Plastoquinol-9 biosynthesis is a pathway that begins in the cytosol and endoplasmic reticulum and ends in the chloroplast by which L-tyrosine and geranylgeranyl diphosphate become plastoquinol-9, ubiquinone analogs and benzoquinone electron carriers. The subpathway that synthesizes homogentisate from L-tryptophan occurs in the cytosol. First, tryptophan aminotransferase uses a pyridoxal 5'-phosphate as a cofactor to convert L-tryptophan into 4-hydroxyphenylpyruvate. Second, 4-hydroxyphenylpyruvate dioxygenase uses Fe2+ as a cofactor to convert 4-hydroxyphenylpyruvate into homogentisate. The subpathway that synthesizes solanesyl diphosphate from geranylgeranyl diphosphate occurs in the endoplasmic reticulum and the single reaction is catalyzed by solanesyl diphosphate which requires a magnesium ion as a cofactor. Solanesyl diphosphate must then be transported out of the endoplasmic reticulum into the cytosol by a yet undiscovered solanesyl diphosphate transporter. The last two reactions are localized to the chloroplast inner membrane (coloured dark green in the image). First, homogentisate solanesyltransferase catalyzes the conversion of solanesyl diphosphate and homogentisate into 2-methyl-6-solanesyl-1,4-benzoquinol, requiring magnesium ion as a cofactor. Second, 2-methyl-6-phytyl-1,4-hydroquinone methyltransferase catalyzes the conversion of 2-methyl-6-solanesyl-1,4-benzoquinol into plastoquinol-9.

Plastoquinol-9 Biosynthesis References