The enzyme pyrophosphatase catalyzes the hydrolysis of inorganic pyrophosphate [PPi] to or...

The enzyme pyrophosphatase catalyzes the hydrolysis of inorganic pyrophosphate [PPi] to orthophosphate [Pi]according to a reaction that can be written

The standard free energy change for this reaction is ΔG°310 = —33.5 kJ/(mol PPi) at pH 7. The pyrophosphatase from E coli bacteria has a molecular weight of 120,000 daltons and consists of six identical subunits. Purified enzyme has a maximum velocity Vmax of 2700 units per milligram of enzyme, where a unit of activity is defined as the amount of enzyme that hydrolyzes 10 μmol of PPi in 15 min at 37°C under standard assay conditions. As the concentration of the substrate PPi is decreased, the initial velocity decreases to 1350 units per milligram of enzyme when [PPi] = 5.0 X 10-6 M.

a. How many moles of substrate are hydrolyzed per second per milligram of enzyme at maximum velocity?

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b. How many moles of active site are there in 1 mg of enzyme? (Assume that each subunit has one active site.)

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c. What is the catalytic constant of each active site of the enzyme?

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d. Estimate the Michaelis constant KM for this enzyme.