4.14 : Globular Proteins

In organisms, proteins are the most abundant macromolecules. They act as the building blocks of life and play various crucial roles in the body. Proteins can be broadly classified into two distinct subtypes based on their shape and solubilities: globular proteins and fibrous proteins.

Globular proteins serve many important physiological functions, such as acting as enzymes, cellular messengers, and molecular transporters. These roles often require the proteins to be soluble in the aqueous cellular environment. Hemoglobin, immunoglobulin, and protein kinase A are examples of globular proteins.

Shape

Globular proteins, also known as spheroproteins, are typically round in shape due to the stacking of amino acid chains to form a compact 3-dimensional or tertiary structure. Larger globular proteins are usually composed of two or more recognizable and distinct structural and functional units called domains or modules. A domain is a self-contained section of a protein capable of independently folding into its three-dimensional structure. However, there are limited ways protein domains can fold since certain specific three-dimensional arrangements of α-helices, β-sheets, and loops are more energetically favorable than others.

Solubility

Globular proteins have higher solubility because of the structural arrangement of the hydrophilic and hydrophobic amino acids. Notably, the hydrophobic amino acids are hidden inside the core of the protein molecule. In contrast, the hydrophilic amino acids are positioned on the protein's outer surface. The polar amino acids on the surface of the globular proteins interact with polar solvents, such as water, enhancing their solubility.

Stability

Non-covalent interactions, including hydrogen bonds, Van der Waals forces, and non-polar hydrophobic interactions, maintain the 3-dimensional structure of globular proteins. Due to these weak interactions, globular proteins have a relatively unstable structure. This makes them more vulnerable to minor changes in pH or temperature. For example, hemoglobin, the oxygen-transporting protein in the body, works only in a narrow pH range of 7.35 to 7.45. If the pH drops below 7.35, it induces conformational changes in hemoglobin's native structure and inhibits its binding to oxygen.