Question

When animals synthesize Met, they typically use Asp as the starting point. Since they cannot incorporate inorganic sulfur, they use Cys as the source of sulfur, going by way of cystathionine. Finally, they use Serine as the source of the methyl group via folate metabolism.

Calculate the the number of ATP consumed (or produced) to drive this overall conversion:   

Asp + Cys + Ser →  Met + Gly + pyruvate + NH₃

Assume that synthesis of cystathionine from homoserine and Cys requires prior activation of the homoserine (e.g., by succinylation),

Answer 1

•The biosynthesis of methionine is started from the aspartate amino acids.

•Aspartate is synthesized by oxaloacetate [ an important Krebs cycle intermediate] through a transamination from glutamate.

•See photo – a chain of reaction from Aspartate to cysteine to methionine. In this process only one step consume the ATP [aspartate to aspartyl- β- phosphate].

•Now let us focus on oxaloacetate , which is a precursor for amino acid synthesis .

•The oxaloacetate is formed during kreb’s cycle. Here is no consumption of ATP.

•Kreb’s cycle start after glycolysis . In glycolysis total 2ATP molecules are consumed .

•So total three ATP molecules are consumed in the given reaction.