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Myoglobin Hemoglobin

Myoglobin (Mb) and Hemoglobin (Hb) serve different functions in the body, with Hb transporting oxygen in red blood cells and Mb facilitating oxygen diffusion in muscle cells. Both proteins contain heme, which binds oxygen, but they differ in structure and quaternary conformations, with Hb existing in T (deoxy) and R (oxy) states. Additionally, CO, NO, and H2S can bind to heme with higher affinity than O2, affecting Hb's oxygen binding capacity.

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11 views22 pages

Myoglobin Hemoglobin

Myoglobin (Mb) and Hemoglobin (Hb) serve different functions in the body, with Hb transporting oxygen in red blood cells and Mb facilitating oxygen diffusion in muscle cells. Both proteins contain heme, which binds oxygen, but they differ in structure and quaternary conformations, with Hb existing in T (deoxy) and R (oxy) states. Additionally, CO, NO, and H2S can bind to heme with higher affinity than O2, affecting Hb's oxygen binding capacity.

Uploaded by

dilloncviljoen
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© © All Rights Reserved
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Myoglobin (Mb) and Hemoglobin (Hb) have

related, but different, roles in the body


Hemoglobin:
• Found in red blood cells
• Promotes diffusion of O2
throughout the body
(binds O2 at lungs,
releases at tissues)

Myoglobin:
• Found in muscle cells
• Promotes diffusion of O2
into and throughout
muscle cell
The oxygen-binding curves of Mb and Hb
reflect their different functions
Myoglobin is a single-subunit, α-helical
protein, with a heme cofactor that binds O2
A porphyrin ring forms the base structure of
heme (with different hemes differing at X)
The heme of myoglobin and hemoglobin is a
protoporphyrin IX with a bound Fe2+
In the globins, the heme iron binds O2 and
the ‘proximal’ histidine of the protein
Heme is held in place by the proximal His
and by hydrophobic residues

distal

proximal
Myoglobin is structurally similar to the
subunits of hemoglobin
Hemoglobin is a heterotetramer with two α-
& two β-subunits (a dimer of αβ protomers)

β2
α2

α1
β1
Hemoglobin can adopt two conformations,
called ‘deoxyhemoglobin’ & ‘oxyhemoglobin’

α2 β1
α2 β1

β2 α1 β2 α1

deoxyhemoglobin oxyhemoglobin
Hb’s conformations are also called ‘T-state’
(for tense) and ‘R-state’ (for relaxed)

(deoxyhemoglobin) (oxyhemoglobin)
Oxygen binding promotes flattening of the
porphyrin ring and shifting of helix F
The proximal His links flattening of the heme
to shifting of helix F in the T→ R transition
Movement of helix F shifts the entire
quaternary structure of hemoglobin
The T and R states have shifted contacts
between α & β subunits

T-state

R-state
The T and R states have shifted contacts
between α & β subunits

T-state R-state
The T and R states have shifted contacts
between α & β subunits
T-state salt bridges are broken in the R-state
Heme also binds CO, NO, and H2S
(with much higher affinity than O2)

P50 of CO binding to free heme is ~20,000x lower


than P50 of O2 binding to free heme
Globin structure reduces heme affinity for CO

P50 of CO binding to globin-bound heme is ~200x lower


than P50 of O2 binding to globin-bound heme
CO is a competitive inhibitor and positive
effector of O2 binding to hemoglobin

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