Hemoglobin Structure and Function — MCQs
Which factor predominantly influences the rightward shift of the oxygen dissociation curve?
Which of the following statements about hemoglobin is true?
What is the primary physiological effect of increased 2,3-DPG on hemoglobin?
Which local anaesthetic is known to cause methemoglobinemia?
All true about interaction of SpO2 reading and methemoglobinemia, except:
Haemoglobin, unlike myoglobin, demonstrates a unique characteristic in its oxygen binding.
The daily production of hydrogen ions from CO2 is primarily buffered by which of the following?
In which type of hemoglobin are zeta 2 and gamma 2 chains present?
Why is blood stored in citrate-phosphate-dextrose considered more beneficial for hypoxic patients compared to blood stored in acidic-citrate-dextrose?
Which of the following statements about sickle cell anemia is false?
Hemoglobin Structure and Function Indian Medical PG Practice Questions and MCQs
Question 1: Which factor predominantly influences the rightward shift of the oxygen dissociation curve?
- A. pH (Bohr effect)
- B. 2,3-Bisphosphoglycerate (2,3-BPG) (Correct Answer)
- C. Temperature increase
- D. Carbon monoxide levels
Explanation: ***2,3-Bisphosphoglycerate (2,3-BPG)*** - **2,3-BPG** is an organic phosphate found in **red blood cells** that serves as the **predominant regulator** of oxygen-hemoglobin affinity under physiological conditions. - An increase in **2,3-BPG** levels binds to the **beta chains of deoxyhemoglobin**, stabilizing the T (tense) state and reducing hemoglobin's affinity for oxygen, thereby shifting the curve to the right and facilitating **oxygen release** to tissues. - **2,3-BPG** is especially important in **chronic adaptations** to hypoxia (high altitude, chronic lung disease, anemia) and is the **primary mechanism** for sustained alterations in oxygen delivery. - Normal RBC 2,3-BPG concentration is approximately equal to hemoglobin concentration, making it a **quantitatively significant** regulatory factor. *pH (Bohr effect)* - A decrease in blood **pH** (increased acidity) due to higher **CO2** and **H+** concentrations also shifts the oxygen dissociation curve to the right via the **Bohr effect**. - While physiologically important for **acute regulation** in metabolically active tissues, the Bohr effect operates in conjunction with other factors rather than as the predominant standalone regulator. - The effect is mediated by **protonation of histidine residues** on hemoglobin, causing conformational changes that reduce oxygen affinity. *Temperature increase* - An increase in **temperature** reduces hemoglobin's affinity for oxygen, shifting the oxygen dissociation curve to the right. - This effect is vital for **oxygen delivery** to actively metabolizing tissues (which generate heat), but is generally a **secondary factor** compared to 2,3-BPG in terms of overall regulation. - The temperature effect is more situational, occurring primarily in tissues with elevated metabolic activity. *Carbon monoxide levels* - **Carbon monoxide (CO)** causes a **leftward shift** of the oxygen dissociation curve, not a rightward shift. - CO binds to hemoglobin with 200-250 times greater affinity than oxygen, forming **carboxyhemoglobin** (COHb). - This not only reduces oxygen-carrying capacity but also **increases hemoglobin's affinity** for the remaining oxygen, making it harder to release oxygen to tissues. - CO poisoning is therefore dangerous both because it displaces oxygen and because it impairs oxygen delivery through leftward shift.
Question 2: Which of the following statements about hemoglobin is true?
- A. Each hemoglobin molecule can bind up to six O2 molecules.
- B. Each hemoglobin subunit contains two heme groups, which bind oxygen.
- C. Hemoglobin consists of two alpha and two beta subunits, each capable of binding one O2 molecule. (Correct Answer)
- D. Each hemoglobin molecule is made of 6 polypeptide chains.
Explanation: ***Hemoglobin consists of two alpha and two beta subunits, each capable of binding one O2 molecule.*** - A **hemoglobin molecule is a tetramer**, meaning it is composed of four protein subunits: two alpha (α) chains and two beta (β) chains. - Each of these four subunits contains one **heme group**, which is an iron-containing porphyrin complex that can reversibly bind one molecule of **oxygen (O2)**. *Each hemoglobin molecule can bind up to six O2 molecules.* - A single hemoglobin molecule, with its **four heme groups**, can bind a maximum of **four O2 molecules**, not six. - The capacity for oxygen binding is directly proportional to the number of heme groups present in the hemoglobin molecule. *Each hemoglobin subunit contains two heme groups, which bind oxygen.* - Each individual **hemoglobin subunit (alpha or beta)** contains **only one heme group**, not two. - Therefore, a complete hemoglobin molecule (with four subunits) contains a total of four heme groups. *Each hemoglobin molecule is made of 6 polypeptides, one for each subunit.* - A hemoglobin molecule is composed of **four polypeptide chains** (two alpha and two beta), not six. - This tetrameric structure is crucial for its function and **cooperative oxygen binding**.
