Question

In muscle tissue, the ratio of phosphorylase a to phosphorylase b determines the rate of conversion of glycogen to glucose 1‑phosphate. Classify how each event affects the rate of glycogen breakdown in isolated muscle tissue. The classifications are Increased rate, Decreased rate, No change.

Answer Bank: AMP allosteric binding activities phosphorylase b, addition of a phosphate inhibitor, addition of phosphorylase b kinase, removal of phosphate groups from phosphorylase a, end of fight-or-flight response, ATP exceeds AMP, treatment with insulin, treatment with glucagon.

Answer 1

Glycogen phosphorylase exists in two forms, unphosphorylated phosphorylase b and phosphorylase a with two Ser-PO4 2– groups. Protein phosphate is added by phosphorylase b kinase in the PKA driven cascade already described. Phosphate is removed from serine in phosphorylase b kinase as well as from phosphorylase a by phosphoprotein phosphatase 1.Glycogen phosphorylase is a dimer of two identical subunits, with a slight difference in the liver and muscle versions. The enzyme is under allosteric regulation, with T and R states, as well as regulation by phosphorylation . This makes the regulatory scheme two dimensional, with phosphorylation on the horizontal axis and allosteric regulation on the vertical axis

The key difference between phosphorylase a and phosphorylase b is that the a form (Ser-PO4 2–) is biased in favour of active R state, whereas the b form (Ser-OH) is biased in favour of T-state.

Phosphorylase b is sensitive to negative allosteric effectors ATP (high energy level means less need to breakdown glycogen) and Glucose-6-phosphate (when present, there's less need to continue glycogen breakdown). AMP is a positive effector for muscle glycogen phosphorylase, overriding the negative effect of ATP. AMP levels rise in active muscle, signalling need for more glucose release. Liver tends to conserve the glycogen reserve if energy is low (AMP high), and rely on other energy substrates such as amino acids.

The liver form of phosphorylase a is sensitive to glucose as a negative effector. This is to conserve glycogen if other glucose sources are currently available. The phosphorylation control of glycogen phosphorylase is a response to messages from outside the cell, signalled by hormone. Allosteric control is a response to conditions inside the cell signalled by molecules that would be present in different energy states of the cell.When blood-glucose levels are high, insulin stimulates the synthesis of glycogen by triggering a pathway that activates protein phosphatase 1

PP1(Protein Phosphatase 1) inactivates phosphorylase kinase and phosphorylase a by dephosphorylating these enzymes.

• Phospholy lase b o Phostolytseb kiny ATP ADP Phoshoylase a - 2x Selon 2x ser-fou (sel 14) • T state (Inactive) Pi a no To state (Inactive) - Prissha Peotein Phosphatase 7 11 the -ve effecta glucose e livel) Phosphorylare - a -ve effections 2x sel-Par Calc-off R-state Cactive) Prestolylase be 2x selon R-state Cactive IV AMP

Protein phosphatase 1 will be continually active. Hence, the level of phosphorylase b will be higher than normal, and glycogen will be less readily degraded.

Protein phosphatase 1 will be much less effective in dephosphorylating glycogen synthase and glycogen phosphorylase. Consequently, the synthase will stay in the less active b form, and the phosphorylase will stay in the more active a form. Both changes will lead to increased degradation of glycogen.