Biochemical Foundations of Human Vital Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemical Foundations of Human Vital Activity
Enzymes: Biological Catalysts
Multiple Forms of Enzymes

Certain complex Enzymes with a quaternary (subunit) Structure are characterized by the presence of Multiple molecular forms within Tissues. While these enzymatic forms may differ in Structure and properties, they catalyze the exact same reaction. A distinction is made between Isoenzymes and multiple enzyme forms.

Isoenzymes are defined as enzymatic forms that differ in their Primary Cell/13.html">Protein Structure and catalytic activity. The existence of isoenzymes is typical for Lactate dehydrogenase, creatine kinase, hexokinase, and other tissue enzymes. For instance, lactate dehydrogenase—which catalyzes the reversible conversion of lactate (lactic acid) into pyruvic acid—occurs in The Human Body in five distinct isoenzymes, conventionally designated as M₄, H₄, М₃Н, М₂Н₂, and МН₃. All five isoforms share the same molecular weight (approximately 134,000) but exhibit varying catalytic activities, alongside a distinct tissue Specificity. Skeletal Muscle is dominated by the M₄ type (muscle), cardiac muscle by the H₄ type (Heart), while other tissues feature combinations of these types. Alterations in the body's functional state, environmental conditions, or age can lead to a redistribution of individual enzyme isoforms among various tissues, signaling shifts in metabolic processes or The Emergence of new isoforms. Changes in the isoform spectrum or the appearance of novel enzyme forms can be utilized for disease Diagnosis or for characterizing the functional state of the Organism—including its adaptation to physical exertion, as systematic training may induce The formation of new enzyme forms.

Multiple enzyme forms arise within tissues due to modifications in the Tertiary Structure of a single enzyme under METABOLISM/18.html">The Influence of various environmental factors, which in turn affects its catalytic activity. Consequently, these forms are not genetically fixed. The emergence of multiple enzyme forms serves as an important regulatory mechanism for adapting metabolic processes to changing environmental conditions.