hemoglobin

Hemoglobin is an Allosteric

Protein

Quaternary structure of hemoglobin

Tetramer with 4 subunits (4 heme groups)

Myoglobin

Does not have a quaternary structure. In the muscle and facilitates oxygen transport. It has only 1 subunit with one oxygen binding site

Heme groups

Fe is bonded to four nitrogen atoms from the pyrrole rings. Fe is in the ferrous oxidation state, Fe2+. Fe can have 2 additional bonds on each side of the heme plane (5th and 6th coordination site)

What happens to the position of Fe when oxygen binds to Heme group

The position of the Fe changes (rearranging position of electrons making iron smaller so it fits in the porphyrin ring)

Fifth coordination site

Occupied by the immidazole group of His

6th coordination site

Available to bind to oxygen

How does oxygen bind to hemoglobin

Cooperatively

Oxygen curve for Hb

Sigmoidal curve representing cooperative oxygen binding

Oxygen is transported from where in the body

From the lungs where the partial pressure of oxygen is high to the tissue where it’s low

Mb oxygen curve

Logarithmic

Why does Hb use cooperativity

Because it enhances oxygen delivery. 66% of potential oxygen binding sites release oxygen in the tissues

Mb doesn’t use cooperatively. Why is this good

Because it stores oxygen in muscles so only 7% of potential oxygen binding sites release oxygen in the tissues

How does oxygen binding change the structure of Hb

Central cavity more closed up in oxygenated form. It’s in the R form with a higher affinity for oxygen.

How does oxygen binding lead to transition from T state to R state

Binding of oxygen shifts the proximal His (moves up 0.4A). This results in movement of the corresponding alpha helix altering the interface of the alpha beta dimers

Allosteric regulation of Hb

2,3 BPG binds a pocket on deoxyhemoglobin and stabilizes it. For T to R transition to happen, 2,3 BPG must be expelled

How does 2,3 BPG work

It binds to the central cavity of deoxyhemoglobin interacting with positively charged residues (bc it’s -vely charged)

How is oxygen transferred form maternal Hb to fetal Hb

Fetal Hb binds to oxygen when it’s released from maternal Hb. Affinity of fetal Hb is greater than maternal Hb for oxygen. Fetal Hb has reduced affinity for 2,3 BPG

Why does fetal Hb have higher affinity for oxygen

Bc of it’s decreased affinity for 2,3 BPG resulting in increased affinity for oxygen.

What kind of tetramer is fetal Hb

2 alpha and 2 gamma chains

Why does fetal Hb use gamma chains

Because it’s 72% dental to beta chain but His is changed to Ser in 2,3 BPG binding pocket

How does metabolizing tissue get more oxygen

Release signal molecules to reduce affinity of Hb for oxygen

Bohr effect

H+ and CO2 regulate oxygen binding to Hb

A decrease in pH promotes

Release of oxygen

How does pH influence oxygen binding

Low pH favours formation of a salt bridge that stabilizes T state (deoxy) of Hb decreasing oxygen affinity and promoting release of oxygen in tissues

Oxygen binding at high pH

His 146 deprotonates disrupting the salt bridge

How does CO2 affect oxygen release

Hb responds by decreasing oxygen affinity

How does CO2 promote release of oxygen

CO2 reacts with terminal amino group of Hb to form negatively charged carbamate.

What does carbamate do

Forms salt bridges at the interface of alpha beta dimers, stabilizing the confirmation of deoxyHb decreasing oxygen affinity

Sickle cell anemia

Caused by a mutation in both copies of the Hb gene. HbS forms fibers that deform the cell (many copies of Hb stuck together). Sickle cells rupture easily leading to anemia

Sickle cell Hb

Has Glu 6 → Val 6 mutation on beta chains. Reduces the solubility of deoxyhemoglobin but not oxyhemoglobin

What happens with HbS when there is a lot of deoxyhemoglobin

Val 6 interacts with Phe 85 and Leu 88 to form aggregates. Forms a sticky patch because it’s hydrophobic on deoxyHb

How does HbS form sticky fibers of cells

DeoxyHb has sticky patch of Val 6 on oxy and deoxy HbS and sticky patch of Phe 85 and Leu 88 on deoxy HbS. The deoxy HbS cells stick together with the Val and Phe and Leu interacting to form fibers