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Phosphorylases, Phosphatases
and Kinases
The enzymes that deal with
phosphate
Phosphate serves biochemical systems in a major
way. High energy phosphate compounds such as ATP,
PEP, 1,3 bisphosphoglycerate owe their energetic
properties to anhydride bonds between neighboring
phosphates or phosphate, oxygen or nitrogen atoms.
This tutorial will introduce you to enzymes that deal with
phosphate in a biosynthetic and regulatory capacity.
These enzymes specifically add or remove phosphate
groups.
Phosphorylase (fos-for-a-lace) The major function of this unique enzyme is to add a phosphate to glycogen and by doing so, forms glucose-1-PO 4. The reaction is classified as a phosphorylysis because of its similarity to a hydrolysis ( click 1 ). The only difference is a phosphate, not a water molecule is placed across the bond ( click 1 ). Click 1 to continue.
H-O-H^ Hydrolysis
O
CH 2 OH
OH
OH
O
CH 2 OH
OH
OH
O O O
O
CH 2 OH
OH
OH
O
CH 2 OH
OH
OH
O OH H O O
O
CH 2 OH
OH
OH
O
CH 2 OH
OH
OH
O O O
O-P-OH
O
O
H
O
CH 2 OH
OH
OH
O O-P-O
O
O
O
CH 2 OH
OH
OH
H^ O^ O
Phosphorylysis
Phosphatase (fos-fa-tace)
Phosphatases are hydrolases. This means these enzymes use a water molecule to remove a phosphate group from a substate. As an example, observe the action of glucose-6-phosphatase, a major enzyme that controls blood sugar. The substrate for enzyme is glucose-6-PO 4 ( click 1 ). A water molecule is used to displace the phosphate group from the molecule ( click 1 ). Phosphatase reactions are NOT reversible. As a guide to avoid confusion, remember that a phosphatase “takes off ”.
O
CH 2 OPO 3 =
OH
OH
OH
HO
+ H 2 O O
CH 2 OH
OH
OH
OH
HO
+ H PO 4 =
You should also recall that a special class of phosphatases remove phosphate groups from proteins. These so called “phosphoprotein phosphatases” control the activity of a large class of enzymes that require phosphate groups for activity. Click 1 to go on.
X
Kinases (ki-naces)
Kinases are enzymes that typically transfer the terminal phosphate group of ATP to an -OH group on a substrate ( click 1 ). This results in a phosphate-ester bond in the product. The reaction is not reversible. Although ATP is the major substrate ( click 1 ), occasionally GTP will be a phosphate group donor. Phosphate groups in ATP are coordinated with a Mg2+^ ion which strains the linkage between the gamma and beta phosphate and facilitates the breakage of this bond ( click 1 ). The reaction is favored by the release of free energy that accompanies bond breakage. Click 1 to go on.
Mg2+
-P-
O
O
HO
O-P-
O
O
O-CH 2 Adenine
HO
O
ATP
R-OH + ATP X R-OPO 3 =^ + ADP
Test you understanding
**1. Name a kinase enzyme in the gluconeogenic pathway that requires GTP as a phosphate donor?
- Why must the substrate for a pyrophosphorylase have a phosphate group on the molecule?
- To add a phosphate group to an unbound glucose molecule, you need ATP. To add a phosphate to a glucose molecule that is part of glycogen, you need only inorganic phosphate. Why?
- Why does a cell require one enzyme to add a phosphate group and another to take it off. Why 2 enzymes?**
PEP carboxykinase.
In order to attach the nucleotide, the enzyme must make a pyrophosphate linkage with the substrate. This requires a phosphate to be present.
To reverse a kinase, ATP must be synthesized. Phosphate ester bond energy does not supply sufficient energy.
Breaking the glycosidic bond provides the energy to drive the phosphorylation. Free glucose has no glycosidic bond
