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Malonate decarboxylase of Klebsiella pneumoniae consists of four different subunits and catalyzes the conversion of malonate plus H' to acetate and CO,. The catalysis proceeds via acetyl and malonyl thioester residues with the phosphribosyl-dephospho-CoA prosthetic group of the acyl carrier protein (ACP) subunit. From a cosmid library of K. pneumoniae, a gene cluster of 9 kb has been isolated and sequenced that included the structural genes for the malonate decarboxylase. The cluster consisted of the eight consecutive genes mdcABCDEFCH and the divergently oriented mdcR gene. The intergenic regions were short (usually < 17 bp, 136 bp between mdcE and mdcF) and ribosome binding sites were found 4-10 bp before each gene. According to N-terminal protein sequencing, the mdcA, C, D and E genes encoded subunits a, 6, / 3 and y of malonate decarboxylase. Data bank searches for related proteins with known function revealed that MdcA represents the ACP-transferase and that MdcD and E together probably function as malonyl-S-ACP decarboxylase. MdcC is the (apo) ACP subunit. MdcB and MdcG could be involved in the synthesis and attachment of the prosthetic group. MdcH is similar to various malonylCoA:ACP-SH transacylases and therefore probably involved in the initial activation of the enzyme by malonylation. MdcF is a membrane protein that could function as a malonate carrier. The mdcR gene encodes a protein of the LysR regulator family. Malonate decarboxylase was functionally expressed in Escherichia coli from plasmids harbouring the entire gene cluster including mdcR. As partial deletion of the mdcR gene impaired growth of the transformants on malonate, MdcR is probably a transcriptional regulator of the mdc genes.

Malonate decarboxylase from Klebsiella pneumoniae consists of four subunits MdcA, D, E, and C and catalyzes the cleavage of malonate to acetate and CO(2). The smallest subunit MdcC is an acyl carrier protein to which acetyl and malonyl thioester residues are bound via a 2'-(5' '-phosphoribosyl)-3'-dephospho-CoA prosthetic group and turn over during the catalytic mechanism. We report here on the biosynthesis of holo acyl carrier protein from the unmodified apoprotein. The prosthetic group biosynthesis starts with the MdcB-catalyzed condensation of dephospho-CoA with ATP to 2'-(5' '-triphosphoribosyl)-3'-dephospho-CoA. In this reaction, a new alpha (1' ' --> 2') glycosidic bond between the two ribosyl moieties is formed, and thereby, the adenine moiety of ATP is displaced. MdcB therefore is an ATP:dephospho-CoA 5'-triphosphoribosyl transferase. The second protein involved in holo ACP synthesis is MdcG. This enzyme forms a strong complex with the 2'-(5' '-triphosphoribosyl)-3'-dephospho-CoA prosthetic group precursor. This complex, called MdcG(i), is readily separated from free MdcG by native polyacrylamide gel electrophoresis. Upon incubation of MdcG(i) with apo acyl carrier protein, holo acyl carrier protein is synthesized by forming the phosphodiester bond between the 2'-(5' '-phosphoribosyl)-3'-dephospho-CoA prosthetic group and serine 25 of the protein. MdcG corresponds to a 2'-(5' '-triphosphoribosyl)-3'-dephospho-CoA:apo ACP 2'-(5' '-phosphoribosyl)-3'-dephospho-CoA transferase. In absence of the prosthetic group precursor, MdcG catalyzes at a low rate the adenylylation of apo acyl carrier protein using ATP as substrate. The adenylyl ACP thus formed is an unphysiological side product and is not involved in the biosynthesis of holo ACP. The 2'-(5' '-triphosphoribosyl)-3'-dephospho-CoA precursor of the prosthetic group has been purified and its identity confirmed by mass spectrometry and enzymatic analysis.

