Essential arginine residues in the active sites of propionyl CoA carboxylase and beta-methylcrotonyl CoA carboxylase

At least one arginine residue is essential for substrate binding in or near the active sites of propionyl CoA carboxylase (PCC) and beta-methylcrotonyl CoA carboxylase (beta MCC) in cultured human fibroblasts. This conclusion is based on studies of enzyme inhibition by phenylglyoxal, a reagent which specifically modifies arginine residues. Human fibroblast PCC both in extracts and in a 20-fold purified preparation was nearly completely protected from phenylglyoxal inhibition following incubation with propionyl CoA or ATP. It appears that a phosphate group from either ATP or the CoA moiety of propionyl CoA reacts with the essential arginine residue(s). beta MCC which was similarly inhibited by phenylglyoxal was protected by beta-methylcrotonyl CoA and ATP. Thus phenylglyoxal may be used to label specific arginine residues within the active sites of previously sequenced carboxylases.     

Molecular cloning of CoA Synthase. The missing link in CoA biosynthesis

Coenzyme A functions as a carrier of acetyl and acyl groups in living cells and is essential for numerous biosynthetic, energy-yielding, and degradative metabolic pathways. There are five enzymatic steps in CoA biosynthesis. To date, molecular cloning of enzymes involved in the CoA biosynthetic pathway in mammals has been only reported for pantothenate kinase. In this study, we present cDNA cloning and functional characterization of CoA synthase. It has an open reading frame of 563 aa and encodes a protein of approximately 60 kDa. Sequence alignments suggested that the protein possesses both phosphopantetheine adenylyltransferase and dephospho-CoA kinase domains. Biochemical assays using wild type recombinant protein confirmed the gene product indeed contained both these enzymatic activities. The presence of intrinsic phosphopantetheine adenylyltransferase activity was further confirmed by site-directed mutagenesis. Therefore, this study describes the first cloning and characterization of a mammalian CoA synthase and confirms this is a bifunctional enzyme containing the last two components of CoA biosynthesis.     

Catalytic role of the conformational change in succinyl-CoA:3-oxoacid CoA transferase on binding CoA

Catalysis by succinyl-CoA:3-oxoacid CoA transferase proceeds through a thioester intermediate in which CoA is covalently linked to the enzyme. To determine the conformation of the thioester intermediate, crystals of the pig enzyme were grown in the presence of the substrate acetoacetyl-CoA. X-ray diffraction data show the enzyme in both the free form and covalently bound to CoA via Glu305. In the complex, the protein adopts a conformation in which residues 267-275, 280-287, 357-373, and 398-477 have shifted toward Glu305, closing the enzyme around the thioester. Enzymes provide catalysis by stabilizing the transition state relative to complexes with substrates or products. In this case, the conformational change allows the enzyme to interact with parts of CoA distant from the reactive thiol while the thiol is covalently linked to the enzyme. The enzyme forms stabilizing interactions with both the nucleotide and pantoic acid portions of CoA, while the interactions with the amide groups of the pantetheine portion are poor. The results shed light on how the enzyme uses the binding energy for groups remote from the active center of CoA to destabilize atoms closer to the active center, leading to acceleration of the reaction by the enzyme.     

HMG CoA reductase inhibitors

Beta-hydroxy-beta-methylglutaryl coenzyme A (HMG CoA) reductase inhibitors (HMG CoA RIs) are now being prescribed in many different countries, and the number of patients treated with these compounds is estimated to be over 1.5 million. The spectrum of indications for these drugs is enlarging. Thus, not only patients with primary hypercholesterolemia are suitable candidates for therapy with HMG CoA RIs, but also patients with other forms of familial and non-familial (secondary) dyslipidemias. The safety and tolerance profile is remarkably good and adverse effects tend to be mild and rare. Currently, several primary and secondary intervention trials are ongoing or in preparation to investigate the influence of HMG CoA RIs on cardiovascular morbidity and mortality.     

