Oxygen-binding proteins

• Oxygen is required for aerobic organisms to carry out cellular respiration

• Oxygen is not very soluble in aqueous environments like blood, so oxygen would not be able to get to the tissues on its own

Why do complex organisms need oxygen-binding proteins?

Prosthetic groups allow a protein to carry out a function it would not be able to do without its prosthetic group. A heme group is a prosthetic group to both myoglobin and hemoglobin.

What is a prosthetic group? Give an example of one.

Oxygen binds to the central iron: Fe (II)

In heme groups, where does oxygen bind?

Fe (II)

What is the oxidation state of Iron in heme groups?

The protein the heme group is bound to (whether its myoglobin or hemoglobin) protects Fe (II) from irreversibly oxidizing to Fe (III)

If the oxidation from Fe (II) to Fe (III) is irreversible, how does the transport of oxygen continue to occur?

X-axis: how much oxygen is present (oxygen concentration)

Y-axis: how much oxygen is bound to the given protein

In fractional saturation curve, what is on the x and y axes?

how much oxygen could be bound to the protein compared to how much oxygen is actually bound to the protein

A fractional saturation curve shows…

Find the halfway point of saturation on the y-axis and trace a line until you hit the curve. Trace a line down from that point to the x-axis, this is your p50.

How do we estimate p50 from a fractional saturation curve?

p50 is the concentration of oxygen needed for the protein to be half saturated with oxygen.

What does p50 mean?

weaker

A protein with a higher p50 than another protein has a __ affinity for oxygen than the other protein

• Myoglobin 1:1

  • 1 Myoglobin: 1 Oxygen

• Hemoglobin 1:4

  • 1 Hemoglobin: 4 Oxygen

Describe the stoichiometry of oxygen binding of myoglobin compared to hemoglobin

weaker, hemoglobin

Which has a higher p50 value, and therefore binds __ to oxygen? Myoglobin or hemoglobin?

Myoglobin delivers oxygen only to actively respiring muscle which has very low oxygen concentration. We know low oxygen concentration decreases affinity, so a higher affinity is needed for myoglobin to hold onto oxygen long enough to give to respiring muscles.

Hemoglobin has a weaker oxygen affinity and therefore an easier time letting go of oxygen so the rest of the body’s tissues can receive oxygen

Why would myoglobin have a stronger affinity to oxygen than hemoglobin?

sigmoidal (S-Shape) curve

What does cooperative binding look like on a fractional saturation plot?

oxygen concentration, Bohr effect (Ph), and 2,3 BPG

O2 binding in hemoglobin is regulated by __, __, and __

Basic

According to the Bohr effect, __ pHs lower the p50 for oxygen in hemoglobin, which means oxygen is more strongly bound to oxygen

microenvironment, protein, bloodstream

In the Bohr effect, the __ is changed in the __, not the __

increase, decrease

According to the Bohr effect, basic conditions __ affinity for oxygen, while acidic conditions __ affinity for oxygen

reduces

2,3 BPG __ hemoglobin’s affinity for oxygen

2,3 BPG

Without __, hemoglobin would bind oxygen too tightly and would not let go

2,3 BPG, 2,3 BPG

__ is very negative, so all the positively charged side chains in the beta subunits of hemoglobin engage in ionic bonds with __

not yet bound, locks in/stabilizes, deoxy state

Because 2,3 BPG only binds when oxygen is __ to hemoglobin, we say that having 2,3 BPG bound to HB __ the __

hyperbolic, sigmoidal

Myoglobin has a __ curve, while hemoglobin has a __ curve.