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Muscle Hexokinase has a lower Km for glucose than live Hexokinase They both work best on Glucose, and convert
Glc->Glc-6-P04. Would you expect their 3D shape to be identical? If not, how different would you expect them to be? Explain your answer.


  • Biochemistry -

    someones taking Dr. K lol

  • Biochemistry -

    hahah yea x), were you able to figure it out?

  • Biochemistry -

    Yes they would be different, but it wouldn't be due to their Km values. Although they do the same thing, which is to convert glucose into glucose-6-phosphate, their 3D structures would be different because they are isoenzymes and are inhibited differently because of how their sequence dictates their 3D structure. Their are four levels to protein structure as you may or may not know: primary, secondary, tertiary, and quaternary structure.The primary structure of a protein refers to the number and sequence of amino acids; the secondary structure of amino acids refer to the making of alpha and beta sheets; the tertiary structure of a protein refers to its 3D conformation; and quaternary structure refers to the joining of several polypeptide chains. Although a protein's primary structure refers to the location and number of amino acids in a protein, the location of those amino acid residues can affect tertiary structure. imagine if you were to take a protein with a 1,000 residues, with 60 of those 1,000 residues being Cys residues and change their location in the protein sequence. Since the covalent bonds in a protein are formed due to disulfide bonds between two Cys residues, the 3D conformation would change drastically. Although the protein before rearrangement has the same number and types of amino acid residues as the altered one, the location of those amino acids would affect the tertiary structure of the protein causing it to look vastly different using X-ray crystallography. The reason why I believe you were told that the Km values for the two enzymes are different is because knowing that their Km values are different should make you investigate why they are different; they are different because they have different primary and tertiary structures, are regulated differently, and although they do the same thing they do it for different needs. The Km value for Hexokinase IV in the liver is high because it tends to act directly in the liver to regulate blood glucose levels to maintain homeostasis, but the role of Hexokinase I,II, and/or III in muscle cells act to consume glucose. Hexokinase I, II, and III in muscle cells are inhibited by glucose 6-phosphate, whereas Hexokinase IV is inhibited by a regulator protein in the liver and also binds an allosteric effector, fructose-6-phosphate, to relive the inhibition caused by the regulator protein; Hexokinase IV is not inhibited by fructose 6-phosphate. In order for the different kinases to bind different allosteric proteins and inhibitors, their tertiary structures must be different.

    I hope this helps.

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