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Chapter 14: Problem 23

How does \(\mathrm{CO}_{2}\) directly and indirectly stabilize the " \(\mathrm{T}\) " state of hemoglobin in venous blood?

Short Answer

Expert verified
CO2 stabilizes the T state by forming carbamates and lowering blood pH.

Step by step solution

01

Understanding Hemoglobin States

Hemoglobin (Hb) can exist in two states: the T state (tense) and the R state (relaxed). In the T state, hemoglobin has a lower affinity for oxygen, making it more conducive for oxygen release in tissues.
02

Direct Stabilization by CO2

CO2 can directly stabilize the T state of hemoglobin by forming carbamates. CO2 reacts with the amino terminal groups of the globin chains forming carbaminohemoglobin, which stabilizes the T state by promoting salt bridge formation.
03

Indirect Stabilization through pH Changes

CO2 indirectly stabilizes the T state by influencing blood pH through the following mechanism: When CO2 dissolves in blood, it forms carbonic acid ( H_2CO_3) , which dissociates into bicarbonate ( HCO_3^- ) and H^+ ions, reducing pH (Bohr effect). Lower pH stabilizes the T state by promoting protonation of certain histidine residues, which further promotes salt bridge formation within Hb.
04

Recap and Analysis

To summarize, CO2 stabilizes the T state of hemoglobin both directly by forming carbamates and indirectly by lowering blood pH, which enhances protonation and salt bridge formation. This stabilization facilitates oxygen release in tissues where it's needed most.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Tense and Relaxed States
Hemoglobin molecules exist in two states known as the tense (T) state and the relaxed (R) state. These states are critical for their function in oxygen transport. In the T state, hemoglobin is tense and tightly packed, making it have a lower affinity for oxygen. This structure is effective for releasing oxygen in tissues.
The R state is more relaxed, allowing hemoglobin to easily bind with oxygen, which is essential in the lungs where oxygen concentration is high. Understanding the balance between these two states helps explain how hemoglobin efficiently carries and releases oxygen depending on the needs of the body.
CO2 Stabilization
Carbon dioxide (CO2) plays an important role in stabilizing the T state of hemoglobin. This stabilization is crucial in the process of oxygen delivery to tissues, especially those with high metabolic activity that produce a lot of CO2.
Directly, CO2 can form chemical compounds known as carbamates by reacting with the amino ends of hemoglobin's globin chains. These carbamates enhance salt bridge formations among the subunits of hemoglobin, stabilizing the T state.
In this stable T state with CO2 bound, hemoglobin is more likely to release its bound oxygen, thus supplying the actively respiring tissues with necessary oxygen.
Bohr Effect
The Bohr effect is a physiological phenomenon where an increase in carbon dioxide or a decrease in pH reduces the affinity of hemoglobin for oxygen. This forms a key part of how CO2 indirectly stabilizes the T state of hemoglobin.
When CO2 enters the blood, it reacts with water to form carbonic acid, which then dissociates to release hydrogen ions (H+). This decrease in pH results in the Bohr effect, leading to protonation of specific amino acid residues like histidine. These additional protons promote salt bridge formation, which stabilizes the T state.
As a result, hemoglobin releases its oxygen more efficiently under these conditions, adapting to the metabolic needs of tissues.
Carbaminohemoglobin
Carbaminohemoglobin refers to a form of hemoglobin bound to carbon dioxide. This complex is key in the direct stabilization of the T state of hemoglobin.
When CO2 binds to the N-terminal amino groups of the alpha and beta chains of deoxygenated hemoglobin, it forms carbamino compounds. This binding is part of the direct mechanism by which CO2 encourages the T state stabilization.
This step is essential because it enhances the release of oxygen by making the hemoglobin less attracted to binding oxygen and more focused on unloading it where it’s needed the most, helping in effective tissue respiration.
Oxygen Affinity
Oxygen affinity refers to hemoglobin's tendency to bind to oxygen molecules. This affinity is influenced by various factors including the presence of CO2, pH levels, and the T/R state of the hemoglobin molecule.
In the R state, hemoglobin has a high affinity for oxygen, which is beneficial for oxygen uptake in the lungs. Meanwhile, the T state has a lower affinity for oxygen which helps in releasing oxygen where it is required.
The transition from high to low oxygen affinity states, facilitated by mechanisms like CO2 binding and the Bohr effect, is essential for hemoglobin to perform its function effectively. This delicate balance ensures that tissues with higher demand for oxygen receive their supply without delay.

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Most popular questions from this chapter

The transcription of a gene is controlled by a transcription factor binding either glucose or lactose. When there is \(0.2 \mathrm{mM}\) of either glucose or lactose in the cell, the gene is transcribed at about \(10 \%\) of the maximum. When there is more than \(2 \mathrm{mM}\) glucose in the cell, the gene is fully induced. However, when there is \(2 \mathrm{mM}\) lactose in the cell, the amount of transcription is approximately half of the maximum. At \(40 \mathrm{mM}\) of either glucose or lactose in the cell, the gene is fully induced. The transcriptional regulation is likely: a. Ultrasensitive with respect to both glucose and lactose. b. Ultrasensitive with respect to lactose but not glucose. c. Graded with respect to glucose. d. Ultrasensitive with respect to glucose but not lactose. e. Hyperbolic with respect to glucose but not lactose.

A cyclist is interested in cheating in a race by delivering more oxygen to his muscles. The cyclist reasons that since bisphosphoglycerate (BPG) stabilizes the "T" state of hemoglobin, which reduces its affinity for oxygen, that reducing the BPG concentration in his blood cells should be good for his performance. How might removing BPG have a detrimental effect on the delivery of oxygen to his muscles?

Two proteins are modified by myristoylation, which targets them to the plasma membrane in a cell at \(25^{\circ} \mathrm{C}\). This changes their effective local concentration from 10 \(\mathrm{nM}\) to \(1 \mu \mathrm{M}\). Assume that any favorable mutation would decrease the value of \(\Delta G^{\circ}\) for binding by \(4 \mathrm{~kJ} \cdot \mathrm{mol}^{-1}\). How many favorable mutations would have to occur in the absence of colocalization to result in an equivalent effective affinity as observed when the two proteins are colocalized?

A dimeric hemoglobin is isolated from a fish. Each subunit contains a binding site for a xenon gas atom. The \(K_{\mathrm{D}}\) for the first binding event is measured to be \(23 \mathrm{nM}\). The \(K_{\mathrm{D}}\) for the second binding event is measured to be \(3.5 \mu \mathrm{M}\). a. What is the Hill coefficient? b. Is this protein positively or negatively cooperative with respect to xenon binding?

A dimeric enzyme, glucokinase, has a binding site for glucose in each subunit. The \(K_{\mathrm{D}}\) for the first binding event is \(1 \mathrm{mM}\) and the \(K_{\mathrm{D}}\) for the second event is \(10 \mu \mathrm{M}\). a. What is the Hill coefficient? b. Is this protein positively or negatively cooperative with respect to glucose binding?
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