 The Figure displays the SDH active site, PLP coenzyme and substrate.According to the series of assays performed by Cleland (1967), the linear rate of pyruvate formation at various concentrations of inhibitors demonstrated that L-cysteine and D-serine competitively inhibit the enzyme SDH.The reason that SDH activity is inhibited by L-cysteine is because an inorganic sulfur is created from L-Cysteine via Cystine Desulfrase and sulfur-containing groups are known to promote inhibition.