Erythrocyte Biochemistry (Pt 1) Flashcards
What are the four chains in adult hemoglobin (Hb)?
2 alpha
2 beta
In heme, what state is the iron in?
ferrous (2+)
Where is the iron atom in heme?
center
What is the purpose of heme?
carries oxygen
Is heme hydrophilic or hydrophobic?
hydrophobic
What comprises the main framework of heme?
a porphyrin ring
4 pyrroles
What happens to the iron molecule when oxygen binds to the heme?
- originally it lies outside the plane of the porphyrin ring
- iron moves into the plane when oxygen binds
- proximal His is pulled
- changes interaction w/ associated globin chains
- distal His stabilizes the oxygen
What happens to the hemoglobin as it enters tissues with low pH (actively respiring tissues)?
-as pH decreases, the binding affinity of Hb for oxygen decreases
- His picks up a H+ ion
- Hb changes conformation and releases oxygen
Between what pressures is the steepest portion of the oxygen-dissociation curve?
20-40 torr
What are the subunits of embryonic hemoglobin?
Gower 1: 2 zeta, 2 epsilon
Gower 2: 2 alpha, 2 epsilon
Portland: 2 zeta, 2 gamma
What are the subunits of fetal hemoglobin (HbF)?
2 alpha
2 gamma
Which has higher oxygen affinity, fetal Hb or maternal Hb?
fetal hemoglobin
Why does fetal hemoglobin have a higher affinity for oxygen?
it doesn’t bind well to 2,3-BPG
What hemoglobin is present in sickle cell disease?
Hb S
What makes Hb S different than normal adult hemoglobin?
a mutation in the beta globin chain
-glutamic acid (in the 6th position) is switched to valine
negative charged aa to a hydrophobic aa
After ferrous iron (Fe2+) is absorbed into the enterocyte, what enzyme converts it to ferric form?
ferroxidase
How is iron stored in the liver?
as ferritin and homosiderin
Since plant-based ferric iron is not easily absorbed, how does it get into the enterocyte?
it’s converted to ferrous form by ferric reductase
What cofactor is needed by ferric reductase?
vitamin C
What transporter does ferric iron use to enter the enterocyte
after being converted to the ferrous form, the iron enters the enterocyte via DMT-1
How is iron exported out of the enterocyte?
ferroportin
How does iron get transported through the blood to its target tissues?
it’s bound to transferrin
Once at the target tissue, how does iron get into the cells?
transferrin binds to transferrin receptors (TfR) and it is endocytosed by clathrin-coated pits
Once in the endosomes, how is the iron released from the transferrin?
- the low pH (5.5) of the endosomes causes transferrin to be released from its receptor
- endosome docks onto mitochondria and transfers Fe3+ in via DMT-1
What process is the main regulator of iron in the body?
absorption
What hormone regulates iron absorption?
hepcidin
Where is hepcidin synthesized?
liver
What controls the level of hepcidin?
human homeostatic iron regulator protein
HFE
What is hepcidin’s mechanism of action?
binds to ferroportin and causes it to be internalized and degraded by lysosomes
Will high levels of iron increase or decrease the amt of hepcidin?
increase
What are some causes of iron deficiency?
- insufficient dietary iron
- insufficient absorption of iron
- heavy menstruation
- GI bleed or ulcers
- aspirin overuse
What type of anemia might an iron deficiency cause?
hypochromic microcytic anemia
What genetic disease might be responsible for an iron overload?
Hereditary Hemochromatosis
What are the characteristics of hereditary hemochromatosis?
- iron accumulates in the liver, heart, and pancreas
- causes cirrhosis, diabetes, arthritis, heart failure
- autosomal recessive gene
-defective HFE (human homeostatic iron regulator protein) which normally controls hepcidin levels
What are the total body iron levels like in a pt. with hereditary hemochromatosis vs. a normal person?
15g
3-5 g normal
What type of anemia do deficiencies in B9 (folate) and/or B12 (cobalamin) cause?
megaloblastic anemia
What are the characteristics of megaloblastic anemia from B12 and B9 deficiencies?
- diminished DNA synthesis in bone marrow
- cell volume > 100 fL (normal 80-100)
- macrocytic, but normochromic
- hypersegmented neutrophils
What is the general structure of folate?
- pteridine (N-containing ring)
- PABA ring (p-amino benzoic acid)
- glutamate residues (lots of them)
What is the active form of folate?
tetrahydrofolate (THF)
What enzyme gets folate into the active form?
dihydrofolate reductase (DHF reductase)
acts on it 2x
In regards to DNA synthesis, why is THF important?
-it is used in the synthesis of purines and in the synthesis of thymine
What drug inhibits dihydrofolate reductase (DHF reductase)?
methotrexate
In what part of the body is B9 absorbed?
jejunum in the small intestines
How much folate is normally stored in the body?
5-10 g
3-6 months worth
What level constitutes a folate deficiency?
< 3 ng/mL
What is the primary form of folate circulating in the bloodstream?
N5-methyl-THF
What is folate’s main duty in biochemical reactions?
transfer of one-carbon groups
When B9 accepts a carbon group from serine, what reaction happens?
- B9 becomes N5, N10-methylene THF
- it donates a carbon and becomes DHF
- DHF reductase comes back in
- regenerates THF
When B9 accepts a carbon group from glycine, what reaction happens?
- B9 becomes N5,N10-methylene THF
- then becomes N5-methyl THF
- then B12 removes the methyl group
- becomes the active form of THF
What is usually the main cause of a B12 deficiency?
lack of intrinsic factor
Delineate the process of B12 absorption.
- binds to R binder proteins in the stomach
- R binders are degraded by pancreatic proteases
- binds to intrinsic factor
- sent into the bloodstream
- carried by transcobalamin III
What does the Schilling Test screen for?
pernicious anemia (lack of intrinsic factor)
How does the first part of the Schilling Test work?
- ingest labeled B12
- saturate liver with regular B12 so labeled isn’t stored
-check urine for labeled B12 indicating that it’s been absorbed out of the GI tract
How does the second part of the Schilling Test work?
- if labeled B12 is not found in the urine, administer the labeled B12 again, but with intrinsic factor this time
- check urine for labeled B12
