Chapter 6.3

Globular Proteins: Tertiary Structure and Functional Diversity

Overview of Globular Proteins

Globular proteins are a major class of proteins that perform most of the chemical work within cells. Their compact, folded structures allow for diverse functions, including catalysis, transport, and regulation.

• Chemical Work: Globular proteins are involved in synthesis, transport, and metabolism within the cell.

• Secondary Structures: They possess regions of α-helix and β-sheet secondary structure.

• Tertiary Structure: The polypeptide chain folds into a compact, three-dimensional shape.

• Prosthetic Groups: Many globular proteins carry prosthetic groups, which are small molecules (e.g., heme in myoglobin) that may be covalently or noncovalently bonded to the protein and are essential for function.

Representations of 3D Structures

Understanding protein structure requires various models to visualize the arrangement of atoms and functional regions.

• Ribbon Model: Shows the path of the polypeptide backbone, highlighting α-helices, β-sheets, and loops.

• Stick Model: Displays the locations of all atoms, emphasizing hydrogen bonds and atomic interactions.

• Surface Models: Illustrate the solvent-accessible surface and charge distribution, which are important for protein interactions.

• Example: Human ubiquitin is a small globular protein that targets other proteins for degradation. Its structure can be visualized using ribbon, stick, and surface models.

Folding into Defined Structures with Diverse Functions

Globular proteins exhibit a wide range of tertiary structures, which enable their diverse biological functions.

• Structural Diversity: Proteins contain varying amounts of α-helix, β-sheet, and loop regions.

• Domains: Larger proteins often have two or more distinct domains—compact, independently folding regions, typically ~200 amino acids each.

• Functional Specialization: Each domain may have a specific function, such as DNA recognition, oligomerization, or cofactor binding.

• Examples: Myoglobin (oxygen storage), neuraminidase (viral enzyme), and triosephosphate isomerase (glycolytic enzyme) all display unique tertiary structures suited to their functions.

Additional info:

• Protein Structure Hierarchy: Globular proteins are classified by their secondary structure content (mainly α, mainly β, or mixed), and by their overall architecture and topology.

• Protein Domains: Domains are evolutionary units and often correspond to functional modules within proteins.