Myoglobin and Hemoglobin

Proteins must be

-folded correctly to have proper function

-must interact with other molecules

reversible binding of ligands

any kind of molecule that reversibly binds to a protein

ligands bind at

binding sites

myoglobin and hemoglobin are both _________-binding proteins

oxygen

Myoglobin's function

facilitates oxygen diffusion to muscle

myoglobin acts as a ______________ ___________ to increase O2 diffusion

molecular bucket

protoporphyrin ring

a cyclic tetrapyrrole organic compound that serves as the immediate precursor to heme.

Myoglobin characteristics

- single polypeptide chain, folding into 8 alpha helixes (tertiary level of structure)

- globular

- one prosthetic group (heme)

- storage and diffusion of oxygen within the cell

- can bind O2, NO, and H2S

- one binding site for O2

heme

-FeI II in protoporphyrin ring

Fe2+ coordination in Heme

four nitrogens, His residue

heme can be used to transport

oxygen

AA side chains do not reversibly bind O2 but

transition metals do. CU + Fe

O2 solubility and transportability is increased when

bound to a soluble carrier (protein)

proximal histidine is His __

F8

His F8 characteristics

-His 93, eighth residue on the F helix

-nitrogen directly coordinates Fe2+

-it anchors the heme to the myoglobin protein

-when O2 binds and iron shifts position, can transmit that movement to the surrounding protein

Distal histidine is His __

E7

His E7 characteristics

-His 64, the seventh residue of the E helix

-does not directly coordinate the iron

-forms a hydrogen bond with bound oxygen

-helps stabilize bound O2 and helps make the binding pocket less favorable for CO to bind in its preferred geometry

when o2 is bound to Heme in myoglobin, it acts as the

sixth ligand to the Fe(II)

Val E11 and Phe CD1 function

hold heme in place, preventing oxidation of Fe II to Fe III and creates binding site for O2

the binding of O2 to the Fe(II)- heme complex alters the _________ state

electronic

oxidation of Fe(II) to Fe(III) prevents

binding of O2

Fe(III) is known as

methemyoglobin

CO, NO, H2S can bind to the Fe(II)- heme complex, making them_________ because...

toxic, their binding affinity is higher than that of O2

affinity is

constant, the ability to bind

Ka

association constant

P+L→ PL depends on which two aspects?

- affinity

- concentration of Ligand

Kd

dissociation constant= affinity

P+L←PL depends on what aspect?

soley on affinity

how to measure affinity

binding curve

[L]

concentration of ligand

θ

fractional saturation

multiplying θ by 100 converts it to

percent saturation

the lower the Kd, the

higher the affinity

Kd is [L] at

0.5 θ AKA half saturation

carbon monoxide poisoning

-light headedness, confusion, headaches, vertigo, flu like affects, significant toxicity of CNS and heart and death

how does carbon monoxide poisoning happen

MB and Hb efficiently bind CO and it outcompetes oxygen

CO likes to bind vertically, not at an angle like O2, therefore....

CO is partially blocked by the distal His E7

Ligand binding is

reversible

θ=

binding sites occupied/ total binding sites

myoglobin ligand

molecular oxygen

henry's law

quantity of gas absorbed by a lipid is proportional to the partial pressure of the fas above the lipid

YO2

[MbO2]/[Mb]+[MbO2]

YO2 is specifically the...

fraction of O2 binding sites occupied by O2

YO2 ranges from

0(no bound O2) to 1 (all binding sites of myoglobin are filed with O2

p50

the partial pressure of oxygen when hemoglobin is 50% saturated aka Kd

YO2 increases as

Kd decreases

Y=

pO2/Kd+pO2

two points on the binding curve are apparant

- the point of half saturation

-the point of full saturation(never attained)

binding is considered weaker when

it takes place at a greater oxygen pressure (larger P50)

Hemoglobin

An α2β2 tetramer formed from a dimer of αβ protomers

hemoglobin's quaternary structure can be viewed as

a dimer of αβ dimers

for this reason, the hemoglobin O2 binding curve is sigmoidal in shape

oxygen binding alters the structure of the entire hemoglobin structure

Hb must be able to bind oxygen in the

lungs

high pO2 forces

Hb to bind to O2

Hb must be able to release oxygen in

capillaries

low pO2

Hb releases o2

If Hb behaved like Mb,

very little oxygen would be released in capillaries

Hb's p50 is __x greater than Mb's p50

10

as more oxygen binds to hemoglobin

affinity becomes greater

why is the difference in O2 binding between Mb and Hb so important?

-Mb's function is to store O2 until its needed

-Hb's function is to take O2 from lungs to tissues

what is cooperativity

the binding sight of a ligand at one site affects the binding of ligands at another site

Hb+ nO2 →

Hb(O2)n

Hb binding O2 at one site ___________ Hb's affinity for O2 at another site

increases

The average P50 value for hemoglobin is

26 torr

Hill equation

if n=1,

-non-cooperative binding

- hyperbolic curve

if n>1

-positive cooperativity

-sigmoidal curve

if n

-negative cooperativity

-sigmoidal curve

log Y/1-Y=

nlog pO2-nlogP50

PO2 of inspired air

158 torr

PO2 of alveolar air

100 torr

PO2 of arterial blood

90 torr

PO2 of capillary

40 torr

PO2 of interstitial

30 torr

PO2 of cytosol

10 torr

in any binding system, a sigmoidal shape indicates

cooperativity

the αβ of hemoglobin associate via

hydrophobic and ionic interactions and hydrogen bonds

the hemoglobin dimer of αβ dimers associate via

ionic interactions and hydrogen bonds

ionic and H-bonds can be disrupted with __ changes but hydrophobic interactions cannot

pH

pH changes will affect _______ _______ interactions but not αβ dimer

dimer dimer

oxygen binding to heme in Mb

Fe(II) coordinated by the 4 porphyrin N atomas and one N from His F8

when O2 is bound to heme in Mb, it acts as

the sixth ligand to the Fe(II)

His E7 ___________ bonds to the oxygen

hydrogen

the cooperativity of binding multiple O2 molecules results from a

structural change

hemoglobin has two stable states:

T, R

oxygen can bind to both, but has a higher affinity for the _ state

R

in the absence of oxygen, the _ state predominates

T

T state

the conformation of deoxyhemoglobin

R state

the conformation of oxyhemoglobin

when deoxy-Hb crystals are exposed to oxygen,

they shatter

T state is stabilized by

a series of ion pairs that lie at the intersection between the two αβ protomers

T to R transition happens by

oxygen binding which initiates a series of movements that result in the structural shift

the T to R transition affects which structure

the quaternary structure of the αβ protomers

T to R transition sliding contacts between

α1-β2, α2-β1 19 residues interacting that change

how does oxygenation cause the T to R transition

by causing the F helix of one αβ dimer to shift relative to the C term of the opposite αβ dimer

in the T to R conformation shift, the Fe+2

cannot move toward the heme without reorienting His F8

His F8 is

tightly packed in a hydrophobic pocket, unable to move without reorienting Helix F

translation of Helix F produces

quaternary shifts in the α1C-β2 FG contacts

PO2 for venous blood

30 torr

PO2 for arterial blood

100 torr