liveonearth (liveonearth) wrote,

Hemoglobin: general

Seems like I'm circling all around these blood topics....because they are circular, they interrelate in many ways.

--this metallo-protein carries oxygen and carbon dioxide in the blood
--hemoglobin is made of a tetramer of globin chains and a heme group.
--different globin chains can add up to the tetramer
--when a person lacks heme, either due to iron deficiency (lacking intake, absorbtion, increased loss) or dt lacking an enzyme to make heme (lead poisoning, genetic lack) then globins are floating around without the iron in the middle to hold the oxygen

--alpha2beta2 is HGB type A1, and is 97% of normal adult HGB
--alpha2delta2 is type A1, 2% (delta is ?? another fetal type or??)
--alpha2gamma2 is type F, 1% (fetal hemoglobin, is the main type during fetal & neonatal periods
--alpha2betaS2 is type S, sickle cell-->hemolytic anemia
--gamma4 is type Bart's

--RELATIONSHIP OF ALPHA-THALASSEMIA TO SICKLE CELL ANEMIA: decreased MCHC in alpha-thalassemia-->milder Sx in Sickle cell pt because lower globin synthesis decreases HGB per cell and decreases sickling
--an old post on thalassemia and vampirism.


--results from a single point mutation in beta globin that causes the globin chains to stack up inside the RBC, forming the sickle shape-->hemolysis in the capillaries and in the spleen
--Hgb S
--another HGB modifying the effect of HbS is HbC
--HbC has a greater tendency to form aggregates with deoxygenated HbS and HbA --> HbSC disease
--HbSC disease has milder Sx than sickle cell anemia
--2-3% of American blacks are asymptomatic HbC/HbA heterozygotes
--about 1/1250 has HbSC disease
--pathogenesis is depending on concentration of Hgb per cell (MCHC): when RBC's are dehydrated, MCHC is increased-->increased sickling and vascular occulsion
--RELATIONSHIP OF ALPHA-THALASSEMIA TO SICKLE CELL ANEMIA: decreased MCHC (ie in alpha-thalassemia)-->milder Sx because lower globin synthesis decreases HGB per cell and decreases sickling
--clinical manifestations dominated by chronic hemolysis and ischemic tissue

--another HGB variant found in 2-3% of African-Americans
--does not cause sickling
--Heterozygotes (HbAC) and homozygotes (HbCC) both said to have "disease"
--HbAC pts usu aSx, might show only TARGET CELLS
--HbCC pts develop anemia, abdominal pain, arthralgias, poss mild jaundice
--splenic enlargment but no infarcts

transport proteins are always globular: hemoglobin, myoglobin, hemoerythrin, immunoglobulins (antibodies). Humans need transport proteins because we are many cells thick and simple diffusion won't supply all our cells adquately.

myoglobin - Mb. Function = storage and release of oxygen, accepts oxygen from arterial blood for delivery to mitochondria. One structural unit. Myo = muscle. The protein portion contributes to reversible oxidation, and the hydrophobic environment on the interior is a "control mecahnism for oxygen" (to keep it from oxidizing stuff). Myoglobin will bind oxygen, but it does not permit oxidation of the metal. The point at which Mb has absorbed half the oxygen that it can hold is p50 ~ K.

K = [Mb][O2] / [MbO2] = unbound / bound

so [MbO2] = [Mb][O2] / K

saturation = Y = sites occupied / total available = [MbO2] / [Mb] + [MbO2]. Problem: pO2 = 1 torr, what is Y? Y = 1 / 28 + 1 = .26. When at p50, K=2.8, and Y = 2.8 / 2.8 + 2.8 = .5 ...halfway saturated.

saturation point = the point at which no more of a gas will be absorbed

hemoglobin - Hb. Function = acquisition and safe transport of oxygen. Four structural units. (???iron is octahedrally coordinated-heme group, has 6 binding sites???). The four subunits have cooperativity. Hemoglobin is a dimer of dimers. Alpha1 and beta2 associate with each other first, then are bound to another similar pair. Hb has a lower affinity for O2 than Mb, so when it gets to the muscle and the partial pressure of oxygen is low, it dumps the O2 for the Mb to pick it up.

cooperativity = positive feedback among subunits, the interaction of the subunits of Hb such that when at low pO2 heme #1 is bound to O2, it is easier for #'s 2-4 to bind. When heme #1 is unbound, 2-4 unbind more easily.

a binary condition: hemoglobin is either fully oxygenated (R state, relaxed, smaller hole in middle) or fully deoxygenated (T state, tense, bigger hole), "two quaternary states":

T state of Hb = deoxygenated. Favored when acid. C terminus of B2 (position 146) is on top of C-helix of alpha (positions 36-42), "tail on loop", held by salt bridges & h-bonds, His97 is in FG corner of B2...etc. Hb in T state has "abnormally high pKa".

