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-PO4. 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.

HydrolysisH-O-H

OCH2OH

OH

OHO

CH2OH

OH

OHOO O

OCH2OH

OH

OHO

CH2OH

OH

OHOHO OHO

OCH2OH

OH

OHO

CH2OH

OH

OHOO O

O-P-OH

O

O

H

OCH2OH

OH

OHO O-P-O

O

O

OCH2OH

OH

OHO OH

Phosphorylysis

Pyrophosphorylases(pyro-fos-for-a-laces)

These enzymes use nucleotide triphosphates (CTP, UTP, ATP, etc) to make products with a pyrophosphate linkage. P~P is split out during the reaction. Recall, a pyrophosphorylase activates glucose by making UDP-glucose from glucose-1-PO4 and UTP (click 1). Note two features of the reaction: (1) the formation of a pyrophosphate group in the product (click 1), and (2) the splitting out of a P~P (click 1). The P~P is cleaved by an ever-present pyrophosphatase yielding energy to drive the reaction (click 1). Pyrophosphorylases only work when a phosphate is already in position on the molecule (click 1). Click 1 to go on.

OCH2OH

OH

OPO3=

OH

HO

+ UTPO

CH2OH

OH

OH

HO

O-P~O-P-O-ribose

O O

O O

uracil

+O O

O OO-P~O-P-O

pyrophosphatase2 HPO4

=

H2O

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-PO4 (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 ”.

OCH2OPO3

=

OH

OHOH

HO

+ H2OO

CH2OH

OH

OHOH

HO

+ HPO4=

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

OO-P-O

O

O

HO

O-P-

O

OO-CH2

Adenine

HO

O

ATP

R-OH + ATP R-OPO3= + ADPX

Summary of what have you learned?

Enzymes that transfer phosphate groups to substrates, relying primarily on ATP as the phosphate donor, are call “kinases”.

A special type of enzyme has the capacity to add inorganic phosphate groups across the glycosidic bonds of glycogen. This enzyme is called “phosphorylase”. A phosphorylase can literally cleave the glycosidic bond with a molecule of phosphate.

Enzymes that use nucleotide triphosphates to transfer a nucleotide to a molecule, splitting out a pyrophosphate group in the transfer are call “pyrophosphorylases”.

Enzymes that remove phosphate from the molecule are called “phosphatases”. (click 1 to go on)

Test you understanding1. Name a kinase enzyme in the gluconeogenic pathway that requires GTP as a phosphate donor?

2. Why must the substrate for a pyrophosphorylase have a phosphate group on the molecule?

3. 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?

4. 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 pyrophosphatelinkage 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