Question 3: What is the primary physiological effect of increased 2,3-DPG on hemoglobin?
- A. Increased affinity of hemoglobin to oxygen
- B. Decreased affinity of hemoglobin to oxygen (Correct Answer)
- C. Left shift of oxygen-hemoglobin dissociation curve
- D. Right shift of oxygen-hemoglobin dissociation curve
Explanation: ***Decreased affinity of hemoglobin to oxygen*** - **2,3-Diphosphoglycerate (2,3-DPG)** binds to the beta subunits of deoxyhemoglobin, stabilizing the **deoxygenated state** and thus **reducing hemoglobin's affinity for oxygen**. - This is the **primary molecular mechanism** by which 2,3-DPG exerts its effect, facilitating **oxygen unloading** in peripheral tissues. - This decreased affinity manifests graphically as a **right shift** in the oxygen-hemoglobin dissociation curve. *Increased affinity of hemoglobin to oxygen* - This is incorrect because 2,3-DPG specifically works to **decrease hemoglobin's affinity** for oxygen, promoting oxygen release. - Increased affinity would mean oxygen is held more tightly, which is counterproductive for **oxygen delivery** to tissues. *Left shift of oxygen-hemoglobin dissociation curve* - A **left shift** indicates **increased affinity** of hemoglobin for oxygen, meaning oxygen is held more tightly. - Since 2,3-DPG decreases affinity, it causes a **right shift**, not a left shift. *Right shift of oxygen-hemoglobin dissociation curve* - While this is the **graphical representation** of 2,3-DPG's effect, it is a **consequence** of the primary molecular mechanism (decreased affinity). - A right shift signifies that for any given partial pressure of oxygen, hemoglobin is **less saturated** with oxygen, reflecting the decreased affinity caused by 2,3-DPG binding.
Question 4: Which local anaesthetic is known to cause methemoglobinemia?
- A. Procaine
- B. Prilocaine (Correct Answer)
- C. Ropivacaine
- D. Etidocaine
Explanation: ***Prilocaine*** - **Prilocaine** is metabolized into **ortho-toluidine**, which can oxidize hemoglobin to **methemoglobin**, especially at higher doses or in susceptible individuals. - **Methemoglobinemia** symptoms include **cyanosis**, **dyspnea**, and in severe cases, central nervous system depression, due to reduced oxygen-carrying capacity of blood. *Procaine* - **Procaine** is an ester-type local anesthetic. It is metabolized to **para-aminobenzoic acid (PABA)**, which can cause allergic reactions, but it is not associated with methemoglobinemia. - It has a relatively **short duration of action** and is less commonly used now compared to amide-type local anesthetics. *Etidocaine* - **Etidocaine** is an amide-type local anesthetic that is known for its **long duration of action** and high potency. - While it can cause systemic toxicity with high doses due to its cardiac and neurological effects, **methemoglobinemia** is not a characteristic side effect. *Ropivacaine* - **Ropivacaine** is an amide-type local anesthetic similar to bupivacaine, known for its **motor-sparing effect** and use in regional anesthesia. - It is associated with a lower risk of **cardiotoxicity** compared to bupivacaine but does not cause methemoglobinemia.
Question 5: All true about interaction of SpO2 reading and methemoglobinemia, except:
- A. Increase in MetHb produces an overestimation when true SpO2 <85%
- B. Does not get affected in Methemoglobinaemia (Correct Answer)
- C. MetHb absorbs red and infrared wavelength of light in a 1:1 ratio corresponding
- D. Increase in MetHb produces an underestimation when true SpO2 >85%
Explanation: ***Correct: Does not get affected in Methemoglobinemia*** - This statement is **FALSE**, making it the correct answer for this EXCEPT question - **Methemoglobinemia significantly affects SpO2 readings** due to MetHb's optical properties - Pulse oximeters cannot distinguish methemoglobin from oxyhemoglobin and deoxyhemoglobin, leading to **inaccurate measurements** - As MetHb levels rise, the SpO2 reading tends to **plateau around 85%** regardless of true oxygen saturation *Incorrect: Increase in MetHb produces an overestimation when true SpO2 <85%* - This statement is TRUE - When **actual SpO2 is below 85%** and MetHb is elevated, the pulse oximeter reads **higher than the true value** (overestimation) - This occurs because MetHb absorption characteristics cause the reading to gravitate toward 85% *Incorrect: MetHb absorbs red and infrared wavelength of light in a 1:1 ratio* - This statement is TRUE - **Methemoglobin** has similar extinction coefficients at both **660 nm (red)** and **940 nm (infrared)** wavelengths - This 1:1 absorption ratio corresponds to an SpO2 reading of approximately **85%** on conventional pulse oximeters - This is why SpO2 readings plateau at 85% in methemoglobinemia regardless of true saturation *Incorrect: Increase in MetHb produces an underestimation when true SpO2 >85%* - This statement is TRUE - When **actual SpO2 is above 85%** and MetHb is elevated, the pulse oximeter reads **lower than the true value** (underestimation) - The reading is pulled down toward the 85% plateau created by MetHb's absorption characteristics
Question 6: Haemoglobin, unlike myoglobin, demonstrates a unique characteristic in its oxygen binding.
