[35] α-Tocopherol and plastoquinone synthesis in chloroplast membranes

Publisher Summary This chapter discusses α-tocopherol and plastoquinone synthesis in chloroplast membranes. The synthesis of α-tocopherol comprises a number of reaction steps, catalyzed by the following enzymes: homogentisate decarboxylase-phytyltransferase, S-adenosylmethionine, methyl-6-phytylquinol methyltransferase, 2,3-dimethylphytylquinolcyclase, S-adenosylmethionine, γ-tocopherol methyltransferase (no EC numbers available). The intermediates obtained in tocopherol synthesis are generally purified by repeated thin-layer chromatography (TLC) or high-pressure liquid chromatography (HPLC). Plastoquinone synthesis occurs essentially through reactions similar for tocopherol; homogentisate (20 μM) and solanesyl diphosphate (80 μM) are condensed to form the equivalent to 1 5 (2-methyl-6-solanesylquinol), which is then methylated by S-adenosylmethionine (70 μM) to yield plastoquinol-9 5 (2,3-dimethylsolanesylquinol). There are various methods for the fractionation and purification of chloroplast components enabled to localize all, but one enzyme in tocopherol and plastoquinone synthesis at spinach chloroplast envelopes. Thylakoids or soluble chloroplast protein had no enzymatic activity. Recombination of membranes with soluble chloroplast extract did not increase prenylquinone synthesis.