The glycine decarboxylase system in higher plant mitochondria: structure, function and biogenesis.

A long period of drought leads to stomata1 closure, thus depriving most of the green cells of CO,. Under these conditions glycolate is formed rapidly inside the chloroplasts, via phosphoglycolate, from ribulose 1,s-bisphosphate by the oxygenase reaction of ribulose1,s-bisphosphate carboxylase/ oxygenase [ l , 21. The resulting glycolate is transported out of the chloroplasts to be metabolized through the oxidative photosynthetic carbon cycle (C, cycle). In the peroxisomes, glycolate is oxidized by the flavoprotein glycolate oxidase, producing glyoxylate which is transaminated to glycine. Glycine, in turn, migrates to the mitochondria where it is converted to serine, NH, and CO,. Serine then returns to the peroxisomes where, via the action of a transaminase and hydroxypyruvate reductase, glycerate is formed. Finally, glycerate reenters the chloroplast where it is phosphorylated by glycerate kinase to give glycerate 3-phosphate, to regenerate ribulose 1,s-bisphosphate after reduction to triose-phosphate via the Calvin-Benson cycle (C, cycle) [I, 21. During photorespiration in higher plants, no net synthesis of storage carbohydrates occurs; the light energy is used to drive the utilization (through the C, cycle) and the formation (through the C, cycle) of ribulose l,S-bisphosphate, thus preventing the formation of the excited triplet state of chlorophyll and excess reactive 0, species (superoxide radicals and singlet oxygen). Generally, the reactions of the C2 cycle are not believed to be regulated after the site of carbon entry (phosphoglycolate synthesis by ribulose-1,s-bisphosphate carboxylase/oxygenase) into the pathway. The conversion of glycine to serine in green-leaf mitochondria is currently considered to be the major source of CO, released during photorespiration [ 1, 21. In this short review, we will consider only the mechanisms of glycine oxidation in green-leaf mitochondria.