Structural Biochemistry/Enzyme Regulation/Isozymes

In other words, Isozymes are enzymes that catalyze the same chemical reactions but have the different in amino acid sequences. They are displayed in different kinetic parameter and different regulatory properties.

Definition

Isozymes (also known as isoenzymes) are homologous enzymes that catalyze the same reaction but differ in structure. The differences in the isozymes allow them to regulate the same reaction at different places in the specie. In particular they differ in amino acid sequences. They display different kinetic parameters as well as regulatory properties. For example, isozymes have different K_M and V_max values, and can be distinguished from one another by biochemical properties such as electrophoretic mobility.

Isozymes are encoded by different genes and expressed in a distinct organelle or at a distinct stage of development. The purpose of isozymes is to allow fine adjustment of metabolism to meet the need of different development stages and help the different tissues and organs function properly depending on their physiology make up and in what kind of environment which they function. For example, the isoenzymes of lactate dehydrogenase in animal organs are different in term of their amino acid sequences and the level of their expression. The level of the different isozymes in a certain organ is related to the level of oxygen supply. Isozymes appear in specific regions of the body; differing in specifics organelles or tissues.

In terms of kinetics, isoenzymes have the capability to fine tune their enzymatic rate constants K_M and K_cat. This adaptation allows for the proper use of the enzyme based on its environment (e.g. lactate dehydrogenase isozymes present in the heart and in the liver, where O₂ is abundant in heart but not so in the liver).

Differentiating Isozymes

As mentioned above, isozymes are enzymes that have different structures but carry out the same tasks. A biochemical assay is needed to differentiate between different isozymes. Another method one could use is gel electrophoresis. This method takes advantage of the fact that isozymes have substituted amino acids and that provides a change in electrical charge of the enzyme. This difference in electrical charge between two different isozymes can be readily detected by gel electrophoresis. This provides a basis for molecular markers because these isozymes can easily be detected.

For identifying isozymes, a crude protein produced from grounded animal/plant tissue and buffer is used. The components of this protein is then extracted according to its electrical charge via electrophoresis. Since all the proteins from the tissue are present in the gel, an assay used to identify the individual enzymes by linking their functions to staining reactions. This method requires the enzyme to be active and functional after separating them via gel electrophoresis.

Applications

Isozymes in general can be used to meet the metabolic needs of different tissues and developmental stages. An example of an enzyme with different isozymes is lactate dehydrogenase (LDH). This enzyme is used to catalyze the synthesis of glucose in anaerobic metabolism of glucose. The isozymes of this enzyme are divided into two forms, the H isozyme and the M isozyme. The H isozyme is expressed more in the heart, whereas the M isozyme is expressed more frequently in the skeletal muscle. Both isozymes have two polypeptide chains, and each isozyme share 75% of the amino acid sequence for the chains. Both isozymes metabolize glucose, but the difference is that the H isozymes have a higher affinity for their substrates than the M isozyme does. Another difference is that the H isozyme functions better in aerobic environments such as the heart, whereas the M isozyme functions better in anaerobic environments such as the muscle, where strenuous activity may deplete the oxygen supplies. For example, when a rat heart is developing, the amount of H and M isozymes in the rat heart tissue begins to change because of the switch from an anaerobic environment to an aerobic one. This can be seen in figure A. This chart describes the rat heart's lactate dehydrogenase isozyme profile changes as the rat heart tissue develops. The H isozyme is shown as squares and the M isozyme is shown as circles. the negative numbers are the days before birth and the positive numbers are the days after birth. The amount of M isozymes decreases dramatically as the rat grows into the adult stage.

Isozymes may also be utilized to diagnose tissue damage such as damaged heart muscle cells during a heart attack or myocardial infarction. When heart muscle cells are damaged, they release the cellular material such as the H isozyme. When taking blood samples, if the H isozymes appear in increased levels, then there is a possibility that the heart cells are damaged.

Another example of an isozyme is hexokinase. The substrate is usually glucose and the product is glucose-6-phosphate. The six-carbon sugar is also known as a hexose. Glucokinase is one isozyme of hexokinase. A kinase is an enzyme which catalyzes the transfer of a phosphoryl group from NTP to NMP. ATP is often used in these types of reactions. Glucokinase is important in metabolism, and regulating carbohydrates in the human body. The difference of the Glucokinase enzyme is that it has a much lower affinity for glucose. Most Glucokinase activity is found in the liver. This is where it catalyzes the conversion of glucose to triglycerides.

References

Berg, Jeremy M. John L. Tymoczko. Lubert Stryer. Biochemistry Sixth Edition. New York: W.H. Freeman, and Company 2007.