Insight into S-adenosylmethionine biosynthesis from the crystal structures of the human methionine adenosyltransferase catalytic and regulatory subunits

Abstract

MAT (methionine adenosyltransferase) utilizes L-methionine and ATP to form SAM (S-adenosylmethionine), the principal methyl donor in biological methylation. Mammals encode a liver-specific isoenzyme, MAT1A, that is genetically linked with an inborn metabolic disorder of hypermethioninaemia, as well as a ubiquitously expressed isoenzyme, MAT2A, whose enzymatic activity is regulated by an associated subunit MAT2B. To understand the molecular mechanism of MAT functions and interactions, we have crystallized the ligand-bound complexes of human MAT1A, MAT2A and MAT2B. The structures of MAT1A and MAT2A in binary complexes with their product SAM allow for a comparison with the Escherichia coli and rat structures. This facilitates the understanding of the different substrate or product conformations, mediated by the neighbouring gating loop, which can be accommodated by the compact active site during catalysis. The structure of MAT2B reveals an SDR (short-chain dehydrogenase/reductase) core with specificity for the NADP/H cofactor, and harbours the SDR catalytic triad (YxxxKS). Extended from the MAT2B core is a second domain with homology with an SDR sub-family that binds nucleotide-sugar substrates, although the equivalent region in MAT2B presents a more open and extended surface which may endow a different ligand/protein-binding capability. Together, the results of the present study provide a framework to assign structural features to the functional and catalytic properties of the human MAT proteins, and facilitate future studies to probe new catalytic and binding functions.

Crystallography captures catalytic steps in human methionine adenosyltransferase enzymes

“…A diverse series of structures are available for MAT enzymes from bacteria, rat, and human (4,(10)(11)(12), which, supported by a range of biochemical evidence, have provided significant insight into the enzymatic mechanism. SAMe synthesis follows an SN 2 catalytic mechanism (13,14) in which the reaction is initiated through a nucleophilic attack by the sulfur atom of methionine on the C5′ atom of ATP, which produces the intermediate tripolyphosphate (PPPi).…”

Towards a systematic analysis of human short-chain dehydrogenases/reductases (SDR): Ligand identification and structure–activity relationships

“…Niflumic acid is a selective COX2 inhibitor and was selected as candidate for co-crystallization studies (see below). Another atypical member of the human SDR enzymes, MAT2B [19], yielded DSF hits like resveratrol (Table 1). MAT2B is an extended SDR member that functions as regulator of MAT2A (MATIIa), an enzyme important in S-adenosylmethionine synthesis.…”

Determination of Autosomal Dominant or Recessive Methionine Adenosyltransferase I/III Deficiencies Based on Clinical and Molecular Studies

“…Notably, the severe types of mutations, including nonsense, frameshift and splicing mutations, have been found exclusively in patients with the AR type (7,11,14,17,(21)(22)(23)(24). Because MAT I/III functions as a dimer, these types of mutations appear be p.Gly257Arg and p.Asp258Gly (7,(17)(18)(19)(20). It was suggested that a mutation in the other allele would exist; however, any mutation in other alleles was not identified by sequence analysis.…”