Overview
This lecture covers the structure and functions of globular proteins, focusing on hemoglobin, insulin, pepsin, and their unique properties compared to fibrous proteins.
Globular Proteins: Key Features
- Globular proteins are roughly spherical in shape and soluble in water.
- They have hydrophobic R groups buried inside and hydrophilic R groups on the outside.
- Their structure allows high solubility in aqueous environments like blood and cells.
- Globular proteins typically have metabolic roles including enzymes, hormones, and transport proteins.
Hemoglobin
- Hemoglobin is a globular protein found in red blood cells responsible for oxygen transport.
- It has a quaternary structure with four polypeptide chains: two alpha-globin and two beta-globin.
- Each polypeptide chain has a prosthetic group called a heme group, which contains an iron ion (Fe2+).
- Each heme group binds one oxygen molecule, so one hemoglobin can carry four oxygen molecules.
- Hemoglobin is a conjugated protein (protein plus non-protein prosthetic group).
Insulin
- Insulin is a hormone produced by the pancreas to help regulate blood glucose concentration.
- It consists of two polypeptide chains (A: alpha helix, B: beta-pleated sheet) linked by disulfide bridges.
- Hydrophilic R groups on the outside make insulin soluble in blood plasma for transport.
- Insulin binds to specific cell receptors and lowers blood glucose when it becomes too high.
Pepsin
- Pepsin is a globular enzyme found in the stomach that breaks down proteins into shorter polypeptides.
- It functions in a very acidic environment (pH ~2), where most proteins would denature.
- Pepsin’s structure contains few basic R groups, preventing it from being affected by low pH.
- Its tertiary structure is stabilized by hydrogen bonds and disulfide links.
Key Terms & Definitions
- Globular Protein — A roughly spherical, water-soluble protein with metabolic roles.
- Hydrophobic R Group — Amino acid side chain that repels water and is found inside globular proteins.
- Hydrophilic R Group — Amino acid side chain attracted to water, found on the protein's exterior.
- Quaternary Structure — Protein structure formed from multiple polypeptide chains.
- Conjugated Protein — Protein with a non-protein prosthetic group attached.
- Prosthetic Group — Non-protein component necessary for a protein's function (e.g., heme).
- Disulfide Link/Bridge — Covalent bond between sulfur atoms in amino acids stabilizing protein structure.
- Enzyme — Protein that catalyzes chemical reactions.
Action Items / Next Steps
- Review the structures and functions of hemoglobin, insulin, and pepsin.
- Understand how specific amino acid properties affect protein structure and function.
- Be prepared to identify and describe key characteristics of globular proteins.