In the first step of glycine, betaine and sarcosine reductases, the substrate is bound to component PB via a Schiff base intermediate. Then the PB-activated substrate is nucleophilically attacked by the selenol anion of component PA to transform it to a carboxymethylated selenoether and the respective amine. By action of component PC, acetyl phosphate is formed, leaving component PA in its oxidized state. Finally component PA becomes reduced by the thioredoxin system to start a new catalytic cycle of reductive deamination (By similarity). [thioredoxin]-disulfide + acetyl phosphate + H2O + NH4(+) = [thioredoxin]-dithiol + glycine + H(+) + phosphate [thioredoxin]-disulfide + acetyl phosphate + H2O + methylamine = [thioredoxin]-dithiol + H(+) + phosphate + sarcosine [thioredoxin]-disulfide + acetyl phosphate + H2O + trimethylamine = [thioredoxin]-dithiol + glycine betaine + H(+) + phosphate Monomer. Component of the glycine, sarcosine and betaine reductase complexes, together with components B and C (By similarity). Belongs to the GrdA family.