The Mechanism of End Product Inhibition of Serine Biosynthesis

A molecular weight of 163,000 daltons was determined by equilibrium sedimentation analysis for crystalline 3-phosphoglycerate dehydrogenase. The enzymatically active enzyme had a s20, w of 7.7 and treatment at alkaline pH values or with silver ions inactivated the enzyme and reduced the s20, w to 3.0. The decrease in sedimentation coefficient indicated dissociation of the enzyme and sedimentation equilibrium measurements made in 3.75 m guanidine-HCl showed that the enzyme was composed of subunits with a molecular weight between 40 and 50,000 daltons. The amino acid composition of the enzyme was determined. A p-hydroxymercuribenzoate titration showed approximately 12 readily available sulfhydryl groups and the fluorometric determination of DPN showed that the enzyme crystallized with 2 moles of DPNH bound per mole of enzyme. The intensity of the peak at 330 and 430 mµ in the fluorescence emission spectrum was altered in a characteristic way by changes in pH. Changes in the fluorescence spectrum were also observed when the enzyme was treated with silver ions or p-hydroxymercuribenzoate.