- A. Parabolic curve of oxygen association
- B. Co-operative index of 81
- C. Hill's coefficient of 1
- D. Co-operative effect of combined O2 (Correct Answer)
Explanation: ***Co-operative effect of combined O2*** - Haemoglobin exhibits **cooperative binding**, meaning the binding of one oxygen molecule to a heme group increases the affinity of the remaining heme groups for oxygen. This results in a **sigmoidal oxygen dissociation curve**. - This **cooperative binding** ensures efficient oxygen uptake in the lungs (high oxygen tension) and efficient oxygen release in the tissues (low oxygen tension). *Parabolic curve of oxygen association* - A **parabolic curve** typically describes processes with a squared relationship and is not characteristic of oxygen binding in haemoglobin. The actual curve for haemoglobin is **sigmoidal**. - This option does not accurately represent the unique binding kinetics of haemoglobin. *Co-operative index of 81* - While haemoglobin does show **cooperativity**, an index of "81" is not a standard or accurate measure for the cooperative effect in haemoglobin. - The **Hill coefficient** is used to quantify cooperativity, and for haemoglobin, it is typically around 2.8 to 3. *Hill's coefficient of 1* - A **Hill's coefficient of 1** indicates no cooperativity, meaning that the binding of one ligand does not affect the binding of subsequent ligands. - This is characteristic of **myoglobin**, which has only one binding site and thus a hyperbolic oxygen-binding curve, not haemoglobin.
Question 7: The daily production of hydrogen ions from CO2 is primarily buffered by which of the following?
- A. Red blood cell bicarbonate
- B. Extracellular bicarbonate
- C. Plasma proteins
- D. Red blood cell hemoglobin (Correct Answer)
Explanation: ***Red blood cell hemoglobin*** - **Hemoglobin is the primary buffer** for the massive daily acid load from CO2 (approximately 12,500 mEq H+ per day). - CO2 diffuses into RBCs where **carbonic anhydrase** rapidly catalyzes: CO2 + H2O → H2CO3 → H+ + HCO3-. - **Deoxygenated hemoglobin** has a higher affinity for H+ than oxygenated hemoglobin (reduced hemoglobin is a weaker acid, thus better H+ acceptor). - This buffering is crucial for CO2 transport: **Hb + H+ → HHb**, preventing significant pH changes despite huge CO2 production. - The bicarbonate produced is then transported out via the **chloride shift** to maintain electrical neutrality. *Extracellular bicarbonate* - While the bicarbonate buffer system is quantitatively the largest extracellular buffer, it is **NOT the primary buffer for CO2-derived H+**. - The extracellular HCO3-/CO2 system primarily buffers **metabolic (non-volatile) acids** produced from dietary and metabolic sources (~50-100 mEq/day). - For CO2-derived acid, the buffering occurs **intracellularly in RBCs** via hemoglobin before bicarbonate enters the plasma. *Red blood cell bicarbonate* - Bicarbonate is produced within RBCs from the dissociation of carbonic acid, but it is **not the buffer itself**. - The bicarbonate is a **product** of the buffering reaction, not the buffering agent. - Most RBC-produced HCO3- is transported to plasma via the **anion exchanger (Band 3 protein)** in exchange for Cl-. *Plasma proteins* - Plasma proteins like **albumin** have buffering capacity due to ionizable groups (imidazole groups of histidine residues). - They contribute only about **1-5%** of total blood buffering capacity. - Far less important than hemoglobin for buffering the large CO2-derived acid load.
Question 8: In which type of hemoglobin are zeta 2 and gamma 2 chains present?
