Methylthioadenosine/S-adenosylhomocysteine nucleosidase, a critical enzyme for bacterial metabolism

Abstract

The importance of methylthioadenosine/S-adenosylhomocysteine (MTA/SAH) nucleosidase in bacteria has started to be appreciated only in the past decade. A comprehensive analysis of its various roles here demonstrates that it is an integral component of the activated methyl cycle, which recycles adenine and methionine through S-adenosylmethionine (SAM)-mediated methylation reactions, and also produces the universal quorum-sensing signal, autoinducer-2 (AI-2). SAM is also essential for synthesis of polyamines, N-acylhomoserine lactone (autoinducer-1), and production of vitamins and other biomolecules formed by SAM radical reactions. MTA, SAH and 5'-deoxyadenosine (5'dADO) are product inhibitors of these reactions, and are substrates of MTA/SAH nucleosidase, underscoring its importance in a wide array of metabolic reactions. Inhibition of this enzyme by certain substrate analogues also limits synthesis of autoinducers and hence causes reduction in biofilm formation and may attenuate virulence. Interestingly, the inhibitors of MTA/SAH nucleosidase are very effective against the Lyme disease causing spirochaete, Borrelia burgdorferi, which uniquely expresses three homologous functional enzymes. These results indicate that inhibition of this enzyme can affect growth of different bacteria by affecting different mechanisms. Therefore, new inhibitors are currently being explored for development of potential novel broad-spectrum antimicrobials.

Fig. 1. Four metabolic pathways produce substrates of MTA/SAH nucleosidase

Four pathways involving SAM: (I) methylation of macromolecules, (II) reactions producing different metabolites by SAM radical enzymes, (III) polyamine (including spermidine) synthesis, and (IV) autoinducer-1 (N-acylhomoserine lactone) synthesis. All result in production of nucleosidase substrates: SAH, MTA or 5′dADO (boxed). Methylation of various macromolecules using specific transmethylases produces SAH as a byproduct. SAM radical enzymes produce vitamins, metabolites and antibiotics with methionine and 5′dADO as byproducts. Polyamine synthesis is a two-step process, (i) decarboxylation of SAM followed by (ii) conversion of putrescine and decarboxylated SAM to spermidine and MTA. Hexanoyl Acyl carrier protein (ACP) donates theAcyl group to SAM to form N-acylhomoserine lactone.

Fig. 2. Recycling of MTA and 5′dADO by MTA/SAH nucleosidase

In the majority of bacteria, MTA is first converted by MTA/SAH nucleosidase into methylthioribose (MTR). In species with a complete methionine salvage cycle, MTR is subsequently phosphorylated by MTR kinase to 5-methylthioribose-1-phosphate (MTRP). The thiomethyl group from MTRP is recycled in a multistep pathway back to methionine. This critical amino acid is also produced as a byproduct by SAM radical enzyme reactions during synthesis of vitamins and other metabolites. The byproduct, 5′dADO, is also a substrate of MTA/SAH nucleosidase and this enzymatic reaction produces 5′deoxyribose and adenine. In eukaryotes and some environmental bacteria, MTA is catabolized by MTA phosphorylase to MTRP in a single step, which is then converted through the same enzymatic steps to methionine. A ribbon diagram of the MTA/SAH nucleosidase with bound substrate, MTA, (marked by oval) is shown.

Fig. 3. MTA/SAH nucleosidase is involved in Activated Methyl Cycle

Methionine is converted into SAM by SAM synthetase (MetK). Donation of the methyl group of SAM to a variety of methyl acceptors results in SAH, which is recycled back to homocysteine in either a one-step or two-step process. In several bacterial species SAH hydrolase directly converts SAH into homocysteine. However, in the majority of eubacteria, MTA/SAH nucleosidase first converts SAH into S-ribosylhomocysteine (SRH), which is then recycled back to homocysteine by LuxS. As a byproduct of this reaction, 4,5 dihydroxy 2,3-pentanedione is produced which spontaneously forms autoinducer-2 (AI-2). In some bacteria, cysteine is produced from homocycteine by multi-step enzymatic reactions since this cycle is the only source of sulfur-containing compounds. Homocycteine is recycled back to methionine in different organisms by the Met E or MetH methionine synthases.