulatory impact of biotin on gene expression must be through alternate mechanisms.

Ligand-binding by catalytically inactive mutants of the cblB complementation group defective in human ATP:cob(I)alamin adenosyltransferase.

MMAB (methylmalonic aciduria type B) is a mitochondrial enzyme involved in the metabolism of vitamin B(12). It functions as the ATP:cob(I)alamin adenosyltransferase for the generation of adenosylcobalamin (AdoCbl), the cofactor of methylmalonyl-CoA mutase (MCM). Impaired MMAB activity leads to the inherited disorder methylmalonic aciduria and is responsible for the cblB complementation group. In this study, the effects on substrate binding of two catalytically inactive patient mutations, R190H and R186W, were investigated using intrinsic fluorescence quenching of MMAB as a measure of ligand-binding. We report the dissociation constant (K(d)) of wild-type MMAB for HOCbl is 51 microM and for ATP is 365 microM and show that cobalamin enhances the affinity of MMAB for ATP, while ATP does not show detectable effects on cobalamin binding. We confirm that residue Arg190 plays a role in the formation of the ATP-binding site as described previously [H.L. Schubert, C.P. Hill, Structure of ATP-bound human ATP:cobalamin adenosyltransferase, Biochemistry 45 (2006) 15188-15196]. Unexpectedly, mutation R186W does not disrupt the binding of HOCbl to MMAB as predicted; instead, both R190H and R186W significantly disrupt the affinity between MMAB and AdoCbl. We surmise that these two residues may be critical for the transfer of the 5'-deoxyadenosyl group from ATP to cob(I)alamin, possibly by contributing to the precise positioning of the two substrates to permit catalysis to occur. Characterization of ligand-binding by MMAB provides insight into the mechanism of cobalamin adenosylation and the effect of patient mutations in the inherited disorder.

Nonenzymatic biotinylation of histone H2A.

Holocarboxylase synthetase (HCS, eukaryotic enzyme) and BirA (prokaryotic) are biotin protein ligases that catalyze the ATP-dependent attachment of biotin to apocarboxylases via the reactive intermediate, bio-5'-AMP. In this study, we examined the in vitro mechanism of biotin attachment to histone H2A in the presence of HCS and BirA. The experiment derives from our observations that HCS is found in the nucleus of cells in addition to the cytoplasm, and it has the ability to attach biotin to histones in vitro (Narang et al., Hum Mol Genet 2004; 13:15-23). Using recombinant HCS or BirA, the rate of biotin attachment was considerably slower with histone H2A than with the biotin binding domain of an apocarboxylase. However, on incubation of recombinant H2A with chemically synthesized bio-5'-AMP, H2A was observed to be rapidly labeled with biotin in the absence of enzyme. Nonenzymatic biotinylation of a truncated apocarboxylase (BCCP87) has been previously reported (Streaker and Beckett, Protein Sci 2006; 15:1928-1935), though at a much slower rate than we observe for H2A. The specific attachment sites of nonenzymatically biotinylated recombinant H2A at different time points were identified using mass spectrometry, and were found to consist of a similar pattern of biotin attachment as seen in the presence of HCS, with preference for lysines in the highly basic N-terminal region of the histone. None of the lysine sites within H2A resembles the biotin attachment consensus sequence seen in carboxylases, suggesting a novel mechanism for histone biotinylation.

Impaired biotinidase activity disrupts holocarboxylase synthetase expression in late onset multiple carboxylase deficiency.

Biotinidase catalyzes the hydrolysis of the vitamin biotin from proteolytically degraded biotin-dependent carboxylases. This key reaction makes the biotin available for reutilization in the biotinylation of newly synthesized apocarboxylases. This latter reaction is catalyzed by holocarboxylase synthetase (HCS) via synthesis of 5'-biotinyl-AMP (B-AMP) from biotin and ATP, followed by transfer of the biotin to a specific lysine residue of the apocarboxylase substrate. In addition to carboxylase activation, B-AMP is also a key regulatory molecule in the transcription of genes encoding apocarboxylases and HCS itself. In humans, genetic deficiency of HCS or biotinidase results in the life-threatening disorder biotin-responsive multiple carboxylase deficiency, characterized by a reduction in the activities of all biotin-dependent carboxylases. Although the clinical manifestations of both disorders are similar, they differ in some unique neurological characteristics whose origin is not fully understood. In this study, we show that biotinidase deficiency not only reduces net carboxylase biotinylation, but it also impairs the expression of carboxylases and HCS by interfering with the B-AMP-dependent mechanism of transcription control. We propose that biotinidase-deficient patients may develop a secondary HCS deficiency disrupting the altruistic tissue-specific biotin allocation mechanism that protects brain metabolism during biotin starvation.

Mice deficient in Neu4 sialidase exhibit abnormal ganglioside catabolism and lysosomal storage.

