Biochem CH. 9

hemoglobin (Hb)

oxygen-binding, transport protein

what kind of protein is hemoglobin

tetrameric transport protein

• allosteric

• RBC

myoglobin, Mb

func = store

• transports oxygen in MUSCLE

• facilitates O2 diffusion for high respiration

hemoglobin carries oxygen from where to where

from LUNGS —> TISSUES

how is hemoglobin an allosteric protein

it displays cooperativity in oxygen binding/release

where does myoglobin bind oxygen? what type of bind?

in MUSCLES

• binding is NOT COOPERATIVE

oxygen binding in Hb & Mb is measured as

a function of the partial pressure of O2

Y_O2, (fractional saturation by O2) describes what?

how much O2 bound

Y_O2 = [HbO₂] / [HbO₂] + [Hb]

myoglobin structure

single polypeptide chain

• consists of many alpha helices

• arranged to form globular structure

myoglobin & hemoglobin bind oxygen at what location

at a heme

heme

a bound prosthetic group

2 things heme groups consists of

• protoporphyrin

• central iron ion (Fe²⁺)

protoporphyrin

org. component in heme group

heme structure

iron (Fe) in middle, bound to 4 Nitrogens

iron can form…

2 additional bonds

• called 5th & 6th coordination series

iron’s 2 additional bonds it can form are called what?

5th & 6th coordination series

the 5th coordination site is occupied by what

proximal histidine

(imidazole ring of histidine)

proximal histidine

occupies 5th coordination sie; imidazole ring of histidine

what does the 6th coordination site bind

oxygen

• O2 binding causes iron to move into the plane of protoporphyrin ring

hemoglobin structrue

tetramer

• 2 alpha subunits

• 2 beta subunits

• EACH subunit has a bound heme

the quaternary structure in Hb is best described as….

a pair of identical alphabeta dimers (a1 B1 a2 B2)

each subunit in Hb has what?

a bound heme

deoxyhemoglobin corresponds to what state

T state of allosteric enzymes

(inactive/low binding affin)

oxyhemoglobin is what state

R state

what causes the transition from deoxyhemoglobin (T state) to oxyhemoglobin (R state)

oxygen binding

• iron moves into the plane of the heme

• proximal histidine (a helix) moves w/ iron

• two aB dimers rotate, forming R state

strucutural changes of Hb protein from O2 binding affects what 

quaternary strucuture

what does the first O2 binding in Hb allow for

allows binding of additional oxygens to be 10x greater

• cooperativity

when O2 binds in Hb the contacts between what are disrupted?

alpha beta subunits

• a1 & B2

• a2 & B1

what 3 new interactions are made from O2 binding in Hb

hydrophobic, ionic, H bonding

O2 binding in Hb affects solvent channel how

narrower solvent channel

2-3 Bisphosphoglycerate (2,3-BPG) 

stabilizes T state of Hb —> facilitates release of O2

where does 2,3-BPG (bisphosphoglycerate) bind to in Hb

a pocket in Hb tetramer that only exists when Hb is in T state

how Hb oxygen affinity is adjusted for fetal/mother

fetal hemoglobin has higher O2 affinity than adult

• binds O2 when mom’s Hb releases

• fetal Hb does not bind 2,3-BPG (reducer) well

func of 2,3-BPG

reduces O2 binding

what kind of amino acid change happens for fetal/mom Hb

histidine —> serine

• (his = mom, ser = fetus; ser reduces BPG affin, alllows to bind O2 more tightly)

Bohr effect

Hb releases protons upon O2 binding

what role do CO2 and H+ have in hemoglobin

enhance O2 release

• (both are produced by actively respiring tissues)

when pO2 is low in TISSUES

H+ binding releases O2

when pO2 is high in LUNGS

O2 binding releases H+

H+ binding releases O2 when

pO2 is low in tissues

O2 binding releases H+ when

pO2 is high in lungs

when low pO2 what is pCO2

pCO2 is high

when high pO2 what is pCO2

low pCO2

low pH allows for what interactions? this enhances…

allows formation of ionic interactions —> stabilize T state of Hb —> enhances O2 release

CO2 reacts w/ what to form what?

• terminal amino groups

• form neg charged carbamate groups

neg charged carbamate groups do what

form salt bridges that stabilize T state

how CO2 & H+ regulate O2 binding by Hb

• CO2: reacts w/ terminal amino groups —> form carbamate groups —> form salt bridges that stabilize T state

• H+: low pH allows ionic interactions —> stabilize T state

(T state stabilize = more O2 release)

why does T state favor carbamate formation more than R state

T state Hb has more avail amine sites

CO2 is transported to lungs as

bicarbonate

what facilitates formation of bicarbonate ions

carbonic anhydrase

sickle cell anemia is caused by

mutation in Hb

• sub valine instead of glutamate at position 6 of B chains

when is sickle cell anemia fatal

when both alleles of the B chain are mutated

(one mutated/one normal = asymptomatic)

hemoglobinopathies (abnormal Hb)

general Hb region altered

change in:

• surface residues

• internally located residues

• stabilizing metHb

• changes at alpha beta contacts

thalassemia (abnormal Hb)

diseases of globin synthesis & processing

effect of CO on Hb O2 transport

CO binds Hb at O2 binding site —> shifts Hb to R-state —> O2 is not released

• COHb almost always fatal

hemoglobin structure 2.0

• 4 subunits

• allosteric

• cavity

hemoglobin oxygen binding

4 & cooperative

hemoglobin func

oxygen delivery

4 negative effectors in hemoglobin

• BPG

• CO

• H+ 

• CO2

(decrease O2 affin; release O2 to tissues)

myoglobin sturcture 2.0

1 subunit

myoglobin oxygen binding

1

myoglobin func 2.0

diffusion & storage

myoglobin negative effectors

CO