In biological systems, oxidases catalyze the following reaction: O2 + 4 H+ + 4 e- -> 2 H2O The react — Physical Chemistry — Kinetics Chemistry Question
Enzyme Catalysis
In biological systems, oxidases catalyze the following reaction:
O2 + 4 H+ + 4 e- → 2 H2O
The reaction is the key to respiration in living systems. The electrons come from Cytochrome c, which contains an iron center. The half reaction is:
Fe II cyt c → Fe III cyt c + e-
The extinction coefficients of Fe II cyt c and Fe III cyt c at 550 nm are 27.7 and 9.2 mM cm-1 (M = mol dm-3), respectively.
The absorbance of cyt c at 550 nm was observed to decrease at a rate of 0.1 A sec-1 in a 5 cm3 solution containing 2.7*10^-9 M oxidase, and sufficient cyt c and oxygen.
How many moles of cyt c were oxidized each second?
Model Answer
A = ε b C; C = A / ε b; mol = C × V (volume)
[0.1 / ((27.7 – 9.2) × 10^3)] × 5×10^–3 = 2.7×10^–8 mol sec^–1
How many moles of oxygen were consumed each second?
Model Answer
Four electrons are needed to reduce one molecule of oxygen, therefore, the oxygen consuming rate is 2.7×10^–8 / 4 = 6.75×10^–9 mol sec^–1
What is the turnover number for oxidase? (Turnover number: the number of product produced by one catalyst per second)
Model Answer
By definition, the turnover number equals 6.75×10^–9 (mol sec^–1) / (2.7×10^–9 M × 5×10^–3 dm3). Therefore, oxidase has a turnover number of 500.