Catalyzes the reversible phosphorolysis of glucosylglycerate into alpha-D-glucose 1-phosphate (Glc1P) and D-glycerate. May be a regulator of intracellular levels of glucosylglycerate, a compatible solute that primarily protects organisms facing salt stress and very specific nutritional constraints. Has a very strict substrate specificity. Cannot catalyze the phosphorolysis of sucrose or synthesize sucrose from Glc1P and D-fructose. (2R)-2-O-(alpha-D-glucopyranosyl)-glycerate + phosphate = (R)-glycerate + alpha-D-glucose 1-phosphate kcat is 0.83 sec(-1) towards 2-O-(alpha-D-glucopyranosyl)-D-glycerate for the phosphorolytic direction (at pH 6.0 and 42 degrees Celsius). kcat is 0.98 sec(-1) towards phosphate for the phosphorolytic direction (at pH 6.0 and 42 degrees Celsius). kcat is 96 sec(-1) towards alpha-D-glucose 1-phosphate for the synthetic direction (at pH 6.5 and 42 degrees Celsius). kcat is 128 sec(-1) towards D-glycerate for the synthetic direction (at pH 6.5 and 42 degrees Celsius). Optimum pH is 6 and 6.5 in phosphorolytic and synthetic directions, respectively. Optimum temperature is 42 degrees Celsius. Retains full activity after incubating at up to 55 degrees Celsius for 10 minutes, but only 35% is left after incubating at 60 degrees Celsius. The excellent specific activity of this enzyme towards glycerate opens up new options for the commercial production of glucosylglycerate. This compound has been reported to be an exceptionally potent and versatile stabilizer of proteins at elevated temperatures. Belongs to the glycosyl hydrolase 13 family. Glucosylglycerate phosphorylase subfamily.