Chapter 5: Protein Function: Oxygen Binding, Immune System, and Molecular Motors

Hemoglobin and myoglobin are explored as model proteins to illustrate oxygen binding, cooperativity, and allosteric regulation. Section 5.1 outlines how myoglobin binds oxygen in a hyperbolic manner due to its single subunit, while hemoglobin exhibits sigmoidal binding due to its quaternary structure and cooperative interactions between subunits. The structural basis for this cooperativity is explained using the T (tense) and R (relaxed) states of hemoglobin, supported by the concerted and sequential models. Section 5.2 discusses the Bohr effect and the influence of pH and CO₂ on hemoglobin’s oxygen affinity, as well as the role of 2,3-bisphosphoglycerate (BPG) in modulating function under hypoxic conditions. The importance of allosteric modulators in protein function is further reinforced. Section 5.3 transitions into the immune system, introducing antibodies (immunoglobulins) as highly specific binding proteins. The structure of antibodies—including light and heavy chains, variable regions, and antigen-binding sites—is analyzed in the context of immune recognition. Section 5.4 focuses on protein interactions with other macromolecules, including actin and myosin in muscle contraction, emphasizing ATP hydrolysis and the cyclic nature of conformational changes that power cellular motion. It also touches on motor proteins and cytoskeletal rearrangement. Through these examples, the chapter demonstrates how proteins perform diverse and highly specialized roles, from oxygen transport to immune defense and mechanical work, all enabled by structural adaptability and finely tuned binding interactions.