A 19-year-old athlete has been vigorously working on the tracks and begins to feel slightly light-headed from hypoglycemia. He drinks a can of soft drinks and is aware of the competition for the glucose to be stored in his liver as glycogen versus used as energy in his muscles. What is the best explanation regarding the fate of the glucose in the soft drink?
The Michaelis constant (Km) is a means of characterising an enzyme's affinity for a substrate. The Km in an enzymatic reaction is the substrate concentration at which the reaction rate is half its maximum speed. Thus, a low Km value means that the enzyme has a high affinity for the substrate (as a "little" substrate is enough to run the reaction at half its max speed). Hexokinase has a low Km for glucose, and in the liver is saturated and acting at a constant rate under all normal conditions. Glucokinase has a considerably higher Km (lower affinity) for glucose, so that its activity increases with increases in the concentration of glucose in the hepatic poal vein. It promotes hepatic uptake of large amounts of glucose after a carbohydrate meal. It is absent from the liver of ruminants, which have little glucose entering the poal circulation from the intestines. Hexokinase is inhibited by glucose 6-phosphate and is most active with low levels of glucose 6-phosphate. Glucokinase found in the liver has a high Km for glucose and is very active after a meal. The glucose in the soft drink would likely be used for ATP production. At normal systemic-blood glucose concentrations (4.5-5.5 mmol/L), the liver is a net producer of glucose. However, as the glucose level rises, the output of glucose ceases, and there is a net uptake. Ref: Bender D.A., Mayes P.A. (2011). Chapter 20. Gluconeogenesis & the Control of Blood Glucose. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.
The lower Km of hexokinase versus the Km of glucokinase will tilt the glucose toward glycolysis.
The bolus of glucose the soda will lead to a higher glucose level, inducing storage of the glucose into glycogen in the liver.
The muscle is using high levels of glucose, leading to an increased level of glucose 6-phosphate thus inhibiting glucokinase.
The glucose will be equally used by muscle for metabolism and liver for glycogen storage.