Understanding reaction mechanism of Mycobacterium tuberculosis Cyclopropane methyltranferases (CMAs)

LUCAS A. DEFELIPE; ADRIAN GUSTAVO TURJANSKI; FEDERICO OSMAN

Buenos Aires

Congreso; ISCB Latin America; 2016

Asociación argentina de bioinformática y biología computacional

A serious concern in Mycobacterium tuberculosis has been the growth of resistance to conventional treatments. In this context that CMAs, a family of methyltranferases in Mycobacterium tuberculosis, shows up. They are responsible for modulating the flow of mycolic acids, the major cell wall´s constituents. The most frequent modifications in mycolic acids are cyclopropane groups, methyl ether, methyl alcohols and methyl olefins. A two-steps mechanism for CMAs has been proposed: first, a common transfer of a methyl group by the SAM (S-Adenosyl-L-methionine) that results on a methyl carbocation. Then, a second step takes place, with a different path for each protein. In this work we used QM/MM methods to study the mechanism of mmaA1, mmaA3 and mmaA4, the responsible for generating methyl olefins, methyl ethers and methyl alcohols, respectively. For mmaA1 and mmaA4, we obtained a 12 kcal/mol barrier for the first step. The transition step consisted of a flat methyl, resembling the mechanism of an SN2 attack. We then examined that the variability in some basics residues resulted on different modifications on the mycolic acids. For example, in mmaA1 a glutamate (which is conserved in mmaA4) was mutated for an aspartate, allowing the desprotonation of the carbocation, instead of a water molecule. We also found that in mmaA3, the structuring of the loop N-terminal didn´t allow to have water molecules in the catalytic site, increasing the basicity of the glutamate.