Catalyzes the reversible phosphorolysis of cellobionic acid (4-O-beta-D-glucopyranosyl-D-gluconate), a probable step in cellulose degradation. May be part of a metabolic pathway where cellobionic acid is converted into alpha-D-glucose 1-phosphate and D-gluconic acid to enter glycolysis and the pentose phosphate pathway, respectively. Produces 4-O-beta-D-glucopyranosyl-D-glucuronate from alpha-D-glucose 1-phosphate and D-glucuronate with low activity in the synthetic direction. 4-O-beta-D-glucopyranosyl-D-gluconate + phosphate = alpha-D-glucose 1-phosphate + D-gluconate kcat is 173 sec(-1) for D-gluconate. kcat is 43 sec(-1) for D-glucuronate. kcat is 127 sec(-1) for alpha-D-glucose 1-phosphate. Glycan metabolism; cellulose degradation. Homodimer. Belongs to the glycosyl hydrolase 94 family. Cellobionic acid phosphorylase subfamily.