Maltose metabolism by pea chloroplasts.

Area of Science:

• Plant Biochemistry
• Chloroplast Metabolism
• Carbohydrate Chemistry

Background:

• Maltose is a disaccharide formed from glucose units.
• Its formation during starch breakdown in plants is not fully understood.
• Chloroplasts play a crucial role in plant carbohydrate metabolism.

Purpose of the Study:

• To investigate the origin of maltose produced during starch degradation in Pisum sativum chloroplasts.
• To determine the enzymatic activities responsible for maltose synthesis in pea leaves and chloroplasts.
• To elucidate the role of specific enzymes in maltose formation during starch breakdown in the dark.

Main Methods:

• Assessed catalytic activities of maltose phosphorylase and maltase in pea leaf extracts.
• Fractionated pea leaves to localize enzyme activities within chloroplasts.
• Incubated isolated chloroplasts with starch in the dark, with and without phosphate, and with [(14)C]glucose to track metabolic pathways.

Main Results:

• Maltose phosphorylase and maltase activities in pea leaves are relatively low.
• Pea chloroplasts lack maltase but possess sufficient maltose phosphorylase activity.
• Starch breakdown and maltose accumulation were reduced without exogenous phosphate; [(14)C]glucose labeling confirmed its incorporation into maltose.

Conclusions:

• Maltose phosphorylase, utilizing glucose-1-phosphate derived from starch by α-glucan phosphorylase, is likely responsible for maltose synthesis in pea chloroplasts.
• Maltase does not appear to contribute to maltose formation within pea chloroplasts.
• This study clarifies a specific pathway for maltose production during starch metabolism in plant chloroplasts.