In mammals there are four hexokinase isozymes with different tissue distributions. Because the isozymes have different kinetic features, this has permitted them to become optimally adapted to the needs of the specific tissue in which they are the major isozyme. Isozyme IV (liver) has a subunit Mr of 52 kDa, and has a poor affinity for glucose, but shows positive cooperativity for binding glucose, and this enables it to act as a glucose sensor. This positive cooperativity is completely due to kinetic affects. Isozymes I (brain), II (muscle) and III (erythrocytes) with an Mr of 100 kDa, are the result of gene duplication plus fusion of the smaller form, which gives them the potential to have two binding sites per subunit for each ligand. Evolved changes in such similar sites enable isozymes I and II to both have negative cooperativity for the allosteric inhibitor glucose- 6-P, while isozyme II has also developed negative cooperativity for the substrate ATP.
KeywordsPositive Cooperativity Negative Cooperativity Monomeric Enzyme Poor Affinity Subunit Size
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