Functional malonate decarboxylase of Klebsiella pneumoniae is an acetyl-S-enzyme with an acetylated phosphoribosyl dephospho-CoA prosthetic group. The mdcH gene product acts as a malonyl-CoA:ACP transacylase and initiates the activation of (deacetyl)malonate decarboxylase by malonyl-transfer to the prosthetic group. The malonyl residue is subsequently decarboxylated to an acetyl residue by the decarboxylase itself. Purified malonate decarboxylase consists of the four subunits MdcA, D, E and C in an apparent 1 : 1 : 1 : 1 stoichiometry. In addition, the preparation contains substoichiometric amounts of MdcH comigrating on SDS/PAGE with MdcD. Malonate decarboxylase isolated from strains with a deletion of the mdcH gene was not activated with malonyl-CoA. Activity could be gained, however, in the additional presence of MdcH that has been synthesized in Escherichia coli and purified from inclusion bodies. Substrates for MdcH are malonyl-CoA or methylmalonyl-CoA but not acetyl-CoA. The enzyme has K m values of 16 mm for both substrates and V max for malonyl-CoA of 190 U´mg ±1 and for methylmalonyl-CoA of 37 U´mg ±1 . Transfer of the methylmalonyl-residue to the prosthetic group proceeds via the covalent methylmalonyl-MdcH intermediate. The transacylase is specifically inhibited by N-ethylmaleimide, and preincubation with malonyl-CoA or methylmalonyl-CoA protects the enzyme from this inhibition.Keywords: enzyme activation; fatty acid biosynthesis; Klebsiella pneumoniae; malonate decarboxylase; malonylCoA:acyl carrier protein transacylase.In spite of the widely used diagnostic criterion of bacterial growth on malonate [1], the enzymic and genetic basis for this behavior was resolved only recently (for a review, see [2]). The key element is a specific malonate decarboxylase that converts malonate directly into acetate and CO 2 . Malonate is chemically rather inert, especially at neutral pH, where both carboxylic residues are dissociated (pK a1 = 2.85, pK a2 = 5.69). The dicarboxylate is therefore activated for the decarboxylation reaction by transiently forming a thioester with the enzyme. The catalytically active enzyme carries an acetyl thioester residue that is exchanged in the first partial reaction by a malonyl thioester residue. This is subsequently decarboxylated with regeneration of the acetyl-S-enzyme (Fig. 1). This malonate decarboxylation mechanism was discovered for the enzyme from Malonomonas rubra, an anaerobic bacterium capable to grow entirely from the free energy of this decarboxylation reaction [3]. Later, aerobic bacteria known to grow on malonate, e.g. Klebsiella pneumoniae [4], Acinetobacter calcoaceticus [5], Pseudomonas putida [6] or Pseudomonas fluorescens [7] were found to dispose of malonate decarboxylases that activate the substrate by the same mechanism forming malonyl-S-enzyme derivates. These aerobic decarboxylases release CO 2 directly from the malonyl-S-enzyme intermediates, forfeiting the free energy of the decarboxylation reaction. In contrast, the M. rubra decarboxylase system catalyze...

Malonate decarboxylase from Klebsiella pneumoniae contains an acyl carrier protein (MdcC) to which a 2'-(5' '-phosphoribosyl)-3'-dephospho-CoA prosthetic group is attached via phosphodiester linkage to serine 25. We have shown in the preceding paper in this issue that the formation of this phosphodiester bond is catalyzed by a phosphoribosyl-dephospho-coenzyme A transferase MdcG with the substrate 2'-(5' '-triphosphoribosyl)-3'-dephospho-CoA that is synthesized from ATP and dephospho-coenzyme A by the triphosphoribosyl transferase MdcB. The reaction catalyzed by MdcG is related to nucleotidyltransfer reactions, and the enzyme indeed catalyzes unphysiological nucleotidyltransfer, e.g., adenylyltransfer from ATP to apo acyl carrier protein (ACP). These unspecific side reactions are favored at high Mg(2+) concentrations. A sequence motif including D134 and D136 of MdcG is a signature of all nucleotidyltransferases. It is known from the well-characterized mammalian DNA polymerase beta that this motif is at the active site of the enzyme. Site-directed mutagenesis of D134 and/or D136 of MdcG to alanine abolished the transfer of the prosthetic group to apo ACP, but the binding of triphosphoribosyl-dephospho-CoA to MdcG was not affected. Evidence is presented that similar to MdcG, MadK encoded by the malonate decarboxylase operon of Malonomonas rubra and CitX from the operon encoding citrate lyase in Escherichia coli are phosphoribosyl-dephospho-CoA transferases catalyzing the attachment of the phosphoribosyl-dephospho-CoA prosthetic group to their specific apo ACPs.