R state of Hb = energetically favored oxygenated state. The C terminus of beta2 is pulled away from alpha chain, salt bridges and h-bonds are broken, histimine 97 of B2 is between Thr38 and Thr41 of alpha chain. pKa is normal (~6.5) in this state, allowing protonation.

"oxygen is nasty stuff" because it rips electrons off of things. We oxidize carbon compounds to live. Oxygen is used in glycolysis, the krebs cycle (releasing lactic acid and lowering blood pH), and the electron transport chain. These processes produce CO2 and H2O. Breathing off CO2 causes blood pH to increase.

carbonic anhydrase = an enzyme that causes CO2 and H2O to form more HCO3- and H3O+

allosteric = "other place" --allosteric effector = ?

carbon monoxide is dangerous because it binds in the oxygen spot on myo & hemoglobin, and won't release, thus cutting the organism's ability to collect, store and release oxygen when needed.

Henry's law = the concentration of oxygen in a liquid is directly proportional to the partial pressure of oxygen in the air adjacent to it. [O2] = pO2.

the binding of oxygen to myoglobin is a factor of:
1. pO2
2. the affinity of Mb for O2 (K = p50 = 2.8 torr)

Gas pressure measurements:
760 torr = 760 mm Hg = 1 atm = 14.7 psi = 10 to the 5th Pa = 1 bar

the difference between Mb and Hb: Hb unloads at low partial pressures and loads at high, whereas Mb always loads

When pH is low it destabilizes oxygenated Hb, reducing its affinity for O2 and causing its release. When pH is high (over 7) it causes Hb to hang onto its oxygen. ???The proton of a free acid binds to His146, breaking its salt bridge to Asp94, moving the hemoglobin to its R (deoxy) state without binding to oxygen?????

2,3 BPG = biphosphoglycerate. Helps hemoglobin release O2. BPG has FIVE negative charges!! Fits into the pocket of Hb in deoxygenated state (T) when + charges are available. When Hb is oxygenated the positive charges shift away and the central cavity can't accomodate BPG. (Hb at low pO2 has high affinity for O2 and when BPG is removed the O2 is retained when BPG is missing.)

fetal hemoglobin = gamma, from a different gene. It reduces BPG binding, so the fetus' Hb has a higher affinity for O2 than the mother's.


HCT = hematocrit = MCV x #RBC / 10
MCV = mean corpuscular volume = Hct% x 10 / millions RBC's
MCH = mean corp hemoglobin = Hgb x 10 / millions RBC's
MCHC = mean corp hemoglobin concentration = Hgb (g/dl) x 100 / Hct %

--Heinz bodies = denatured Hgb, could be congenital glucose-6-phosphate deficiency, drug-induced hemolytic anemia, unstable abnormal hemoglobins. Test pt for G6PD before giving high vitamin C doses in cancer tx. fava beans?

1. Know heme and globin synthesis
hemoglobin structure = tetrameric, four heme groups, metallo-porphyrin ring, iron and red pigment, binds O2 and CO2 and CO and others, globin is made of four aa chains, 2 alpha and 2 beta

Normal types of hemoglobin:
--HbA1 (97%) A1 = alpha-beta
--HbA2 (2%) A2 = alpha-delta
--HbF (1%) fetal = alpha-gamma
--Alpha from chromosome 16 and beta
--delta and gamma chains from chromosome 11

Abnormal hemoglobins:
--HbS (sickle cell anemia)
--HbC (results in mild hemolytic anemia)
--most common abnormality is increase in HbA2, which is diagnostic of thalassemia esp Beta-thalassemia trait.
--more than 250 variants of Hb have been recognized.

