The following vitamin acts as a cofactor for glycogen phosphorylase:

Rate limiting enzyme of glycogenolysis is glycogen Phosphorylase which requires PLP i.e. vitamin B6. This vitamin is required as donor of phosphate as Glycogen phosphorylase transfers phosphate to glucose, releasing glucose-1-phosphate. Pyridoxal phosphate is also cofactor for transamination and decarboxylation reaction. Here is a difference to be kept in mind. Transamination Decarboxylation Aldehyde group of pyridoxal phosphate (PLP) plays a role Phosphate group of PLP plays a role