Question

qual 5 In this case? Why or why not? 5.8. The hemoglobin in blood establishes an equilibrium with oxygen gas very quickly. The equilibrium can be represented as heme + 02 = heme-Oz where "heme" stands for hemoglobin and "heme-O," stands for the hemoglobin-oxygen complex. The value for the equilibrium constant for this reaction is about 9.2 X 1018. Carbon monoxide also binds with hemoglobin by the following reaction: heme + CO heme.CO This reaction has an equilibrium constant of 2.3 X 1023. Which reaction's equilibrium lies farther toward products? Does your answer justify the toxicity of CO?

Answer 1

Let us denote the reaction constant for the O₂ binding as K_O2 and that for CO binding as K_CO.

The reaction for Oxygen is.

$heme+O_2\rightleftharpoons heme\cdot O_2$

Here, the equilibrium constant is given as,

$K_{O2}=\frac{[heme\cdot O_2]}{[heme][O_2]}\qquad(1)$

Here, big bracket [] denotes the concentration.

The reaction for Carbon monoxide is-

$heme+CO\rightleftharpoons heme\cdot CO$

Here, the equilibrium constant is given as,

$K_{CO}=\frac{[heme\cdot CO]}{[heme][CO]}\qquad (2)$

In blood, [heme] is same for Oxygen and Carbon monoxide. Further, we assume that Carbon Monooxide and Oxygen is present in the same concentration. Then, dividing (2) by (1), we have,

$\frac{[heme\cdot CO]}{[heme\cdot O_2]}=\frac{K_{CO}}{K_{O_2}}\qquad\\ \Rightarrow \frac{[heme\cdot CO]}{[heme\cdot O_2]}=\frac{2.3\times 10^{23}}{9.2\times10^{18}}=25000$

This means that at same concentration, Carbon monoxide can bind 25000 times more than the Oxygen.

Thus, higher the value of equilibrium constant, higher is the binding capacity of the gas and the Further the equilibrium lies towards the product. Thus, equilibrium for the second reaction involving CO lies further towards the product. As can be seen from the calculations, Carbon monoxide can bind to heme 25000 times better than the Oxygen. This will mean that CO will take up most of the heme itself by binding to it leaving a small number of heme for O₂. Thus, due to reduction in oxygen carrying capacity of heme in the blood, organs stop getting Oxygen and hence CO is a toxic gas,