Lecture 5: Reversible Binding of a Protein to a Ligand- Oxygen Binding Proteins Part I

Chapter 5.1

What group does oxygen bind to?

a heme group

What are some important characteristics of oxygen?

• poorly soluble in aqueous solutions like blood

• a non-polar molecule

• diffusion through tissues is ineffective over large distances

• transition metals such as iron and copper have a strong tendency to bind

True or False: If copper or iron bind to oxygen alone, then they will form radical hydroxylates, but amino acids cannot bind oxygen well—proving the need for a heme group.

true

What does a heme group consist of?

• complex organic ring structure

• protoporphyrin

• bound Fe2+ atom

True or False: Fe3+ doesn’t bind oxygen well, so a heme group is needed to turn Fe3+ into Fe2+.

true

True or False: Histidine binds to the iron when the heme group is part of the protein.

true

How many coordination bonds do iron atoms form?

6 bonds

What type of protein family are globins?

oxygen-binding proteins

Where, in the body, is rich with myoglobin?

muscles

What is important to know about globins?

• highly conserved tertiary structure

• 8 alpha-helical segments connected by bends (globin fold)

• most function in O2 transport or storage

What are the 4 types of globins?

• myoglobin

• hemoglobin

• neuroglobin

• cytoglobin

What is myoglobin?

a monomeric protein that facilitates O2 diffusion in muscle tissue

What is hemoglobin?

a tetrameric protein responsible for O2 transport in the bloodstream

What is neuroglobin?

a monomeric protein expressed largely in neurons to protect the brain from low O2 or restricted blood supply

What is cytoglobin?

a monomeric protein that regulates levels of nitric oxide

What is nitric oxide?

a localized signal for muscle relaxation

True or False: Myoglobin has a single binding site for oxygen.

true

What is important to know about the structure of myoglobin?

• 153 residues

• 1 molecule of heme

• contains bends

True or False: Myoglobin and hemoglobin are enzymes that catalyze oxygen binding.

true

What is the K_a (association constant)?

provides a measure of the affinity of the ligand L for the protein

What does higher K_a mean?

higher affinity

What is K_a equivalent to?

the ratio of the rates of the forward (association) and the reverse (dissociation) reactions that form the PL complex

Why is the concentration of ligand considered constant?

there’s a higher concentration of oxygen compared to myoglobin, but oxygen binding to myoglobin in the body doesn’t change the concentration of myoglobin much

In the equation Y = [L] / ([L] + (1/K_a)), what does “Y” equal?

“Y” is the amount of oxygen saturation

What does it mean when Y = 0.5?

½ of the available ligand-binding sites are occupied

What is the reciprocal of K_a?

the dissociation constant (K_d)

What is the dissociation constant?

the equilibrium constant for the release of the ligand

What does lower K_d mean?

higher affinity and tight binding

What does high K_d mean?

weak binding

How is oxygen measured?

in partial gas pressure (partial pressure is easier to measure than oxygen concentration)

True or False: Carbon monoxide binds free heme more than 20,000 times better than O₂ does due to differences in the orbital structures (CO has a linear angle and binds tighter).

true

What increases heme’s affinity for O₂ in myoglobin?

the distal His

Why does the distal His increase heme affinity?

there’s a hydrogen bond between the imidazole side chain of His E7 that binds O₂ electrostatically stabilizing the Fe-O₂ polar complex

True or False: Due to myoglobin’s distal His, CO's 20,000-fold stronger binding affinity of the free heme compared to myoglobin reduces to 40-fold.

true