How does lead interfere? In 7 step heme synthesis process it inhibits a bone marrow enzyme (#2): ALAD = delta-aminolevulinic acid dehydratase, which stops heme synthesis, accumulated precursor is hepatocarcinogenic also blocks a later enzyme (#7, protoporphyrinogen oxidase, ferrochetalase). Visible basophilic stippling. (if ALAD is blocked, precursor accumulates and causes liver cancer)

2. Identify lab screening for hemoglobinopathies:
--screen for Hgb distribution using Hgb Electrophoresis

Normal hemoglobin values:
--meaured in g/dL
--spectophotometer lyses RBC's into hemoslysate which is read by shining light through it
--normal for males is 14.0-17.4g/dL
--normal for females 12-16
--normal for children variable: newborn has 50%-90% HbF that decreases to under 5% by 6 mo

Abnormal hemoglobins:
--HbF in adult more than 1% suggests thalassemia
--HbS = sickle
--HbC = dz in 3% of African Americans, results in mild hemolytic anemia
--HgM = methemoglobin, ferric form of iron can't bind oxygen-->anoxia, cyanosis, 30% causes headaches, 70% death
--Sulfhemoglobin = abnormal pigment formed by combo w/ sulfides, oft with drug induced HgM, also causes cyanosis
--Carboxyhemoglobin - bound to CO2, Hb affinity for CO 240x greater than for O2
--Haptoglobin = a tgransport glycoprotein synthesized in liver, carried free Hb in plasma, preserves iron, if decreased indicates intravascular hemolysis
--Bart's Hgb = unstable w/ high O2 affinity, hereditary homozygous thalassemia from heterozygous parents, stillborn or dies soon after birth

Interpretation of abnormal hemoglobin values:
--over 16-17: high
--under 12-14: low
--depends on gender, men usu have more, also postmenopausal women

Factors that interfere with measurement of hemoglobin:
--low RBC production
--high RBC destruction
--increased blood volume (preg 1st trimester, overhydrated)
--increased RBC production

1) Know heme & globin synthesis, hemoglobin structure, types of normal & abnormal hemoglobin.

Heme Synthesis:
1) succinyl CoA + glycine -> 2-delta-aminolevulinate
enzyme = delta-aminolevulinate synthetase
2) 2-delta-aminolevulinate -> porphobilinogen
enzyme = delta-aminolevulinate dehydratase*
3) porphobilinogen -> linear tetrapyrrole
enzyme = uroporphyrinogen I synthase
4) linear tetrapyrrole -> uroporphyrinogen III
enzyme = uroporphyrinogen II synthase and uroporphyrinogen III cosynthase
5) uroporphyrinogen III -> coproporphyrinogen III
enzyme = uroporphyrinogen decarboxylase
6) coproporphyrinogen III -> protoporphyrin IX
enzyme = coproporphyrinogen oxidase
7) protoporphyrin IX -> heme
enzyme= protoporphyrinogen oxidase* and ferrochetalase*
* key enzymes

Important take-home message: delta-aminolevulinate dehydratase and ferrochetalase are both inhibited by lead, thus lead poisoning can lead to the build-up of 2-delta-aminolevulinate (a highly reactive oxyradical that is reported to cause liver cancer and yield basophilic stippling in the RBCs) and protoporphyrin IX; this decreases heme production and leads to anemia

Heme Structure: a metallo-porphyrin; contains iron atoms and the red pigment porphyrin; binds respiratory gases (CO2 and O2)

Globin Synthesis and Structure: composed of 4 amino acid chains: 2 alpha and 2 beta chains
Hemoglobin Structure: a tetrameric hemeprotein composed of heme and globin

Hemoglobin Types: Chain Structure:
A1 97% alpha2beta2
A2 2% alpha2delta2
F 1% alpha2gamma2
S alpha2betaS2
C alpha2betaC2
F = fetal
S = sickle

Normal Hemoglobin Types:
A1: 95-98%
A2: 2-3%
F: 0.8-2%
S: 0%
C: 0%
* S and C are abnormal types

Hemoglobin F in children:
Newborn = 50-80%
6 months = 8%
over 6 months = 1-2%

2) Identify the lab screening for hemoglobinopathies, normal hemoglobin values, abnormal hemoglobins, interpretation of abnormal hemoglobin values, factors that interfere with measurement of hemoglobin.

Lab Screening for Hemoglobinopathies: hemoglobin electrophoresis (measures distribution of hemoglobin types by weight)

Normal Hemoglobin Values:
males: 14.0-17.4 g/dL
females: 12.0-16.0 g/dL
children: variable based on age

Lab Test for Identifying Hemoglobin Values: RBCs lyse into hemosylate and hemosylate is read by the spectrophotometer; the amount of light absorbed by the spectrometer is directly proportional to the amount of hemoglobin released from the lysed RBCs

Interpretation of Abnormal Hemoglobin Values & Interfering Factors:
Increased values may be due to:
- increased RBC production
- dehydration
Decreased values may be due to:
- decreased RBC production
- increased RBC destruction
- hemorrhage
- increase in blood volume (ie- pregnancy or overhydration)
Tags: anemia, blood, hemoglobin, thalassemia

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