the enzyme hexokinase acts on both glucose and fructose. the km and vmax values
ID: 34742 • Letter: T
Question
the enzyme hexokinase acts on both glucose and fructose. the km and vmax values are given in the table. compare and contrast the interaction of hexokinase with both of these substrates.
glucose
Km (M) : 1 X 10^-4
Vmax: 1.0
fructose
Km (M) : 7 X 10^-4
Vmax: 1.8
a. if the amount of hexokinase is 5 microgram , what is the kcat for glucose and fructose?
i did vmax/5 and got 0.2 and 0.36. is that correct?
b. which substrate is better?
c. how much better is the best substrate than the poorer substrate?
Explanation / Answer
a) Kcat is the turnover number. Its the number of substrate molecules which gets converted to product by enzyme per unit time. To calculate Kcat, first we need to calculate specific activity (SA) of the enzyme which would be Vmax/amounbt of protein in micrograms.
Glucose
SA = 1/5 = 0.2 uM/min/mg.
To get Kcat we need to divide SA by molecularweight of hexokinase.
Kcat = SA/mol wt of enzyme
Kcat = 0.2/100 = 0.002 moles/min
Fructose
SA = 1.8/5 = 0.36 uM/min/mg
Kcat = 0.36/100 = 0.0036 moles/min
What you have got is specific activity. To calculate you have to divide it by mol wt of the enzyme. So, your method need to be corrected.
b) As per the Kcat results, 0.0036 moles of fructose gets converted to products (higher) than 0.002 moles of glucose (lesser). So, that makes fructose is a better substrate than glucose.
c) How much better ...!!! Morethan thousand molecules of fructose molecules gets converted to products than that of glucose per min.
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