Effect of insulin on association of acetyl CoA carboxylase phosphatase and acetyl CoA carboxylase

Insulin promotes an association between acetyl CoA carboxylase and acetyl CoA carboxylase phosphatase. The association between rat epididymal fat tissue carboxylase and the phosphatase occurs in both a tissue culture system and in vivo and is accompanied by an increase in acetyl CoA carboxylase activity.     

Succinyl CoA Synthetase of Tobacco

The isouition and partial purification of succinyl CoA synthetase from Nicotiana tabacum is described. Enzynio activity had a pH optimum of pH 7.0 and apparent Km values for succinic acid. ATF and MgCl(2) . are 3.5 × 10(-2) 5.7 × 10(-3) and 4.0 × 10(-3) M, respectively. The euzyme is specific in requirement for ATP. Enzyme activity was low in seed, decreased during germination and seedling development, then increased to maximum in physiologically mature leaf tissue. Plauts with the apical apical meristem removed had less euzyme activity in leaves but more enzyme activity in the roots than plants with apical meristem intact. Most enzyme activity observed was associated with the soluble fractions of the cell.     

Cloning of acetyl CoA carboxylase cDNA and the effects of spirodiclofen on the expression of acetyl CoA carboxylase mRNA in Panonychus citri

The citrus red mite, Panonychus citri (McGregor) (Acari: Tetranychidae), is an important pest worldwide. Previous studies showed that acetyl CoA carboxylase (ACCase) may be a critical target of spirodiclofen, a recently introduced insecticide. Therefore, we cloned the PcACCase gene and obtained a full‐length cDNA (Genbank accession number KJ675439 ). The open reading frame was 6 972 bp, coding for 2 323 amino acids. The PcACCase protein had a theoretical molecular weight of 262.82 kDa and an isoelectric point of 6.31, and it contained six conserved domains and one low‐complexity region. Quantitative real‐time PCR showed that spirodiclofen can up‐regulate the expression levels of ACCase mRNA during all four developmental stages.     

Ribozyme-catalyzed aminoacylation from CoA thioesters

Coenzyme A (CoA) thioesters play essential roles in modern metabolism. To demonstrate plausible biochemical functions of thioesters in the RNA world, we have isolated a new class of ribozymes (ACT) that catalyze self-aminoacylation from a number of CoA thioesters with catalytic efficiencies ranging from 7000 to 24 000 M(-1) x min(-1). Active thioester substrates are required to contain both a free alpha-amino group in the acyl moiety and a CoA as the thiol component. We hypothesize ribozyme-based aminoacylation systems using aminoacyl thioesters of CoA as the ancestors of modern aminoacyl tRNA synthetases. On the basis of our previous results [Huang et al. (2000) Biochemistry 39, 15548-15555; Coleman and Huang (2002) Chem. Biol. 9, 1227-1236], an extensive RNA-catalyzed "metabolic pathway" involving CoA and its thioesters is proposed. Complex contemporary metabolic systems could have evolved from the proposed ribozyme pathways.     

Structure of the mammalian CoA transferase from pig heart

Ketoacidosis affects patients who are deficient in the enzyme activity of succinyl-CoA:3-ketoacid CoA transferase (SCOT), since SCOT catalyses the activation of acetoacetate in the metabolism of ketone bodies. Thus far, structure/function analysis of the mammalian enzyme has been predicted based on the three-dimensional structure of a CoA transferase determined from an anaerobic bacterium that utilizes its enzyme for glutamate fermentation. To better interpret clinical data, we have determined the structure of a mammalian CoA transferase from pig heart by X-ray crystallography to 2.5 A resolution. Instrumental to the structure determination were selenomethionine substitution and the use of argon during purification and crystallization. Although pig heart SCOT adopts an alpha/beta protein fold, resembling the overall fold of the bacterial CoA transferase, several loops near the active site of pig heart SCOT follow different paths than the corresponding loops in the bacterial enzyme, accounting for differences in substrate specificities. Two missense mutations found associated with SCOT of ketoacidosis patients were mapped to a location in the structure that might disrupt the stabilization of the amino-terminal strand and thereby interfere with the proper folding of the protein into a functional enzyme.     

