Question

When deoxyhemoglobin was first isolated in crystalline form, the researcher who did so noted that the crystals changed color from purple to red and also changed shape as he observed them under a microscope. What is happening on the molecular level? Hint: The crystals were mounted on a microscope slide with a loosely fitting cover slip.

Short answer

Deoxyhemoglobin binds to oxygen, turning into oxyhemoglobin and changing color from purple to red and altering shape due to structural changes.

Step-by-step solution

- Identifying the Crystals

Understand that the crystals in question are deoxyhemoglobin, which is hemoglobin without oxygen bound to it. Recognize that hemoglobin undergoes a color change when it binds to oxygen.

- Environmental Conditions

The researcher observed the crystals on a microscope slide with a loosely fitting cover slip. This setup likely allowed air (and thus oxygen) to come into contact with the deoxyhemoglobin crystals.

- Oxygen Binding

When deoxyhemoglobin is exposed to oxygen, it binds to the oxygen molecules, transforming into oxyhemoglobin. This binding causes a conformational change in the hemoglobin structure.

- Color and Shape Change

The binding of oxygen to deoxyhemoglobin (forming oxyhemoglobin) results in a color change from purple to red and an associated change in the crystal's shape. These changes are due to the different structural conformations of deoxyhemoglobin and oxyhemoglobin.

Key concepts

Deoxyhemoglobin

Deoxyhemoglobin is a form of hemoglobin that is not bound to oxygen. This form is crucial for transporting carbon dioxide from tissues to the lungs. Deoxyhemoglobin appears purple under certain lighting conditions and is the 'spent' form of hemoglobin after it has released its oxygen load.
When deoxyhemoglobin is isolated in crystalline form, it retains its purple coloration due to the absence of oxygen. This coloration is distinct, and it helps in identifying the deoxy form of hemoglobin.
Understanding deoxyhemoglobin is fundamental in comprehending how hemoglobin functions in oxygen transport. The molecule is in a 'tense' state, making it more likely to release any bound oxygen.

Oxyhemoglobin

Oxyhemoglobin is the oxygen-bound form of hemoglobin. When hemoglobin binds to oxygen in the lungs, it transforms into oxyhemoglobin, which is bright red in color. This color change is a key indicator that oxygen has been successfully bound to the hemoglobin molecules.
Oxyhemoglobin travels via the bloodstream from the lungs to the tissues, where it releases oxygen to be used in cellular respiration. The transition from deoxyhemoglobin to oxyhemoglobin is essential for keeping tissues supplied with oxygen.
When observed under a microscope, as in the case mentioned, the change in color from purple (deoxyhemoglobin) to red (oxyhemoglobin) signifies that oxygen is binding to the hemoglobin crystals.

Conformational Change

Conformational changes refer to the structural changes that occur in a molecule when it undergoes a functional shift. In the context of hemoglobin, these changes happen when it binds or releases oxygen.
When deoxyhemoglobin binds oxygen, a significant conformational change occurs, shifting the structure from the 'tense' (T) state to the 'relaxed' (R) state. This shift is what makes oxygen more readily bind to the remaining iron atoms in the hemoglobin molecule.
These structural adjustments are crucial because they facilitate efficient oxygen binding and release. Without conformational changes, hemoglobin would be less effective at transporting oxygen throughout the body.
During the experiment described, the observed shape change in the crystals is a direct result of these conformational changes occurring at the molecular level.

Oxygen Binding

Oxygen binding is the process by which oxygen molecules attach to hemoglobin. Each hemoglobin molecule can bind up to four oxygen molecules, making it highly efficient in oxygen transport.
The binding of oxygen to hemoglobin is a reversible process, which means that hemoglobin can pick up oxygen in the lungs and release it in tissues where it is needed. When oxygen binds to the iron atoms in hemoglobin, it causes a notable change in the molecule's structure, promoting further oxygen binding.
This process is what maintains the balance of oxygen delivery and carbon dioxide transport in the body. The initial binding of oxygen promotes further binding due to conformational changes, a concept known as cooperative binding.
In the exercise, the observable changes in color and shape of the hemoglobin crystals are indicators of oxygen binding, transforming deoxyhemoglobin into oxyhemoglobin.