Mammalian sialidase Neu4, ubiquitously expressed in human tissues, is located in the lysosomal and mitochondrial lumen and has broad substrate specificity against sialylated glycoconjugates. To investigate whether Neu4 is involved in ganglioside catabolism, we transfected beta-hexosaminidase-deficient neuroglia cells from a Tay-Sachs patient with a Neu4-expressing plasmid and demonstrated the correction of storage due to the clearance of accumulated GM2 ganglioside. To further clarify the biological role of Neu4, we have generated a stable loss-of-function phenotype in cultured HeLa cells and in mice with targeted disruption of the Neu4 gene. The silenced HeLa cells showed reduced activity against gangliosides and had large heterogeneous lysosomes containing lamellar structures. Neu4(-/-) mice were viable, fertile and lacked gross morphological abnormalities, but showed a marked vacuolization and lysosomal storage in lung and spleen cells. Lysosomal storage bodies were also present in cultured macrophages preloaded with gangliosides. Thin-layer chromatography showed increased relative level of GD1a ganglioside and a markedly decreased level of GM1 ganglioside in brain of Neu4(-/-) mice suggesting that Neu4 may be important for desialylation of brain gangliosides and consistent with the in situ hybridization data. Increased levels of cholesterol, ceramide and polyunsaturated fatty acids were also detected in the lungs and spleen of Neu4(-/-) mice by high-resolution NMR spectroscopy. Together, our data suggest that Neu4 is a functional component of the ganglioside-metabolizing system, contributing to the postnatal development of the brain and other vital organs.

Metabolic derangement of methionine and folate metabolism in mice deficient in methionine synthase reductase.

Hyperhomocyst(e)inemia is a metabolic derangement that is linked to the distribution of folate pools, which provide one-carbon units for biosynthesis of purines and thymidylate and for remethylation of homocysteine to form methionine. In humans, methionine synthase deficiency results in the accumulation of methyltetrahydrofolate at the expense of folate derivatives required for purine and thymidylate biosynthesis. Complete ablation of methionine synthase activity in mice results in embryonic lethality. Other mouse models for hyperhomocyst(e)inemia have normal or reduced levels of methyltetrahydrofolate and are not embryonic lethal, although they have decreased ratios of AdoMet/AdoHcy and impaired methylation. We have constructed a mouse model with a gene trap insertion in the Mtrr gene specifying methionine synthase reductase, an enzyme essential for the activity of methionine synthase. This model is a hypomorph, with reduced methionine synthase reductase activity, thus avoiding the lethality associated with the absence of methionine synthase activity. Mtrr(gt/gt) mice have increased plasma homocyst(e)ine, decreased plasma methionine, and increased tissue methyltetrahydrofolate. Unexpectedly, Mtrr(gt/gt) mice do not show decreases in the AdoMet/AdoHcy ratio in most tissues. The different metabolite profiles in the various genetic mouse models for hyperhomocyst(e)inemia may be useful in understanding biological effects of elevated homocyst(e)ine.

Human methionine synthase reductase is a molecular chaperone for human methionine synthase.

Sustained activity of mammalian methionine synthase (MS) requires MS reductase (MSR), but there have been few studies of the interactions between these two proteins. In this study, recombinant human MS (hMS) and MSR (hMSR) were expressed in baculovirus-infected insect cells and purified to homogeneity. hMSR maintained hMS activity at a 1:1 stoichiometric ratio with a K(act) value of 71 nM. Escherichia coli MS, however, was not activated by hMSR. Moreover, hMS was not significantly active in the presence of E. coli flavodoxin and flavodoxin reductase, which maintain the activity of E. coli MS. These results indicate that recognition of MS by their reductive partners is very strict, despite the high homology between MS from different species. The effects of hMSR on the formation of hMS holoenzyme also were examined by using crude extracts of baculovirus-infected insect cells containing hMS apoenzyme (apoMS). In the presence of MSR and NADPH, holoenzyme formation from apoMS and methylcobalamin was significantly enhanced. The observed stimulation is shown to be due to stabilization of human apoMS in the presence of MSR. Apoenzyme alone is quite unstable at 37 degrees C. MSR also is able to reduce aquacobalamin to cob(II)alamin in the presence of NADPH, and this reduction leads to stimulation of the conversion of apoMS and aquacobalamin to MS holoenzyme. Based on these findings, we propose that MSR serves as a special chaperone for hMS and as an aquacobalamin reductase, rather than acting solely in the reductive activation of MS.

Homozygous nonsense mutation in the MCEE gene and siRNA suppression of methylmalonyl-CoA epimerase expression: a novel cause of mild methylmalonic aciduria.

