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CENTRAL CARBON → ASPARTATE / SULFUR FAMILY → INDIVIDUAL PATHWAY

Methionine biosynthesis

Follow the carbon skeleton from L-Aspartate to L-Methionine.

What it needs

Aspartate-family precursors, activated acyl donor, sulfur and a methyl donor.

What it supplies

L-Methionine. This product can enter downstream cellular reactions; its synthesis depends on the precursor and helper supplies shown.

The pathway

Read from top to bottom. Select an enzyme box to jump to its reaction. Cosubstrates and selected coproducts appear in the account below.

Methionine biosynthesisL-Aspartate to L-Aspartyl-4-phosphate via Aspartate kinase; L-Aspartyl-4-phosphate to L-Aspartate-4-semialdehyde via Aspartate-semialdehyde dehydrogenase; L-Aspartate-4-semialdehyde to L-Homoserine via Homoserine dehydrogenase; L-Homoserine to O-Succinyl-L-homoserine via Homoserine O-succinyltransferase (MetA); O-Succinyl-L-homoserine + cysteine to L-Cystathionine via Cystathionine γ-synthase (MetB); L-Cystathionine to L-Homocysteine via Cystathionine β-lyase (MetC); L-Homocysteine to L-Methionine via Methionine synthase (MetE or MetH)L-Aspartate1. Aspartate kinaseATP → ADPRead reaction 1L-Aspartyl-4-phosphate2. Aspartate-semialdehyde dehydrogenaseNADPH → NADP⁺Read reaction 2L-Aspartate-4-semialdehyde3. Homoserine dehydrogenaseReducing power is consumedRead reaction 3L-Homoserine4. Homoserine O-succinyltransferase (MetA)Succinyl-CoA → CoARead reaction 4O-Succinyl-L-homoserine5. Cystathionine γ-synthase (MetB)Read reaction 5L-Cystathionine6. Cystathionine β-lyase (MetC)Pyruvate and ammonia releasedRead reaction 6L-Homocysteine7. Methionine synthase (MetE or MetH)5-Methyl-THF supplies a methyl groupRead reaction 7L-Methionine

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Reactions and enzymes

1 · Aspartate kinase

L-Aspartate → L-Aspartyl-4-phosphate

Aspartate kinase · EC 2.7.2.4

Inputs, helpers or coproducts: ATP → ADP

2 · Aspartate-semialdehyde dehydrogenase

L-Aspartyl-4-phosphate → L-Aspartate-4-semialdehyde

Aspartate-semialdehyde dehydrogenase · EC 1.2.1.11

Inputs, helpers or coproducts: NADPH → NADP⁺

3 · Homoserine dehydrogenase

L-Aspartate-4-semialdehyde → L-Homoserine

Homoserine dehydrogenase · EC 1.1.1.3

Inputs, helpers or coproducts: Reducing power is consumed

4 · Homoserine O-succinyltransferase (MetA)

L-Homoserine → O-Succinyl-L-homoserine

Homoserine O-succinyltransferase (MetA) · EC 2.3.1.46

Inputs, helpers or coproducts: Succinyl-CoA → CoA

5 · Cystathionine γ-synthase (MetB)

O-Succinyl-L-homoserine + cysteine → L-Cystathionine

Cystathionine γ-synthase (MetB) · EC 2.5.1.48

6 · Cystathionine β-lyase (MetC)

L-Cystathionine → L-Homocysteine

Cystathionine β-lyase (MetC) · EC 4.4.1.8

Inputs, helpers or coproducts: Pyruvate and ammonia released

7 · Methionine synthase (MetE or MetH)

L-Homocysteine → L-Methionine

Methionine synthase (MetE or MetH)

Inputs, helpers or coproducts: 5-Methyl-THF supplies a methyl group

MetH is cobalamin-dependent; MetE is an alternative enzyme.

Connected pathways

Threonine biosynthesisCysteine biosynthesisAspartate from oxaloacetate

Evidence and scope

The illustrated succinylhomoserine / transsulfuration route is one bacterial option. Acetylhomoserine and direct-sulfhydrylation routes are alternatives. This page explains selected documented biochemical reactions. It does not establish that every displayed enzyme occurs in Aquifex, that the pathway is universally essential, or that the book image records a complete organism. Gene symbols name common enzyme activities; organism-specific protein records require separate matching.

Original explanatory diagram and reaction account, based on the records below. Arrows show the biosynthetic reading direction; they do not claim that every reaction is irreversible. Selected protons, water and cofactors are omitted from the overview and are not implied absent.

Compare the related Aquifex annotation group →