Globular Proteins

Overview of Globular Protein Functions

Globular proteins are a diverse class of proteins characterized by their compact, spherical shapes and solubility in aqueous environments. They perform a wide range of essential biological functions in living organisms.

• Storage of ions and molecules: myoglobin, ferritin

• Transport of ions and molecules: hemoglobin, serotonin transporter

• Defense against pathogens: antibodies, cytokines

• Muscle contraction: actin, myosin

• Biological catalysis: chymotrypsin, lysozyme

Oxygen-Binding Proteins

Importance of Oxygen Binding

Oxygen is essential for cellular metabolism in all tissues. However, oxygen is poorly soluble in water and cannot be efficiently transported or stored without specialized proteins. Myoglobin and hemoglobin are key oxygen-binding proteins that facilitate oxygen storage and transport, respectively.

• Myoglobin (Mb): Stores O2 in muscle tissue.

• Hemoglobin (Hb): Transports O2 in the blood.

Heme Group and Oxygen Binding

Both myoglobin and hemoglobin contain a heme prosthetic group, which is responsible for their oxygen-binding properties and the red color of blood.

• Heme (protoporphyrin IX): A planar molecule with four pyrrole rings coordinated to a central Fe2+ ion.

• Fe2+ Coordination: The iron ion has six coordination sites: four with pyrrole nitrogens, one with a proximal histidine (His F8), and one available for O2 or other ligands.

• Oxygen Binding: When O2 binds, the complex is called oxyhemoglobin or oxymyoglobin. Without O2, it is deoxyhemoglobin or deoxymyoglobin. If CO binds, it forms carboxyhemoglobin.

Structural Representation

The globin fold is a conserved tertiary structure found in both myoglobin and hemoglobin, consisting mainly of alpha helices arranged to create a pocket for the heme group.

Spectroscopic Detection of Oxygen Binding

Principles of Detection

The heme group acts as a chromophore, absorbing light in the ultraviolet and visible spectrum. The absorption properties of hemoglobin differ depending on whether it is oxygenated or deoxygenated.

• Oxyhemoglobin: Found in arterial blood, has a distinct absorption spectrum.

• Deoxyhemoglobin: Found in venous blood, with a different spectrum.

• Application: This difference forms the basis for pulse oximetry, a non-invasive method to monitor blood oxygen saturation.

Carbon Monoxide (CO) vs. Oxygen Binding

Affinity and Toxicity

Carbon monoxide (CO) binds to the heme iron with an affinity approximately 105 times greater than oxygen. Even at very low concentrations, CO can outcompete O2 for binding sites, leading to carbon monoxide poisoning.

• CO Binding: In hemoglobin and myoglobin, CO binds in a more kinked geometry, which slightly reduces its affinity compared to free heme, but it still binds 250 times more strongly than O2.

• Physiological Impact: Small amounts of CO can prevent O2 from binding, impairing oxygen transport and delivery to tissues.

Example: Pulse Oximetry

Pulse oximeters use the spectral differences between oxyhemoglobin and deoxyhemoglobin to estimate blood oxygen saturation, but cannot distinguish CO-bound hemoglobin, which can lead to falsely normal readings in CO poisoning cases.

Summary Table: Functions of Selected Globular Proteins

Additional info:

• The globin fold is highly conserved among oxygen-binding proteins, allowing for efficient and reversible oxygen binding.

• CO poisoning is a medical emergency due to its high affinity for hemoglobin, which prevents oxygen delivery to tissues.