Carbohydrate Metabolism › Glycolysis

Hexokinase

Notes

Hexokinase

Sections

HEXOKINASE

  • Catalyzes phosphorylation of glucose to form glucose-6P
  • Traps glucose inside the cell
  • 1st regulated enzymes in glycolysis – catalyze an irreversible reaction
  • 4 isozymes – I, II, III, and IV (glucokinase)

HEXOKINASE (I, II & III) vs. GLUCOKINASE (IV)

  • Tissue distribution
  • Kinetics (Km and Vmax)
  • Regulation (allosteric vs hormonal)

Hexokinase

  • Ubiquitous in mammals
  • Low Km & low Vmax
  • Allosteric regulation – inhibited by glucose-6P

Glucokinase

  • Liver & pancreatic beta cells
  • High Km & high Vmax: spares glucose for brain, muscle & other tissues (glucose sensor)
  • Hormonal regulation: inhibited by glucagon, activated by insulin
  • Glucokinase regulatory protein (GKRP): nuclear protein that reversibly binds/inactivates glucokinase
  • High [glucose] inhibits GKRP & promotes glucokinase release
  • Fructose-6P (glycolytic intermediate in equilibrium with glucose-6P): promotes GKRP-GK binding
  • Liver glucose-6P shunts into one of three pathways: glycolysis, glycogen or fatty acid synthesis

Full-Length Text

  • Here we will learn about hexokinase, the first regulated enzyme in glycolysis.
    • We will also learn about the hexokinase isozyme glucokinase.
  • To begin, start a table to learn some key features of hexokinase.
  • Denote that it is the first of three regulated enzymes in glycolysis.
    • It catalyzes the first glycolytic reaction: the irreversible phosphorylation of glucose.

First, let's draw the reaction catalyzed by hexokinase.

  • Indicate that hexokinase phosphorylates glucose to form glucose 6-phosphate via an irreversible reaction (a single-headed arrow).
    • This reaction, along with the rest of glycolysis, occurs in the cytosol.
  • Indicate that one ATP is consumed in this process.
  • Finally, indicate that glucose cannot cross the phospholipid bilayer once it is phosphorylated; it is trapped in the cell.
  • Denote that in mammals there are four hexokinase isozymes: different proteins that catalyze the same reaction.
    • Hexokinase I through III (which we'll just lump as "hexokinase" because they're so similar)
    • Hexokinase IV, called glucokinase.
  • Now, denote the key differences between hexokinase and glucokinase:
    • Tissue distribution
    • Kinetics
    • Regulation.

Let's start with tissue distribution.

  • Show that our hexokinase reaction occurs in skeletal muscle, which is the most common example of where it occurs even though it occurs throughout most of the body.
  • Indicate that glucokinase is found in:
    • The liver
    • The pancreas (its beta cells)
    • Less notably, also in select cells of the intestine and brain.
  • Now, for kinetics, draw a graph.
  • Label the x-axis glucose concentration (mmol/L), numbered from 0 to 30.
  • Label the y-axis enzyme activity.
  • For hexokinase, draw a short, steep curve that quickly plateaus (its Vmax).
  • Indicate Km as dashed lines; it's the concentration at which hexokinase is 50% saturated.
    • This line is close to a glucose concentration of 0 mmol/L.
  • For glucokinase, draw a slowly rising curve that intersects the hexokinase graph at about 3 mmol/L and eventually plateaus (its Vmax).
  • Indicate the Km of glucokinase.
  • When we compare the two enzymes, write that:
    • For Km, glucokinase is greater than hexokinase – hexokinase has a greater affinity for glucose than glucokinase.
    • Also, for Vmax, glucokinase is greater than hexokinase.
  • Thus, write that the high Km of glucokinase spares glucose stores for hexokinase in the brain, muscle and other tissues.
    • The liver only takes up glucose after these other tissues have satisfied their own requirements. We will learn why shortly.
  • To emphasize this point, highlight a vertical line at approximately 3-5 mmol/L.
  • Label this portion of the graph as fasting blood glucose.
  • Write that at fasting glucose levels, hexokinase is at Vmax, whereas glucokinase activity varies according to blood glucose concentration.
    • Thus, glucokinase is a glucose sensor.

Now, for regulation, indicate that hexokinase undergoes allosteric regulation.

  • Show that glucose-6-phosphate allosterically inhibits hexokinase.
    • This feedback inhibition prevents excess hexokinase activity when the muscle is at rest and ATP levels are sufficient.
  • Now, show that glucokinase catalyzes the same reaction as hexokinase in the liver.
  • Indicate that it experiences hormonal regulation.
  • Draw a representative blood vessel between the liver and the pancreas.
  • Show that when blood glucose is high, glucokinase in pancreatic beta cells phosphorylates and sequesters glucose.
    • The glucose transporters (GLUT2) on the surface of beta and liver cells also have a high Km like glucokinase to preserve glucose for other tissues.
  • Indicate that glucose-6-phosphate continues through glycolysis and produces ATP.
    • We will not draw the additional steps of glucose metabolism.
  • Show that beta cells use this ATP to secrete the hormone insulin when blood glucose is high.
  • Show that insulin travels to the liver and stimulates glucokinase expression.
  • Show that that glucokinase expression is inhibited by glucagon, which is secreted by pancreatic alpha cells when blood glucose is low.
    • Thus, glucokinase activity responds to blood glucose levels.
  • Indicate that glucose-6-phosphate in the liver is shunted into one of three pathways:
    • It can continue through glycolysis and produce pyruvate, like skeletal muscle.
    • It can enter glycogen biosynthesis, a mechanism for glucose storage.
    • It can enter fatty acid biosynthesis, which also stores energy.
    • Glucose-6-phosphate does not accumulate in the liver and does not inhibit glucokinase; instead, it shunts into storage via biosynthetic pathways.
  • As a clinical correlation, write that the liver prevents hyperglycemia (excess blood glucose) via these pathways.
  • Now, write that patients with diabetes mellitus are treated with insulin to help lower their pathologically high blood glucose levels. Why?
    • As we have seen, one of insulin's many effects is to stimulate glucokinase, which shunts blood glucose into storage or glycolysis in the liver.

This brings us to the final regulatory mechanism of glucokinase.

  • Draw a representative nucleus in the liver.
  • Draw a protein called glucokinase regulatory protein (GKRP) in the nucleus.
  • Next, show that GKRP reversibly binds and inactivates glucokinase (GK).
    • Glucokinase is inactive when bound to GKRP.
  • Indicate that increased glucose (such as after a meal), promotes glucokinase release into the cytosol.
  • Indicate that fructose-6-phosphate (F6P), a glycolytic intermediate that is in equilibrium with glucose-6-phosphate, promotes GKRP-GK binding.
    • As glycolytic intermediates build up and intracellular glucose decreases, GK binds GKRP.

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