Question 7 of 10 macmillan learning under appropriate...

Question 7 of 10 © Macmillan Learning Under appropriate conditions, hemoglobin dissociates into its four subunits. The isolated $\alpha$ subunit binds oxygen, but the $O_2$-saturation curve is hyperbolic rather than sigmoid. In addition, the binding of oxygen to the isolated $\alpha$ subunit is not affected by the presence of $H^+$, $CO_2$, or BPG. What do these observations indicate about the source of the cooperativity in hemoglobin? O Cooperativity arises from the bicarbonate buffer system. O Individual subunits are capable of binding $O_2$ cooperatively. O BPG regulates the cooperative behavior of hemoglobin. O The cooperative behavior of hemoglobin arises from subunit interactions.

Transcript

00:02 So we're in this situation where we know that ordinarily hemoglobin is composed with four subunits and displays positive co -operativity.

00:12 And we're in this new circumstance where the subunits of our hemoglobin have dissociated from each other to form single units, and we no longer see cooperativity, and we need to know why.

00:25 So let's just cover what we know real quickly.

00:27 We know that under normal circumstances, hemoglobin has a sigmoidal curve, but that at this point it has a high, hyperbolic curve.

00:47 We also know that protons carbon dioxide and bpg do not affect the binding of oxygen to our dissociated form of hemoglobin.

01:06 So let's first analyze the difference between the sigmoid curve and this hyperbolic curve.

01:13 We know that in normal circumstances we'll see a sigmoid curve that looks something like this.

01:21 But right now we have a hyperbolic curve with no cooperativity, so our curve looks something like that, and in the case of negative cooperativity, we also see a hyperbolic curve that tapers off a bit more quickly than in the case of no cooperativity.

01:37 But why is this? so we analyze this graph real quickly.

01:42 We'll see that it is a comparison of the pressure of oxygen versus the percent saturation of her hemoglobin.

01:49 So basically how many of our binding sites currently contain oxygen.

01:52 And as we go up, the pressure of oxygen will see a greater saturation of our binding sites.

01:59 In the case of positive cooperativity, when we have one oxygen bound to our hemoglobin, this encourages further binding to other subunits.

02:10 So we'll start out with 0 % saturation in a relatively narrow, shallow slope.

02:16 And as we go in this direction, we see that our slope increases.

02:19 Our rate of oxygen binding is also increasing.

02:24 And then it tapers out as we start to run out of binding sites.

02:29 In the case of no cooperativity, our slope only depends on the total ratio of binding sites that can still take in oxygen.

02:38 So we'll start out with a steeper slope because there are more total binding sites available and taper out as these become more scarce.

02:45 In the case of negative cooperativity, it's the same circumstance.

02:48 We'll start out with the greatest slope at the most available binding sites.

02:52 But we taper out more quickly because when one oxygen has bound, discourages further binding of other oxygens to other subunits, thus causing us to taper out more quickly...

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