Heme Biochem Flashcards
What are the functions of hemoglobin and myoglobin? Describe their structures and how they differ.
Both hemoglobin and myoglobin function to bind oxygen.
Hemoglobin: Tetrameric protein, with 2 alpha + 2 beta subunit. Each subunit has a single heme-binding pocket. These TRANSPORT oxygen.
Myoglobin: Globular protein with multiple alpha helical structures. Has 1 heme-binding pocket. These STORE oxygen and have a higher affinity for oxygen than hemoglobin does.
How does carbon dioxide travel from the tissues to the lungs to be exhaled?
Primarily it is carried in plasma as bicarbonate, but some binds to the terminal amine groups of the deoxy form; note that it does not bind to the heme site.
Compare the p50 of myoglobin to the p50 of hemoglobin - what does this mean?
The p50 value of hemoglobin is almost 10x higher than the p50 of myoglobin, meaning it takes a much higher partial pressure of O2 to saturate a molecule of hemoglobin. This means that hemoglobin has a lower affinity for oxygen than myoglobin does.
Compare the shape of an oxygen binding curve for myoglobin versus hemoglobin.
The oxygen binding curve for hemoglobin is sigmoidal because there is cooperative binding occurring among 4 binding sites.
An oxygen binding curve for myoglobin is hyperbolic because there is no cooperative binding occurring - only one binding site.
For each of the following, describe how they affect the binding of oxygen to hemoglobin, and what it does to an oxygen binding curve:
Protons
BPG
CO2
All of these are allosteric modifiers of hemoglobin and stabilize a conformation change of either T (deoxy) or R (oxy).
Protons: T state is stabilized via protonation of histadine residues and subsequent strengthening of ionic interactions; creates right shift.
BPG: Stabilizes T state; this is helpful in environments like high altitude: hemoglobin is less saturated in the lungs because it has a lower affinity for oxygen, but it delivers more oxygen to the tissues because it parts with it more readily. Creates right shift.
CO2: binds to the N-terminal amino groups of and hemoglobin chains and stabilizes the T state. Also a right shift.
In the lungs, as [O2] rises and competes for binding sites, CO2 is kicked off and we breathe it off.
How does fetal hemoglobin differ from adult hemoglobin, and why is it important?
HbF (fetal form) has α2γ2 chains. It has a lower affinity for 2,3-BPG than HbA (adult form), which results in a higher affinity for oxygen. This is important because it allows the mother’s hemoglobin to deliver/lose oxygen to the fetus.
Note that there is another form of HbA: HbA2; has slightly higher oxygen affinity (because of decreased affinity for 2,3-BPG) and has important implications in thalassemia.
Where is heme synthesized, and what is it needed for?
Can occur in all types of cells, but most active in bone and liver cells:
In bone marrow, most heme synthesis occurs in erythroid tissue for hemoglobin production
In liver, heme is necessary for synthesis of cytochrome P450 enzymes
In other cells with mitochondria, heme is necessary for the synthesis of cytochromes used in the ETC (also necessary for the synthesis of catalase, peroxidase, and nitric oxide synthase).
Describe the rate-limiting step of heme synthesis.
Succinyl CoA gets pulled out of the TCA cycle to be combined with glycine via the enzyme delta-ALA synthase to make delta-ALA.
This enzyme is PLP (=B6!) dependent.
Deficiency in B6 results in decreased heme synthesis –> reduced hemoglobin production –> microcytic, hypochromic anemia with deposits of iron accumulation in cells (sideroblastic anemia)
How is the rate-limiting step of heme synthesis regulated?
By iron levels.
When iron levels drop, delta-ALA synthase expression is decreased so we don’t make a bunch of protoporphyrin rings that we can’t fill with iron.
How is heme synthesized from delta-ALA? What can interrupt this process?
8 delta-ALA molecules are condensed –> 4 porphobilinogen molecules.
This step can be inhibited by lead.
These 4 porphobilinogen molecules get attached together to make linear tetrapyrrole.
This gets cyclized and modified, eventually becoming a protoporphyrin ring.
The enzyme ferrochelatase catalyzes a reaction which puts iron in the center of the ring.
