L-methionine biosynthesis (from homoserine)

De novo biosynthesis of L-methionine from L-homoserine through three ordered stages: homoserine activation, sulfur incorporation to form L-homocysteine, and methylation of L-homocysteine. Bacteria can activate homoserine with an acetyl or succinyl group, incorporate sulfur through two-step trans-sulfuration or one-step direct sulfhydrylation, and complete the pathway with a cobalamin-independent or cobalamin-dependent methionine synthase. Substrate-matched alternatives are retained explicitly because enzyme-family membership alone does not reliably distinguish acetyl from succinyl chemistry or forward biosynthetic from related PLP-dependent lyase reactions.

The boundary begins with L-homoserine and ends with L-methionine. Upstream aspartate-family synthesis of homoserine, sulfur assimilation, cobalamin supply, methionine salvage, the methionine/SAM cycle, and transcriptional or riboswitch regulation are outside the module. PANTHER terms orient exact exemplars but do not override experimentally established substrate specificity. Family identifiers are omitted where the locally available family does not distinguish the leaf chemistry. No PTN is asserted because an appropriate ancestral function-bearing node has not been established locally for every route.