This document discusses the regulation of enzymes through covalent modification, with a focus on phosphorylation. It states that phosphorylation is the most common form of covalent modification, occurring in 30% of eukaryotic proteins. Phosphorylation involves the addition of phosphoryl groups to amino acids by protein kinases and removal by protein phosphatases. This effectively regulates enzyme activity and structure. A key example is regulation of glycogen phosphorylase in muscle and liver through interconversion between active phosphorylated form A and less active form B by phosphorylation and dephosphorylation. Glycogen phosphorylase breakdown is regulated by controlling the ratio of these two forms.

2. INTRODUCTION
Many proteins and enzymes like regulatory
enzymes can be regulated via covalent
modification.
Covalent Modification involves the addition of
some sort of functional group onto the enzyme by
another molecule.
Over 500 different types of covalent modification
have been found in proteins.
Common modifying groups include
phosphoryl,acety,adenylyl,methyl,amide,carboxyl,h
yd-roxyl,sulphate etc. There are even entire
proteins that are used as specialized modifying

3. Phosphorylation is the most common and important means
of regulatory modification and covalent modification . It is
estimated that 30% of all the proteins in a eukaryotic cell
are phosphorylated.
Target
residues

4. Why is phosphorylation such a common mechanism of
regulation???
• The reason has to do with the fact that it’s a highly
effective means of regulation.

5. Phosphoryl groups affect the structure and
catalytic activity of enzymes
Protein kinases
• The attachment of
phosphoryl groups to
specific amino acid
residues of a protein is
catalyzed by protein
kinases.
Protein phosphatases
• Removal of phosphoryl
group from specific amino
acid is catalyzed by protein
phosphatases.
An important example of enzyme regulation by phosphorylation is seen in
glycogen phosphorylase (Mr 94,500) of muscle and liver , which catalyzes
the reaction:

7.  The glucose-1-phosphate so formed can be used for
the synthesis of ATP in muscle or converted to free
glucose in the liver.
 Glycogen phosphorylase occurs in 2 forms:
The more active phosphorylase a
The less active phosphorylase b
 Phosphorylase a has 2 subunits , each with a specific
Ser residue that is phosphorylated at its hydroxyl
group. Phosphoprotein phosphatase 1
phosphorylase kinase

9. SOME IMPORTANTPOINTS
• Glycogen phosphorylase contains pyridoxal phosphate as a prosthetic
group.
• The breakdown of glycogen in skeletal muscles and the liver is
regulated by variations in the ratio of the 2 forms of glycogen
phosphorylase.
• The a and b forms differ in their secondary , tertiary and quaternary
structures; the active site undergoes changes in structure and
consequently , changes in catalytic activity as the 2 forms are
interconverted.
• The activity of glycogen phosphorylase in muscle is subjected to a
multilevel system of regulation , involving covalent
modification(phosphorylation), allosteric regulation and a regulatory
cascade sensitive to hormonal status that acts on the enzymes
involved in phosphorylation and dephosphorylation.

10. Regulation of muscle glycogen
phosphorylase activity by multiple
mechanisms

11. • Terms expansion:
• PPI-1 = Phosphoprotein phosphatase inhibitor 1
• PKA = Protein kinase A
• PP2B = Phosphoprotein phosphatase 2 B
• PP1 = phosphoprotein phosphatase 1

12. Thanks for your kind patience……