Phosphoprotein phosphatase nonspecifically hydrolyzes CoA

CoA hydrolysis was studied by a homogenous phosphoprotein phosphatase (EC 3.1 3.16) preparation from bovine spleen nuclei at pH 5.8. Phosphoprotein phosphatase catalyzed hydrolysis of the CoA 3'-phosphoester bond to form dephospho-CoA and Pi. The Km value for phosphoprotein phosphatase with CoA as substrate was 3.7 mM, the specific activity - 0.26 mmol Pi.min-1.mg-1. Phosphoprotein phosphatase did not essentially catalyze the calcium pantothenate hydrolysis (not more than 2% as compared with the CoA hydrolysis rate).     

Identification and characterization of a succinyl-coenzyme A (CoA):benzoate CoA transferase in Geobacter metallireducens

Geobacter metallireducens is a Fe(III)-respiring deltaproteobacterium and serves as a model organism for aromatic compound-degrading, obligately anaerobic bacteria. In this study, a genetic system was established for G. metallireducens using nitrate as an alternative electron acceptor. Surprisingly, disruption of the benzoate-induced bamY gene, encoding a benzoate coenzyme A (CoA) ligase, reproducibly showed an increased biomass yield in comparison to the wild type during growth with benzoate but not during growth with acetate. Complementation of bamY in trans converted the biomass yield back to the wild-type level. Growth of the bamY mutant with benzoate can be rationalized by the identification of a previously unknown succinyl-CoA:benzoate CoA transferase activity; it represents an additional, energetically less demanding mode of benzoate activation. The activity was highly enriched from extracts of cells grown on benzoate, yielding a 50-kDa protein band; mass spectrometric analysis identified the corresponding benzoate-induced gene annotated as a CoA transferase. It was heterologously expressed in Escherichia coli and characterized as a specific succinyl-CoA:benzoate CoA transferase. The newly identified enzyme in conjunction with a benzoate-induced succinyl-CoA synthetase links the tricarboxylic acid cycle to the upper benzoyl-CoA degradation pathway during growth on aromatic growth substrates.     

Mammalian myristoyl CoA: protein N-myristoyltransferase

Myristoyl CoA:Protein N-myristoyltransferase (NMT) is the enzyme which catalyses the covalent transfer of myristate from myristoyl CoA to the amino-terminal glycine residue of protein substrates. Although NMT is ubiquitous in eukaryotic cells, the enzyme levels and cellular distribution vary among tissues. In this article, we describe the properties of mammalian NMT(s) with reference to subcellular distribution, molecular weights, substrate specificity and the possible involvement of NMT in pathological processes. The cytosolic fraction of bovine brain contains multiple forms of NMT activity whereas bovine spleen contains only a single form. In bovine brain and spleen, the cytosol contained majority of NMT activity. In contrast, rabbit colon and rat liver NMT activity was predominantly particulate. Regional differences in NMT activity have been observed in both rabbit intestine and bovine brain. Results from our laboratory along with the existing knowledge, provide evidence for the existence of tissue specific isozymes of NMT.     

The presence and possible function of methylmalonyl CoA mutase and propionyl CoA carboxylase in Spirometra mansonoides.

Both spargana and adult forms of Spirometra mansonoides were shown to accumulate lactate, succinate, acetate, and propionate upon in vitro incubation. Adults differed markedly from the spargana in that quantitatively the most significant products of the former were acetate and propionate while the latter formed primarily acetate and lactate. The adults accumulated approximately 32 times more propionate than the spargana per gram of tissue. In accord with this propionate formation, propionyl CoA carboxylase and methylmalonyl CoA mutase have been found to be present in both stages of the parasite. As might be predicted, however, the activities of the carboxylase and mutase were 100-fold and 10-fold higher, respectively, in the adults as compared to the larvae. A possible physiological relationship between propionate formation and energy generation is suggested. Accordingly, inorganic 32P was incorporated into ATP upon incubation of methylmalonyl CoA with a homogenate obtained from adult S. mansonoides. Since methylmalonyl CoA mutase requires vitamin B12 coenzyme, a relationship between vitamin B12 content and propionate formation in helminths is suggested.