This step can be inhibited by lead.
This forms heme.
With what signs and symptoms will a patient with lead poisoning present?
Often a child, since children absorb much more lead than adults.
Heme levels will be low. Peripheral blood smear will show microcytic and hypochromic cells because they cannot make heme. Will have sideroblastic anemia (accumulation of iron in cells) and will have adequate iron level.
Patient may exhibit abdominal pain, irritability, and/or learning difficulties.
Patients with lead poisoning are treated with chelation therapy - EDTA is a common chelating agent.
What can trigger a sporadic porphyria?
Anything that would cause heme biosynthesis to increase, such as induction of cytochrome P450 enzymes in the liver (alcohol consumption can do this).
You have a very distressed patient who reports that any time he gets ill, drinks alcohol, or takes ibuprofen he has symptoms including psychosis, hallucinations, and red-colored urine. Yeah, okay, he waited a long time to seek help. What is a likely diagnosis and what is happening to cause these symptoms?
Acute intermittent porphyria is often triggered when consumption of drugs or alcohol or onset of illness stimulates the increase in synthesis of cytochrome P450 enzymes in the liver. This necessitates the synthesis of heme, and the enzyme porphobilinogen deaminase is deficient in this pathway.
Porphobilinogen accumulates, and it is excreted with urine. When exposed to oxygen, it turns red.
The intermediate is neurotoxic, causing the psych symptoms.
What is the deficient enzyme in porphyria cutanea tarda, and what are the symptoms?
The enzyme uroporphyrinogen decarboxylase of the heme synthesis pathway is deficient in PCT. it causes photo-reactive intermediates to accumulate in the blood. When the skin is exposed to UV light, it triggers a photoreaction, creating reactive oxygen species. This damages skin tissue, may cause blisters or scarring, and may cause excessive growth of hair in sun-exposed areas. They may also have dark red urine; if so, uroporphyrinogen (one of the intermediates) may be detectable in it.
Your patient’s lab shows an elevated indirect bilirubin level. What does this indicate?
High levels of unconjugated bilirubin - not bound to glucuronate. This is lipid soluble and more able to pass through membranes than conjugated bilirubin (bad news).
Why would a patient have pale stools?
Stercobilin - the conjugated bilirubin that passes through the GI tract - gives feces a brown color. If the liver cannot secrete bilirubin into the bile, or if the bile duct is obstructed and cannot secrete its contents into the intestine, this could cause pale stools.
Why might a patient have dark, cola-colored urine?
If an excess amount of bilirubin is passing into the kidneys from the blood stream, urine may be dark-colored. This could be caused by hemolytic or obstructive jaundice, or some forms of hepatocellular jaundice.
Describe two issues that might arise with the enzyme glucuronyltransferase.
This is the enzyme responsible for conjugating bilirubin.
In newborns, the enzyme glucuronyltransferase is not yet fully expressed- it might take a couple of days to be expressed. This can cause an accumulation of unconjugated bilirubin which can be treated with phototherapy (this causes the unconjugated bilirubin to become more soluble and able to be secreted.
In some adults there is a deficiency of glucuronyltransferase. This is usually asymptomatic but can cause elevated indirect bilirubin levels on labs. If the liver is stressed by alcohol, illness, fasting, or the use of some drugs, hyperbilirubinemia might occur.
What might cause a microcytic, hypochromic anemia?
These are due to impaired hemoglobin synthesis. Some causes include:
Iron deficiency
Thalassemia
Lead poisoning
Very severe, rare B6 deficiency
What might cause a macrocytic, normochromic anemia?
This is due to impaired DNA synthesis. Some causes include:
B12 (cobalamin) deficiency
Folic acid (B9) deficiency
Erythroleukemia
What might cause a noromocytic, normochromic anemia?
These are due to red cell loss. Causes might include: Acute bleeding Hemolysis via sickle cell disease Red cell metabolic defects Red cell membrane defects
What is a dietary source of folic acid?
Leafy green vegetables (think “foliage”)
What is the key enzyme that is necessary in converting folic acid into tetrahydrofolate?
Dihydrofolate reductase - uses NADPH
