M. M. Blanke
Fruit taste in terms of fruit quality is dominated by the sugar:acid ratio. Photosynthesis and carbon metabolism of fruit differ from that of leaves in that organic acids contribute to the carbon storage pool despite starch and sugar. Malic acid is the predominant organic acid in pome fruits, induces their acidic taste and is produced by a two-step enzymic reaction. It involves the synthesis of (1) the instable oxaloacetic acid (OAA) by phosphoenolpyruvate carboxylase (PEPC, EC or phosphoenolpyruvate carboxykinase (PCk; EC, and (2) malic acid by malic dehydrogenase (MDH). These enzymes were purified from apple fruit cvs 'Golden Delicious'and 'Cox's Orange Pippin' at Long Ashton, University of Bristol, England. This collaborative work has shown that the CO2 concentrating mechanism by PEPC and the kinetic properties of apple fruit PEPC, both resulting in malic acid synthesis, suggests the definition of a new type of 'fruit photosynthesis' rather than its categorisation within an existing type.

The activity of MDH extracted from apple fruit was ca. 80fold in excess of that of PEPC, leaving the regulation largely at the PEPC level. Regulation of malic acid synthesis is by glucose-6-phosphate stimulation and end product inhibition of PEPC by malic acid in a strongly pH-dependent manner. PEPC had a pH optimum at pH 7.8 and was salt-sensitive at concentrations above 160 mM salt. PEPC exhibited a Ki of between 10 μM at pH 6.8 and 46 mM L-malate at pH 8. This mechanism also provides fine regulation of cytoplasmic pH in pome fruit.

Blanke, M. M. (1998). FRUIT PHOTOSYNTHESIS AND POME FRUIT QUALITY. Acta Hortic. 466, 19-22
DOI: 10.17660/ActaHortic.1998.466.2
Malus domestica Borkh., malate dehydrogenase, phosphoenolpyruvate carboxykinase, phosphoenolpyruvate carboxylase, malic acid, sugar

Acta Horticulturae