Let's first examine the structural features of hexokinase that mediate catalysis. The yeast hexokinase is 50 kD in size and consists of two lobes, one large and one small, illustrated here in this ball-and-stick representation.
The binding site for glucose lies within the deep cleft at the interface of these two lobes. Hydrogen bonding between the hydroxyl groups of glucose and amino acids within the active site contribute to binding. The carbonyl oxygens that may contribute to these hydrogen bonds are shown in white.
Prior to glucose binding, hexokinase is said to be in the open configuration. ATP is bound within the large lobe, but relatively far from the glucose binding site, and in a different position than it will assume in the operational active site.
When glucose binds to the enzyme, a large conformational change occurs that closes the two lobes around the sugar substrate. This structural change occurs as a result of a new pattern of hydrogen bond contacts within the enzyme and between the enzyme and substrates. Move the molecule around to visualize the white carbonyl oxygens that may be involved in these hydrogen bonds. ATP binding is enhanced in this so-called closed conformation, and the gamma phosphate is now close enough to the C-6 hydroxyl for the reaction to proceed. Such a mechanism is referred to as induced fit since the enzyme molds around its substrate. Engulfment of the active site creates a hydrophobic environment that facilitates the nucleophilic attack of glucose on ATP.
Like many enzymes at the start of biochemical pathways, hexokinase helps regulate flow through glycolysis by maintaining a balance between substrate and products. In higher organisms, the brain utilizes a large percentage of circulating glucose. Therefore, under conditions of adequate glucose levels, tight regulation of hexokinase is required to prevent misdirection of resources.
Mammalian brain hexokinase is 100 kD in size and consists of a C-terminal catalytic domain and an N-terminal regulatory domain connected by an alpha helix. Each domain is similar in structure to the yeast enzyme.
As product levels rise, G6P binds the N-terminal domain, at a region (shown in yellow) distinct from the active site where it was formed.
G6P binding transmits a signal across the entire molecule that helps to turn off further conversion of glucose to G6P. This is referred to as allosteric feedback inhibition because binding of product at one site affects the activity of the enzyme at a completely different site. Some researchers hypothesize that G6P can also bind to the C-terminal domain and displace ATP to enhance the allosteric inhibition.
Another type of hexokinase, glucokinase, specifically catalyzes glucose conversion in the pancreas and liver. Since glucokinase has a lower affinity for glucose than hexokinase, it can sense small changes in physiological levels of glucose. Therefore, under conditions of starvation, the pancreas and liver will not use up glucose needed for brain function. Unlike other hexokinases, glucokinase is not inhibited by G6P. Mutations in hexokinase have been implicated as a cause of diabetes.
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