Unit 15 - Myoglobin, Hemoglobin ...

What is the role of myoglobin (Mb)?

Oxygen storage and diffusion in muscle tissue.

What is the role of hemoglobin (Hb)?

Oxygen transport in blood via erythrocytes.

How many hemoglobin molecules are in one erythrocyte?

Approximately 300 million.

What is the quaternary structure of hemoglobin?

α2β2 tetramer.

What cofactor allows O2 binding in Mb and Hb?

Heme group (a porphyrin with Fe2+).

Where does the heme bind in myoglobin?

Between the E and F helices.

Which residues interact with heme in myoglobin?

HisF8 (proximal) coordinates Fe2+; HisE7 (distal) controls O2 access.

What structure dominates the secondary structure of myoglobin?

80% α-helical.

Why does O2 bind at an angle in the heme pocket?

To reduce Fe2+ oxidation and enable reversible binding.

Why is carbon monoxide (CO) dangerous?

It binds heme 200x more tightly than O2.

What is the shape of the O2 binding curve for myoglobin?

Hyperbolic.

What is YO2 in the context of O2 binding?

Fractional saturation: fraction of O2 binding sites occupied.

What is the formula for YO2 in myoglobin?

YO2 = pO2 / (pO2 + P50)

What does P50 represent?

pO2 at which YO2 = 0.5.

What is the typical P50 value for myoglobin?

~3 Torr.

How can oxy vs. deoxy states of hemoglobin be distinguished?

By their different absorption spectra.

What are the four subunits in hemoglobin?

Two α and two β subunits.

What type of protein structure does hemoglobin exhibit?

Quaternary structure.

What is the difference in O2 binding curves for Mb vs. Hb?

Mb = hyperbolic, Hb = sigmoidal.

Why is hemoglobin’s binding curve sigmoidal?

Due to cooperative O2 binding among subunits.

What is the Hill equation for hemoglobin?

YO2 = (pO2^n) / (pO2^n + P50^n)

What does the Hill coefficient (n) indicate?

The degree of cooperativity in O2 binding.

What is the Hill coefficient for hemoglobin?

~3 (high cooperativity).

What is the Hill coefficient at low or high pO2?

Approximately 1 (independent binding).

Why does the fourth O2 bind more easily to Hb than the first?

Cooperative transition from T to R state increases affinity.

What is the P50 for the first and fourth O2 binding to Hb?

First: ~30 Torr; Fourth: ~0.3 Torr.

Why does Hb deliver O2 efficiently to muscle?

Because Hb binds O2 at high pO2 and releases it at low pO2, where Mb binds it.

What is the T state of hemoglobin?

“Tenser” conformation; low O2 affinity.

What is the R state of hemoglobin?

“Relaxed” conformation; high O2 affinity.

What structural change occurs at heme upon O2 binding?

Fe2+ moves into the heme plane (~0.6 Å).

What structural change occurs in the F helix upon O2 binding?

F helix shifts ~1 Å due to movement of HisF8.

What changes at the α1-β2 interface during T→R?

Subunit rearrangement and salt bridge disruption.

What stabilizes the T state in Hb?

A network of inter-subunit salt bridges.

What breaks the T state salt bridges?

Energy from Fe–O2 bond formation.

What is the net rotational change during T→R transition?

~15º rotation between αβ dimers.

What concept explains positive cooperativity?

Binding of one O2 enhances affinity for others.

What is the physiological relevance of T↔R switching?

Enables O2 loading in lungs and unloading in tissues.

What model explains Hb’s allosteric behavior via symmetry?

MWC model (Monod-Wyman-Changeux).

What does the MWC model assume?

All subunits exist in T or R state, not mixed.

What model explains sequential binding changes?

Koshland’s sequential model.

What does the sequential model propose?

Ligand binding induces a conformation change in each subunit independently.

How do the MWC and sequential models differ?

MWC = concerted all-or-none; sequential = stepwise, subunit by subunit.

How does Hb actually behave in terms of models?

Exhibits features of both MWC and sequential models.

What kind of protein is hemoglobin considered?

An allosteric protein.

What are allosteric effects?

Binding at one site affects binding at other sites (can be positive or negative).

What causes the T→R transition in Hb?

O2 binding alters Fe2+ position, shifting F helix and subunit interfaces.

What is a Hill plot used for?

To visualize cooperativity via log(YO2/(1-YO2)) vs. log(pO2).

What is the slope of a Hill plot at YO2 = 0.5?

Equals the Hill coefficient (n).

Why is a sigmoidal curve ideal for Hb function?

It allows tight control over O2 binding and release across physiological pO2 range.

What kind of binding curve would non-cooperative Hb have?

A hyperbolic curve similar to myoglobin’s.