Glycogen phosphorylase (R-state). Computer graphic of one molecule of glycogen phosphorylase, an enzyme involved in breaking down glycogen. The action of glycogen phosphorylase provides fuel for muscle contraction; in the liver this enzyme maintains blood glucose balance. Here, the R- or active" state of this enzyme is seen. It is a protein made up of two polypeptide subunits (blue and green). Thus this dimer molecule has a two fold axis. When active, N- terminal tails (white strands) twist and lie at centre, and pink/orange parts of the molecule alter. The active site of the enzyme is inside the green subunit."