Methylmalonyl-CoA epimerase (MCE) catalyzes the interconversion of D- and L-methylmalonyl-CoA in the pathway responsible for the degradation of branched chain amino acids, odd chain-length fatty acids, and other metabolites. Despite the occurrence of metabolic disorders in the enzymatic step occurring immediately upstream of MCE (propionyl-CoA carboxylase) and downstream of MCE (adenosylcobalamin-dependent methylmalonyl-CoA mutase), no disease-causing mutations have been described affecting MCE itself. A patient, formerly identified as belonging to the cblA complementation group of vitamin B12 disorders but lacking mutations in the affected gene, MMAA, was tested for mutations in the MCEE gene. The patient's fibroblasts had normal levels of adenosylcobalamin compared to controls, whereas other cblA cell lines typically had reduced levels of the cofactor. As well, this patient had a milder form of methylmalonic aciduria than usually observed in cblA patients. The patient was found to be homozygous for a c.139C>T (p.R47X) mutation in MCEE by sequence analysis that was confirmed by restriction digestion of PCR products. One sibling, also with mild methylmalonic aciduria, was homozygous for the mutation. Both parents and one other sibling were heterozygous. A nearby insertion polymorphism, c.41-160_161insT, heterozygous in both parents, showed the wild-type configuration on the mutant alleles. To assess the impact of isolated MCE deficiency in cultured cells, HeLa cells were transfected with a selectable vector containing MCEE-specific small interfering RNA (siRNA) to suppress gene expression. The reduced level of MCEE mRNA resulted in the reduction of [14C]-propionate incorporation into cellular macromolecules. However, siRNA only led to a small reduction in pathway activity, suggesting that previously postulated non-enzymatic conversion of D- to L-methylmalonyl-CoA may contribute to some flux through the pathway. We conclude that the patient's MCEE defect was responsible for the mild methylmalonic aciduria, confirming a partial requirement for the enzymatic activity in humans.

Mutation and biochemical analysis of patients belonging to the cblB complementation class of vitamin B12-dependent methylmalonic aciduria.

Methylmalonic aciduria, cblB type (OMIM 251110) is an inborn error of vitamin B(12) metabolism that occurs due to mutations in the MMAB gene. MMAB encodes the enzyme ATP:cobalamin adenosyltransferase, which catalyzes the synthesis of the coenzyme adenosylcobalamin required for the activity of the mitochondrial enzyme methylmalonyl CoA mutase (MCM). MCM catalyzes the isomerization of methylmalonyl CoA to succinyl CoA. Deficient MCM activity results in methylmalonic aciduria and a susceptibility to life-threatening acidotic crises. The MMAB gene was sequenced from genomic DNA from a panel of 35 cblB patients, including five patients previously investigated. Nineteen MMAB mutations were identified, including 13 previously unknown mutations. These included 11 missense mutations, two duplications, one deletion, four splice-site mutations, and one nonsense mutation. None of these mutations was identified in 100 control alleles. Most of the missense mutations (9/11) were clustered in exon 7; many of these affected amino acid residues that are part of the probable active site of the enzyme. One previously described mutation, c.556C >T (p.R186W), was particularly common, accounting for 33% of pathogenic alleles. It was seen almost exclusively in patients of European background and was typically associated with presentation in the first year of life.

Impact of cblB mutations on the function of ATP:cob(I)alamin adenosyltransferase in disorders of vitamin B12 metabolism.

ATP:cob(I)alamin adenosyltransferase (MMAB protein; methylmalonic aciduria type B) is an enzyme of vitamin B(12) metabolism that converts reduced cob(I)alamin to the adenosylcobalamin co-factor required for the functional activity of methylmalonyl-CoA mutase. Mutations in the human MMAB gene result in a block in adenosylcobalamin synthesis and are responsible for the cblB complementation group of inherited vitamin B(12) disorders. In this study, we examined the impact of several mutations, previously identified in cblB patients and clustered within a small, highly conserved region in MMAB. We confirmed mitochondrial expression of MMAB in human cells and showed that two mutations, R186W and E193K, were associated with absent protein by Western blot, while one, R191W, coupled with another point mutation, produced a protein in patient fibroblasts. Wild type MMAB and all four mutant proteins were stably expressed at high level as GST-fusion proteins, but only the R191W protein was enzymatically active. It showed an elevated K(m) of 320 microM (vs 6.8 microM for wild type enzyme) for ATP and 60 microM (vs 3.7 microM) for cob(I)alamin, with a reduction in k(cat) for both substrates. Circular dichroism spectroscopy revealed that three mutant proteins examined retained a alpha-helical structure as for the wild type protein. Characterization of MMAB will contribute to our understanding of cobalamin processing in mammalian cells and of disease mechanisms in the genetic disorders.

Identification of the gene responsible for methylmalonic aciduria and homocystinuria, cblC type.

Methylmalonic aciduria and homocystinuria, cblC type (OMIM 277400), is the most common inborn error of vitamin B(12) (cobalamin) metabolism, with about 250 known cases. Affected individuals have developmental, hematological, neurological, metabolic, ophthalmologic and dermatologic clinical findings. Although considered a disease of infancy or childhood, some individuals develop symptoms in adulthood. The cblC locus was mapped to chromosome region 1p by linkage analysis. We refined the chromosomal interval using homozygosity mapping and haplotype analyses and identified the MMACHC gene. In 204 individuals, 42 different mutations were identified, many consistent with a loss of function of the protein product. One mutation, 271dupA, accounted for 40% of all disease alleles. Transduction of wild-type MMACHC into immortalized cblC fibroblast cell lines corrected the cellular phenotype. Molecular modeling predicts that the C-terminal region of the gene product folds similarly to TonB, a bacterial protein involved in energy transduction for cobalamin uptake.