Biochem Lect 22: Myoglobin and Hemoglobin

Hemoglobin location/function/structure

• blood

• O₂ transport

• tetrameric (2 alpha chains, 2 beta chains)

Myoglobin location/function/structure

• muscle

• O₂ storage

• monomeric

Both Hb and Mb contain ____ and have ___ alpha helices

heme protein, 8

Mb binding curve shape

hyperbolic (binds 1 thing)

like MM

Hb binding cure shape

sigmoid (cooperativity)

Heme structure contains _____ and ____ at center

protoporphyrin IX ring (4 pyroles), Fe²⁺

Heme structure binds to _____ above and _____ below

His F8 (8th residue on F helix), O₂

In total, Fe²⁺ is bound to 5/6 things:

3 Ns from ring, N from histidine, O₂

O₂ binding causes Fe²⁺ to be drawn closer to/further from the ring

closer to

O₂ binding has a bigger effect on Mb or Hb?

Hb (causes major conformational change)

ferrous ion

Fe²⁺

ferric ion

Fe³⁺

a myoglobin with a ferrous ion is called…

metmyoglobin (does not bind O₂)

Hb is a ____, and some/all part of it are touching

ɑβ dimer (2 alpha 2 beta parts), some

How many oxygens (n) can be bound to Hb? How many usually are?

n = 4 → maximum = perfect cooperativity

n = 2.8 → average

n = 1 → no cooperativity

What is T state?

deoxy-Hb

What is R state?

oxy-Hb

Does T or R state have salt bridges and how many? 

T state, 8 (broken when transition to R state)

Hb cooperativity has both MWC and KNF features. Define each.

MWC = all or none (all subunits in same conformation)

KNF = sequential model (one changes neighbor, etc.)

ɑ/β cannot bind until all ɑ/β are bound. 

β,ɑ

What happens when 1 O₂ binds? Which model is this? 

binds to 1st ɑ, neighboring subunit relaxes

-

KNF (sequential)

What happens when 2 O₂ binds? Which model is this? 

binds to 2nd ɑ, entire molecule relaxes

-

MWC (all or nothing)

Mb/Hb has higher affinity for O₂ at all pressures.

Mb (due to non-cooperativity)

Why is cooperative binding helpful for Hb function? (lungs vs capillaries)

pick up/drop O₂ as needed

• lungs have high pO₂ → Hb fills with O₂

• capillaries have low pO₂ → Hb drops O₂

pH effect on O₂ binding

high H⁺ = low pH → makes Hb tense → decrease O₂ affinity

-

aka H⁺ bad

CO₂ effect on O₂ disocciation in tissues

high CO₂ → shifts to make more H⁺ → decrease O₂ binding → O₂ dropped into tissues

CO₂ effect on O₂ disocciation in lungs

low CO₂ → shifts to make less H⁺ → increase O₂ binding → O₂ taken into Hb

CO₂ also favors O₂ release from Hb by stabilizing deoxy-Hb → reacts with _____ to form _____

terminal amino groups, carbamate group (negative charged, forms salt bridges)

2,3 BPG activates/inhibits Hb

inhibits (causes sigmoid bonding)

2,3 BPG binds at same/different site as O₂

different (allosteric)

Fetal Hb has higher/lower affinity for O₂ compared to Adult Hb

higher

Fetal Hb has __ chains instead of __ chains

ɣ,β

ɣ chains have __ amino aicd instead of His

Ser

What is the functional difference between ɣ and β?

Serine is more negative than His → 2,3BPG binds less tightly (less inhibited) → fetal Hb has higher affinity for O₂