Protein Structure and Function

Overview and Learning Outcomes

Proteins are essential macromolecules in all living organisms, responsible for a vast array of biological functions. Understanding protein structure and function is fundamental in biology. By mastering this topic, students will be able to:

• Describe how the structure of an amino acid affects protein function and how amino acids polymerize to form polypeptides.

• Explain the four levels of protein structure.

• Predict how bond disruption or amino acid changes affect protein structure.

Proteins: Biological Importance and Diversity

General Properties of Proteins

Proteins account for more than 50% of the dry weight of cells and are responsible for nearly every biological process in organisms. Their structural diversity allows for a wide range of functions.

• Abundance: There are approximately protein molecules per liver cell.

• Diversity: The human body can produce between 50,000 and 2 million different proteins.

• Largest Known Protein: Titin (connectin) contains 34,350 amino acid residues in its canonical form.

Major Functions of Proteins

Proteins perform a variety of functions, each vital to cellular and organismal health.

Amino Acids: Building Blocks of Proteins

Structure of Amino Acids

All amino acids share a common core structure, but differ in their side chains (R groups), which determine their chemical properties and functions.

• Core Structure: Each amino acid contains a central carbon (alpha carbon) bonded to an amino group (), a carboxyl group (), a hydrogen atom, and a unique R group.

• R Group: The R group (side chain) is what makes each amino acid unique and influences its chemical behavior.

General formula:

Classification of Amino Acids

Amino acids are classified based on the properties of their side chains:

Essential Amino Acids: These are amino acids that cannot be synthesized by the human body and must be obtained from the diet.

Protein Synthesis: Polymerization of Amino Acids

Formation of Polypeptides

Proteins are polymers constructed from amino acids linked by peptide bonds. The sequence and chemical properties of amino acids determine the final structure and function of the protein.

• Peptide Bond: A covalent bond formed between the carboxyl group of one amino acid and the amino group of another, releasing water (condensation reaction).

• Polypeptide: A linear chain of amino acids; the primary structure of a protein.

• Protein: The functional molecule resulting from the folding of a polypeptide into its correct three-dimensional shape.

• Structure = Function: The specific shape of a protein determines its biological activity.

Peptide bond formation equation:

Levels of Protein Structure

Primary Structure

The primary structure is the unique sequence of amino acids in a polypeptide, determined by the genetic code.

• Sequence: The order of amino acids from the amino (N) terminus to the carboxyl (C) terminus.

• Importance: Even a single amino acid change can dramatically affect protein function.

Secondary Structure

Secondary structure refers to local folding patterns stabilized by hydrogen bonds between backbone atoms.

• Alpha Helix (-helix): A coiled structure stabilized by hydrogen bonds.

• Beta Pleated Sheet (-sheet): Sheet-like structure formed by hydrogen bonds between parallel or antiparallel strands.

Tertiary Structure

Tertiary structure is the overall three-dimensional shape of a polypeptide, resulting from interactions among R groups.

• Interactions: Includes hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.

• Function: Determines the protein's specificity and activity.

Quaternary Structure

Quaternary structure arises when two or more polypeptide chains (subunits) aggregate to form a functional protein.

• Examples: Hemoglobin (four subunits), collagen (three subunits).

• Not all proteins have quaternary structure.

Protein Folding and Disease

Importance of Correct Folding

Proper folding is critical for protein function. Misfolded proteins are associated with several diseases, known as amyloid diseases.

• Diseases: Alzheimer's Disease, Parkinson's Disease, Amyotrophic Lateral Sclerosis (ALS), Type II Diabetes, Bovine Spongiform Encephalopathy.

• Chaperones: Molecular chaperones assist in correct protein folding.

Example: A single amino acid substitution in hemoglobin leads to sickle cell anemia.

Summary Table: Levels of Protein Structure

Additional info: The notes have been expanded to include definitions, examples, and context for each topic, as well as reconstructed tables for amino acid classification and protein structure levels.