- A flat ring that contains an iron atom in the center that binds the oxygen atom in a reversible reaction to be able to release oxygen.
- The heme group binds to myoglobin 
- Heme binds four times to hemoglobin 
- Oxygenation of heme causes hemoglobin to change color from dark purple to a scarlet color. 
- In addition to oxygen, however, it can bind things like NO, CO, and H2S and they bind to a higher affinity than oxygen which is evidence of the toxicity of CO.
- CO has a 200-fold greater affinity for hemoglobin than oxygen. 

Heme

- The hemoglobin curve has a sigmoidal (S-shaped) curve instead of the hyperbolic curve for myoglobin.
- Cooperative interaction between the binding sites is displayed, because once a single oxygen binds to hemoglobin the affinity for oxygen at the other sites increases affinity for oxygen at the other subunits. 
- As more oxygen atoms bind to hemoglobin, the slope of the curve increases. 
- Hemoglobin is more saturated with oxygen in arterial oxygen pressures and in venous oxygen pressures hemoglobin is only about half saturated with oxygen. 
- Additionally, the background indicates the color change that occurs with oxygen binding to hemoglobin (blue to red). 


Hemoglobin binding curve

There are many diseases associated with mutations in hemoglobin.
- Sickle-cell anemia 
- Polycythemia 
- Cyanosis
- Hemolytic anemia

Mutations in hemoglobin

- There are additional compounds in the blood stream that effect the oxygen levels besides CO2
- The compound 2,3-bisphosphoglycerate (BPG) binds to deoxyhemoglobin (empty O2 bound hemoglobin) 
- BPG in blood decreases the oxygen affinity by keeping hemoglobin in the deoxyhemoglobin conformation. 
- In the absence of BPG, O2 would not release from the hemoglobin.

2,3-bisphosphoglycerate (BPG)

-  The T-state is the hemoglobin conformational state of deoxyhemoglobin, or the hemoglobin without oxygen bound.
- The iron is situated 6 Angstroms out of the heme and allows for the iron to access oxygen. 


T-State of Hemoglobin

- R-state is the conformational state of oxyhemoglobin.
- When oxygen binds, it “flattens” the iron complex. 
- To prevent steric clash during flattening of the heme, there is an alpha helix that tilts as well.
- Overall, changes in hemoglobin's conformational states at the tertiary level cannot occur without causing quaternary structure changes in the whole protein. 

R-State of Hemoglobin

Hemoglobin is a transport protein with a quaternary structure composed of four globular subunits: two $$\alpha$$ and two $$\beta$$ polypeptide chains, each containing an iron-based heme group.

Grand Valley State University

Google

- The first protein with structure determined by X-ray crystallography 
- Smaller protein with oxygen binding behavior 
- Contains the heme group 
- Facilitates oxygen diffusion in muscles by increasing its solubility in muscle cells.


Myoglobin

The quaternary structure of a protein is the overall multi-subunit configuration formed by the assembly of several separate polypeptide chains (protein subunits), which must all be present and correctly configured for the protein to function. This overall quaternary structure is stabilized by relatively weak interactions.

Quaternary Structure

Voet, D., Voet, J. G., & Pratt, C. W. (2013). Chapter 7: Protein Function: Myoglobin and Hemoglobin, Muscle Contractions, and Antibodies. In Fundamentals of biochemistry: Life at the molecular level (4th ed., pp. 176-216). Hoboken, NJ: Wiley. Retrieved February 21, 2021, from https://archive.org.

Fundamentals of biochemistry: Life at the molecular level, 4th edition: Chapter 7

https://openstax.org/details/books/microbiology/

Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

Microbiology is produced through a collaborative publishing agreement between OpenStax and the American Society for Microbiology Press. The book aligns with the curriculum guidelines of the American Society for Microbiology.

OpenStax Microbiology Textbook

Hemoglobin

- At low oxygen levels, little oxygen is binding to myoglobin so the YO2 is low.
    - YO2 is the level of saturation of the myoglobin.
- With more oxygen in the system, more oxygen binds to the myoglobin.
- It levels off all oxygen binding sites are filled and myoglobin becomes saturated. 


Learn Before

Related

Learn After