Methionine synthase: Difference between revisions

[[Image:Overall.jpg]]

MS is a B12-dependent enzyme responsible for regenerating methionine from homocysteine. MS uses vitamin B12 Cobalamin as a cofactor. The change from homocysteine to methionine is an SN2 reaction where the methyl group on N-5 from 5-me THF is donated to Cob(I)alamin forming methylcobalamin (or Me-Cob(III)alamin). This is a complex reaction as THF, a product, is a poor leaving group and requires the "super nucleophile", Cob(I)alamin, to carry out the reaction<ref>DOI:10.1146/annurev.biochem.72.121801.161828</ref><ref name="Kung et al">DOI: 10.1038/nature10916</ref> as a methyl carrier.

5-me THF is a product of [[methylenetetrahydrofolate reductase]] ([[MTHFR]]) from the folate cycle.

[[Image: MS_cycle.gif]]

'''Cob(II)alamin''': Cob(I)alamin is highly reactive towards oxygen so occasionally under aerobic conditions, Cob(I)alamin will undergo oxidation leading to an inactive Cob(II)alamin enzyme in the +2 oxidation state. This is regulated by reductive methylation by using Flavodoxin as an electron donor to reactivate Cob(I)alamin, and subsequently regenerates Me-Cob(III)alamin with a methyl being donated from SAM<ref>DOI:10.1073/pnas.1133218100</ref>.

MS is an important enzyme responsible for generating methionine, required by our bodies for healthy cell and tissue growth, and protein synthesis. Any MS and/or B12 deficiencies can result in diseases such as abnormal birth defects or anemia<ref name="Kung et al"/>.

<StructureSection load='' size='400' side='right' scene='90/907471/Superposition_1/3'>

Methionine synthase contains four domains, each with a unique function that bind to Cob(I)alamin as the methyl carrier. In the N-terminal, 5-me THF donates a methyl in the catalytic cycle to Cob(I)alamin, which then donates it to homocysteine to form methionine. However, every 2,000 cycles or so, Cob(I)alamin becomes oxidized (as shown below in the darker yellow color) and now requires reduction and remethylation triggering the reactivation cycle. In the C-terminal, S-adenosylmethionine or SAM donates methyl with Flavodoxin as the electron donor<ref name="Bandarian et al">DOI: 10.1038/nsb738</ref>.

During each cycle, the domains must be positioned close enough to Cobalamin in order for methyl transfers to be successful. Conformations of MS allows substrates to be presented to Cobalamin for reactions to occur.

[[Image:cob_1_alamin.jpeg|300px]]

DMB, in the lower ligand of the <scene name='90/907471/Bindingdomain2/2'>B12 binding domain</scene> is displaced from the Cobalt by a Histidine residue to be 'uncapped' to form Me-Cob(III)alamin<ref>DOI:10.1146/annurev.biochem.72.121801.161828</ref>.

When B12 is not engaged with one of the other three substrate binding domains, it is protected by a <scene name='90/907471/Cap/1'>cap</scene>.

</jmol>

Every 2000 or so cycles, cobalamin needs to be <scene name='90/907471/B12_activation_w_sah/1'>reactivated</scene> through methylation by S-adenosyl methionine (SAM). To determine the structure of the reactivation conformation, the mutant H759G was used. This mutation maximises the fraction of enzyme with the B12 domain in the cap-off conformation bound to the activation domain. The approach of the B12 domain and the activation domain has to be carefully regulated because methylating homocysteine with methyl groups from S-adenosyl methionine results in a futile cycle. Thus, this step should be reserved to rescue B12 out of the +2 cobalt oxidation state, and then methylation of homocysteine using a methyl group from 5-me THF resumes.

</StructureSection>

Many thanks to Dr. Theis, Anna, Mike, and Shaylie for their enthusiasm and assistance on creation of 3D structural images of the B12 domain for MS.

Also thanks to Dr. Drennan for taking the time to review the page and providing helpful suggestions for improvement.

<references/>

[[Category:One-carbon metabolism]]