In the first step of sarcosine reductase, 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. [thioredoxin]-disulfide + acetyl phosphate + H2O + methylamine = [thioredoxin]-dithiol + H(+) + phosphate + sarcosine Heterotetramer of two alpha and two beta subunits. Component of the sarcosine reductase complex, together with components A and C. PB is substrate specific. Belongs to the GrdB/GrdF/GrdH family. Truncated C-terminus.