ABSTRACT
3-Methylcrotonylglycinuria is an inborn error of leucine catabolism and has a recessive pattern of inheritance that results from the deficiency of 3-methylcrotonyl-CoA carboxylase (MCC). The introduction of tandem mass spectrometry in newborn screening has revealed an unexpectedly high incidence of this disorder, which, in certain areas, appears to be the most frequent organic aciduria. MCC, an heteromeric enzyme consisting of alpha (biotin-containing) and beta subunits, is the only one of the four biotin-dependent carboxylases known in humans that has genes that have not yet been characterized, precluding molecular studies of this disease. Here we report the characterization, at the genomic level and at the cDNA level, of both the MCCA gene and the MCCB gene, encoding the MCC alpha and MCC beta subunits, respectively. The 19-exon MCCA gene maps to 3q25-27 and encodes a 725-residue protein with a biotin attachment site; the 17-exon MCCB gene maps to 5q12-q13 and encodes a 563-residue polypeptide. We show that disease-causing mutations can be classified into two complementation groups, denoted "CGA" and "CGB." We detected two MCCA missense mutations in CGA patients, one of which leads to absence of biotinylated MCC alpha. Two MCCB missense mutations and one splicing defect mutation leading to early MCC beta truncation were found in CGB patients. A fourth MCCB mutation also leading to early MCC beta truncation was found in two nonclassified patients. A fungal model carrying an mccA null allele has been constructed and was used to demonstrate, in vivo, the involvement of MCC in leucine catabolism. These results establish that 3-methylcrotonylglycinuria results from loss-of-function mutations in the genes encoding the alpha and beta subunits of MCC and complete the genetic characterization of the four human biotin-dependent carboxylases.
Subject(s)
Amino Acid Metabolism, Inborn Errors/genetics , Carbon-Carbon Ligases/genetics , Leucine/metabolism , Adult , Amino Acid Metabolism, Inborn Errors/enzymology , Amino Acid Metabolism, Inborn Errors/pathology , Amino Acid Sequence , Aspergillus nidulans/drug effects , Aspergillus nidulans/genetics , Aspergillus nidulans/growth & development , Base Sequence , Blotting, Northern , Carbon-Carbon Ligases/metabolism , Child, Preschool , Chromosome Mapping , Chromosomes, Human, Pair 3/genetics , Chromosomes, Human, Pair 5/genetics , DNA/chemistry , DNA/genetics , DNA Mutational Analysis , DNA, Complementary/chemistry , DNA, Complementary/genetics , Exons , Female , Gene Expression Regulation, Enzymologic , Genes/genetics , Humans , In Situ Hybridization, Fluorescence , Infant , Introns , Isoenzymes/genetics , Isoenzymes/metabolism , Leucine/pharmacology , Molecular Sequence Data , Mutation , Protein Subunits , RNA/genetics , RNA/metabolism , Radiation Hybrid Mapping , Sequence Analysis, DNA , Tissue Distribution , Transcription, GeneticABSTRACT
Alkaptonuria (AKU), the prototypic inborn error of metabolism, was the first human disease to be interpreted as a Mendelian trait by Garrod and Bateson at the beginning of last century. AKU results from impaired function of homogentisate dioxygenase (HGO), an enzyme required for the catabolism of phenylalanine and tyrosine. With the novel 7 AKU and 22 fungal mutations reported here, a total of 84 mutations impairing this enzyme have been found in the HGO gene from humans and model organisms. Forty-three of these mutations result in single amino acid substitutions. This mutational information is analysed here in the context of the HGO structure and function using kinetic assays performed using purified AKU mutant enzymes and the crystal structure of human HGO. HGO is a topologically complex structure which assembles as a functional hexamer arranged as a dimer of trimers. We show how the intricate pattern of intra- and inter-subunit interactions and the extensive surfaces required for subunit folding and association of this oligomeric enzyme can be inactivated at multiple levels by single-residue substitutions. This explains, in part, the predominance of missense mutations (67%) in AKU.
Subject(s)
Alkaptonuria/genetics , Dioxygenases , Oxygenases/genetics , Alkaptonuria/metabolism , Alkaptonuria/pathology , Amino Acid Sequence , Amino Acid Substitution , Aspergillus nidulans/genetics , Aspergillus nidulans/metabolism , Catalytic Domain , Homogentisate 1,2-Dioxygenase , Humans , Models, Molecular , Molecular Sequence Data , Mutation , Oxygenases/chemistry , Oxygenases/metabolism , Protein Conformation , Protein Folding , Sequence Alignment , Structure-Activity RelationshipABSTRACT
A key step in the elimination of invading pathogens from the body is the covalent binding of complement proteins C3b and C4b to their surface. However, many pathogens have evolved mechanisms to avoid the complement system of the host. Understanding how these mechanisms work may lead to more efficacious forms of therapy. Here we provide an insight into the molecular basis of how Streptococcus pyogenes binds human plasma C4b-binding protein (hC4BP), a complement regulatory molecule that may decrease C3b and C4b deposition on the streptococcal surface. We show that streptococcal surface molecules bind to a site on hC4BP that is indistinguishable from the C4b binding site. This site involves multiple binding surfaces that span short consensus repeats 1 to 3 of the alpha-chain of hC4BP. We propose that hC4BP is bound to the bacterial surface because the streptococcal surface molecules involved in the interaction mimic human C4b epitopes.