Photoaffinity labeling and stoichiometry of the coenzyme A ester sites of transcarboxylase

Photolabeling acyl CoA binding proteins in microsome preparations

To identify key enzymes that participate in acylglycerol metabolism, 12-[(5-iodo-4-azidosalicyl)amino]dodecanoyl-coenzyme A (ASACoA) was employed as a photoaffinity label for those enzymes that use fatty acyl CoA as a co-substrate. ASACoA inhibited diacylglycerol acyltransferase activity in liver microsomes and was incorporated into triacylglycerol in a microsome dependent reaction. When photoactivated, ASACoA labeled four proteins in rat liver (75, 54, 50 and 37 kDa) and in epididymal fat cell (75, 64, 54 and 37 kDa) microsomes. Photolabeling was sensitive to palmitoyl CoA. After solubilization in Triton X-114, all four proteins were concentrated into the detergent phase, indicating that they are integral membrane proteins.

Photoaffinity labeling of mitochondrial proteins with 2-azido [32P]palmitoyl CoA

A long-chain fatty acyl CoA photolabel, 2-azido [32P]palmitoyl CoA, was synthesized and its covalent interaction with mitochondrial membrane proteins examined. On binding of 2-azido [32P]palmitoyl CoA to beef heart mitochondria, two polypeptides were primarily labeled, the 30 kDa ADP/ATP carrier and a 41 kDa protein of unknown identity. Carboxyatractyloside and palmitoyl CoA completely protected against labeling of the 30 kDa protein indicating that it was the ADP/ATP carrier. With inverted submitochondrial particles, only the 30 kDa polypeptide was labeled by 2-azido [32P]palmitoyl CoA. The labeling was inhibited by bongkrekic acid and palmitoyl CoA but not carboxyatractyloside, providing evidence that the ADP/ATP carrier was covalently bound from the matrix side of the membrane. In brown adipose tissue mitochondria, 2-azido [32P]palmitoyl CoA photolabeled the ADP/ATP carrier and the 32 kDa uncoupling protein with some minor labeling of 36 and 68 kDa polypeptides. The results indicated that this physiological photolabeling reagent with the azido group on the CoA portion of the molecule interacts like 2-azido ADP with nucleotide binding sites of a number of important enzymes in cell metabolism. Moreover, the evidence strongly supports the hypothesis that long chain fatty acyl CoA esters are natural ligands for key nucleotide binding proteins.

Primary structure of the monomer of the 12S subunit of transcarboxylase as deduced from DNA and characterization of the product expressed in Escherichia coli

Transcarboxylase from Propionibacterium shermanii is a complex biotin-containing enzyme composed of 30 polypeptides of three different types: a hexameric central 12S subunit to which 6 outer 5S subunits are attached through 12 1.3S biotinyl subunits. The enzyme catalyzes a two-step reaction in which methylmalonyl coenzyme A and pyruvate serve as substrates to form propionyl coenzyme A (propionyl-CoA) and oxalacetate, the 12S subunit specifically catalyzing one of the two reactions. We report here the cloning, sequencing, and expression of the 12S subunit. The gene was identified by matching amino acid sequences derived from isolated authentic 12S peptides with the deduced sequence of an open reading frame present in a cloned P. shermanii genomic fragment known to contain the gene encoding the 1.3S biotinyl subunit. The cloned 12S gene encodes a protein of 604 amino acids and of M(r) 65,545. The deduced sequence shows regions of extensive homology with the beta subunit of mammalian propionyl-CoA carboxylase as well as regions of homology with acetyl-CoA carboxylase from several species. Two genomic fragments were subcloned into pUC19 in an orientation such that the 12S open reading frame could be expressed from the lac promoter of the vector. Crude extracts prepared from these cells contained an immunoreactive band on Western blots (immunoblots) which comigrated with authentic 12S. The Escherichia coli-expressed 12S was purified to apparent homogeneity by a three-step procedure and compared with authentic 12S from P. shermanii. Their quaternary structures were identical by electron microscopy, and the E. coli 12S preparation was fully active in the reactions catalyzed by this subunit. We conclude that we have cloned, sequenced, and expressed the 12S subunit which exists in a hexameric active form in E.coli.

Preliminary crystallographic data and quaternary structural implications of the central subunit of the multi-subunit complex transcarboxylase

The hexameric central subunit (Mr = 360,000) of the multi-subunit complex transcarboxylase has been crystallized by bulk dialysis against 250 mM-sodium acetate (pH 5.5). The crystals are cubic, a = 193.1 A, space group P4(1)32 or enantiomorph. The number of molecules per unit cell is four and was deduced from the density of the crystals (1.10 g cm-3) and the mother liquor (1.01 g cm-3) and the specific volume of the protein calculated from molecular dimensions obtained from electron microscopy studies. Four molecules per cell requires the central subunits to lie on 3-fold axes, which are perpendicular to 2-fold rotation axes, so that the molecules satisfy 32 symmetry giving one subunit as the asymmetric unit. Of the four possible models that have been considered for the quaternary structure of transcarboxylase, only that with antiparallel subunits, two sets of isologous binding sites and D3 symmetry is in agreement with the symmetry requirements of the cubic crystals.

The anatomy of transcarboxylase and the role of its subunits

Biotin enzymes in general catalyze the fixation of CO2 and in a few instances decarboxylations yielding CO2. Transcarboxylase is an exception; it catalyzes the transfer of a carboxyl group from one compound to another and CO2 is not involved. This enzyme plays an essential role in the formation of propionic acid by propionibacteria and its structure and catalytic mechanism have been extensively investigated including studies of the quaternary structure by electron microscopy. The structure is complex, consisting of three types of subunits: (1) a central hexameric subunit, (2) six dimeric outside subunits, and (3) twelve biotinyl subunits which bind the outside subunits to the central subunit. There are 12 substrate sites on the central subunit (2 per polypeptide) and 2 substrate sites on each of the dimeric outside subunits. The carboxyl is transferred between these sites via the biotin of the biotinyl subunit. The biotinyl subunit (approximately 123 residues) has been completely sequenced and it has been shown that the first 42 residues serve in binding the outside subunits to the central subunit and the remainder of the sequence is involved in placing the biotin between the subunits so that it may serve as the carboxyl carrier between the substrate sites on the central and outside subunits. It is proposed that the dual sites on the polypeptides of the central subunit have arisen as a consequence of gene duplication and fusion. An intriguing question is why such a complicated structure is required for catalysis of a rather simple reaction.