Heme Synthesis & Hemoglobin Flashcards

Question

* What enzymes are particularly sensitive to lead poisoning? * What will be seen in the urine during lead poisoning?

Answer 1

* **Ferrochelatase** and **ALA dehydratase** are particularly sensitive to lead poisoning * **Protoporphyrin** and **ALA** accumulate in the urine

Answer 2

* Hemoglobin is a specialized protein designed to **transport oxygen (O₂) from the lungs,** a region of high O₂concentration, **to peripheral ****tissues, **where oxygen is low * Metabolism in the peripheral tissues generates CO₂ and H⁺ that are transported back to the lungs, in part, by hemoglobin. * **O₂ has very low solubility in plasma** * As a consequence, \>98% of the O₂ that reaches tissues is carried in red blood cells (RBCs) bound to Hemoglobin

Answer 3

* **RBCs contain carbonic anhydrase** which catalyzes the rapid reversible hydration of CO₂ to carbonic acid (H₂CO₃). * H₂CO₃ then rapidly and spontaneously dissociates to bicarbonate (HCO₃^-) and a H⁺ * CO₂ and HCO₃^- are soluble in plasma and RBC cytosol * most of the CO₂ made in tissues returns to the lungs as those species * about 14% of the CO₂ made is carried bound to Hb

Answer 4

* **Hemoglobin is a heterotetrameric protein **(αβ)₂ * Both subunits are **evolutionarily related to myoglobin** * a monomeric protein abundant in muscle that is designed to store O₂ * myoglobin: 1 heme groups * hemoglobin: 4 heme groups * **Fe²⁺ is the ferrous form** of iron that is capable of **binding O₂** * **Fe³⁺ is the ferric form** of iron that **CANNOT bind O₂** and is present in an **INACTIVE** form * **methemoglobin** (metHb)

Answer 5

* **Myoglobin** gives a normal binding curve which is **hyperbolic** in shape * **Hemoglobin** shows **sigmoidal** cooperative binding of oxygen * direct result of its more complex subunit structure * **P₅₀: **partial pressure of oxygen yielding 50% saturation of binding * analogous to Km for the binding of substrates to enzymes

Answer 6

**Sequential cooperativity** for oxygen binding: * binding of oxygen to one subunit induces a conformational change that is partially transmitted to adjacent subunits * transmission of the partial conformational change induces an increased affinity for oxygen by these adjacent subunits * R=relaxed=high affinity; T=taut=low affinity

Answer 7

* Carbon monoxide (CO) has **~250-fold higher affinity** for Hb than does O₂ * When bound to the heme group of one subunit, it **causes all four subunits to "lock" in the R conformation** * ****limiting oxygen release in peripheral tissues

Answer 8

**Without O2 bound:** * heme Fe²⁺ is **pulled away** from the plane of the porphyrin ring by a His residue of the Hb polypeptide chain * a His ring N is bound to the Fe²⁺ **When O2 binds:** * it **pulls the Fe²⁺ back into the plane of the ring** * moves the His residue and its whole section of the polypeptide chain * That in turn causes the Hb subunits to shift relative to one another to an arrangement that **favors the R-conformation**

Answer 9

* **Reduces affinity: ** * **H⁺, CO₂, and 2,3-diphosphoglycerate (DPG)** ALL can bind to Hb * **leftward curve shift** * **Increases affinity:** * **high O₂ ** * causes H⁺, DPG and CO₂ to dissociate from Hb * **rightward curve shift**

Answer 10

**H⁺** and **CO₂** are **heterotropic negative** allosteric effectors that **decrease the affinity** of Hb for O2

Answer 11

**During catabolism** * In the blood of tissues, **CO₂ and H⁺** are high * **HbO₂ will release its O₂ load** ⇒catabolism can continue

Answer 12

**Oxygen** is a **positive homotropic** allosteric effector of O₂ binding

Answer 13

**Reciprocal relationship between O₂ and H⁺ binding ** * **Oxygen is a negative allosteric effector of H⁺ and CO₂ binding** * Changes in H⁺ binding result from a _shift in the pKa of specific residues_ (mostly histidines) due to microenvironment effects triggered by conformational changes in the hemoglobin molecule

Answer 14

**2,3-diphosphoglycerate** is a **negative** allosteric effector of O₂ binding

Answer 15

* **2,3-DPG binds to a specific site in a central cavity between the β subunits ** * Binding is by _ionic interactions_ * Special regulatory mechanisms exist in RBCs to control the concentration of 2,3-DPG in order to fine tune the affinity of hemoglobin for O₂ in response to changes in metabolism and environment

Answer 16

1. **Without any 2,3-DPG** * ****Hb would be much more like **myoglobin** * **2,3-DPG stabilizes the T-state**, making it easier for Hb to release O₂ 2. **2,3-DPG levels increase at high altitudes** * Because there is **less O₂ at high altitudes**, tissues tend to become somewhat **hypoxic** * By increasing the concentration of 2,3-DPG ⇒ **RBC adapt to hypoxia** ⇒ easier for O₂ to dissociate from Hb * anemia and smoking also cause an increase in 2,3- DPG * Changes in 2,3-DPG occur over hours and days * takes most people a few days to adapt to high altitude * exercise or strenuous activity will be difficult until then

Answer 17

↑ temp = ↓O₂ affinity ↓ temp = ↑ O₂ affinity

Answer 18

* Each chromosome **16** has two **α-globin** genes * a person has 4 total, each is active and codes ~1/4 of expressed α-globin subunit * Each chromosome **11** has a single **β-globin** gene

Answer 19

* ~95% HbA (α₂β₂) * ~3% HbA₂ (α₂δ₂) * ~2% HbF (α₂γ₂) **Note:** all forms have an α-globin

Answer 20

* most common hemoglobinopathy ⇒ **sickle cell anemia** * pattern of inheritance ⇒ **autosomal recessive**

Answer 21

* Caused by a **point mutation** in the adult **β-globin gene** that causes **substitution of valine (Val) for ****glutamic acid (Glu) at amino acid 6** * Patient's RBCs containing mainly **hemoglobin S (HbS)** * ****two normal adult α-globin subunits and two sickle adult β-globin subunits. * **Valine is hydrophobic** and its presence **creates a sticky patch on deoxyHb** * leads to polymerization of Hb tetramers into long chains * intracellular fibers cause the sickle cell shape and reduced deformability of the RBCs * leads to problems in their passage through the microcirculation

Answer 22

1. **degree of deoxygenation** * which can be affected by subtle changes in pH, ionic strength and temperature * Deoxygenated HbS forms insoluble polymers 2. **intracellular hemoglobin concentration** 3. **relative amount of HbF present** * _HbF inhibits polymerization_ owing to a GLU residue at position 87 of the gamma chain, which prevents a critical lateral contact in the sickle cell fiber * HbF decreases with post-partum age but _varies from 1-30% of total Hb in sickle cell individuals_

Answer 23

a heterogeneous group of disorders caused by **inherited mutations that decrease the synthesis of adult hemoglobin HbA** (α₂β₂)

Answer 24

1. **β-Thalassemias** * Caused by mutations that diminish the synthesis of β-globin chains 2. **α-Thalassemias** * Caused by mutations that result in reduced or absent synthesis of α-globin chains