- A. Gower I
- B. Gower II
- C. Portland (Correct Answer)
- D. Fetal hemoglobin
Explanation: ***Portland*** - **Portland hemoglobin** is a primitive embryonic hemoglobin composed of **zeta (ζ) 2 and gamma (γ) 2 chains** (ζ2γ2). - It plays a role in early fetal oxygen transport, particularly in the yolk sac stage. *Gower I* - **Gower I hemoglobin** is another embryonic hemoglobin, but it consists of **zeta (ζ) 2 and epsilon (ε) 2 chains** (ζ2ε2). - This composition is crucial for oxygen delivery during the very initial stages of embryonic development. *Gower II* - **Gower II hemoglobin** is an embryonic hemoglobin made up of **alpha (α) 2 and epsilon (ε) 2 chains** (α2ε2). - It represents a transitional form as the embryo develops and starts producing alpha globin chains. *Fetal hemoglobin* - **Fetal hemoglobin (HbF)** consists of **alpha (α) 2 and gamma (γ) 2 chains** (α2γ2). - It is the predominant hemoglobin during the second and third trimesters of pregnancy and has a higher affinity for oxygen than adult hemoglobin.
Question 9: Why is blood stored in citrate-phosphate-dextrose considered more beneficial for hypoxic patients compared to blood stored in acidic-citrate-dextrose?
- A. The fall in 2,3-DPG is less. (Correct Answer)
- B. It has a higher pH level than acidic-citrate-dextrose.
- C. It is more effective in oxygen delivery.
- D. It has a longer shelf life than acidic-citrate-dextrose.
Explanation: ***The fall in 2,3-DPG is less.*** * **Citrate-phosphate-dextrose (CPD)** better preserves levels of **2,3-bisphosphoglycerate (2,3-DPG)** in stored red blood cells. * Higher 2,3-DPG levels are crucial for **oxygen unloading** from hemoglobin in tissues, which is particularly beneficial for hypoxic patients who need efficient oxygen delivery. *It has a higher pH level than acidic-citrate-dextrose.* * While CPD does maintain a **less acidic pH** than acid-citrate-dextrose (ACD), which is generally favorable for red blood cell viability, the most direct benefit for hypoxic patients relates to 2,3-DPG. * The slightly higher pH indirectly contributes to better 2,3-DPG preservation but isn't the primary reason for improved oxygen delivery. *It is more effective in oxygen delivery.* * While the *consequence* of using CPD is **more effective oxygen delivery** due to better 2,3-DPG preservation, this option describes the outcome rather than the underlying mechanism compared to the more specific answer regarding 2,3-DPG. * The increased efficacy in oxygen delivery is directly attributable to the preserved 2,3-DPG levels. *It has a longer shelf life than acidic-citrate-dextrose.* * The storage solutions primarily impact red blood cell viability and function, but the **shelf life** (typically 21-35 days depending on the anticoagulant/preservative) is generally determined by other factors, including the additive solutions used with the anticoagulant. * While CPD improves red blood cell quality, the primary advantage for hypoxic patients specifically lies in oxygen affinity rather than overall storage duration.
Question 10: Which of the following statements about sickle cell anemia is false?
- A. Sickle cells are present in sickle cell anemia.
- B. Target cells are commonly seen in sickle cell anemia.
- C. Ringed sideroblasts are associated with sickle cell anemia. (Correct Answer)
- D. Howell Jolly bodies can be found in sickle cell anemia.
Explanation: ***Ringed sideroblast*** - **Ringed sideroblasts** are not typically associated with sickle cell anemia; they are indicative of disorders like **sideroblastic anemia**. - In sickle cell anemia, the primary findings include **hemolysis** and ineffective erythropoiesis, not ringed sideroblasts [3]. *Howell jolly bodies* - These bodies are remnants of nuclear material and can be found in individuals with **spleen dysfunction**, which can occur in sickle cell anemia [1]. - They are actually a common finding due to **hyposplenism** or **asplenia** in patients with sickle cell disease [2]. *Sickle cells* - The presence of **sickle-shaped red blood cells** is a hallmark of sickle cell anemia, caused by the mutation in the **beta-globin chain** [3]. - These sickle cells are responsible for the characteristic complications of the disease, such as **vaso-occlusive crises** [1][3]. *Target cells* - Target cells, or **codocytes**, are often seen in disorders like **thalassemia** and liver disease, and can also be present in sickle cell anemia. - They are formed due to an increase in the **surface area to volume ratio** of red blood cells, often secondary to **membrane abnormalities** seen in sickle cell changes [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 644-646. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Diseases Of The Urinary And Male Genital Tracts, pp. 570-571. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 598-599.
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