Protein Structure and Function

Introduction

Proteins are essential macromolecules that perform a vast array of functions within living cells. Their structure and function are determined by the sequence and properties of their building blocks, the amino acids.

Key Concepts

• Most cellular functions depend on proteins: Proteins act as enzymes, structural components, transporters, and more.

• Amino acids are the building blocks for proteins: Each protein is a polymer made from a unique sequence of amino acids.

• Proteins vary widely in structure: Protein structure is described in a hierarchy of four levels.

• Proteins vary widely in function: Most proteins are enzymes (catalysts), but they also serve in defense, movement, signaling, and transport.

What Do Proteins Do?

Protein Functions

Proteins are crucial for most tasks required for cells to exist. Their functions are diverse and depend on their structure.

Amino Acids: The Building Blocks of Proteins

Functional Groups in Amino Acids

• Amino functional group: (acts as a base to attract a proton)

• Carboxylic acid functional group: (acts as an acid to donate a proton)

• Organic chemistry shorthand: The generic "R" is used to mean any "substituent" or side chain.

Structure of Amino Acids

Each amino acid contains a central carbon atom (the alpha carbon) bonded to:

• An amino group ()

• A carboxyl group ()

• A hydrogen atom

• A variable side chain (R group)

Amino acids can exist in non-ionized and ionized forms, depending on the pH of the environment.

Classification of Amino Acids

There are twenty naturally occurring amino acids in proteins. They are generally grouped according to the chemical properties of their side chains:

• Nonpolar (hydrophobic)

• Polar (hydrophilic)

• Acidic (negatively charged)

• Basic (positively charged)

Examples of Amino Acid Side Chains

Amino Acid Interactions with Water

Amino acids with hydrophobic side chains tend to be found in the interior of proteins, while hydrophilic side chains are often exposed on the surface.

Optical Isomers of Amino Acids

Chirality

Optical isomers (enantiomers) exist when the alpha carbon of an amino acid is bonded to four different groups. These isomers are mirror images and cannot be superimposed, similar to left and right hands.

Protein Polymers: Formation and Breakdown

Polymerization of Amino Acids

Proteins are macromolecules (polymers) made by linking many amino acid monomers through covalent bonds called peptide bonds.

Building a Polymer: Condensation Reaction

• Condensation (dehydration) reaction: Monomer in, water out

• Equation:

Degrading a Polymer: Hydrolysis

• Hydrolysis reaction: Water in, monomer out

• Equation:

Peptide Bond Characteristics

• Amino acids are linked by peptide bonds, which have partial double-bond character and do not rotate freely.

• Peptide bonds are planar due to resonance between the carbonyl and amide groups.

Protein Backbone Flexibility

While the peptide bond itself is rigid, the bonds on either side of the peptide bond can rotate, allowing the protein backbone to be flexible and adopt many shapes.

Protein Diversity: Size and Shape

Structural Diversity

• Proteins can be globular, fibrous, or have other complex shapes.

• Examples: TATA-box binding protein (butterfly shape), Porin (doughnut shape), Pyrophosphatase (globular), Collagen (fibrous).

Basic Levels of Protein Organization

Hierarchy of Structure

• Primary Structure: Linear sequence of amino acids.

• Secondary Structure: Distinctive shaped sections of proteins made by hydrogen bonds between backbone atoms (e.g., alpha helices and beta sheets).

• Tertiary Structure: Three-dimensional shape of the complete protein, determined by interactions among R groups.

• Quaternary Structure: Multiple polypeptide subunits assemble to make a single combined structure.

Example: Hemoglobin

Hemoglobin is a transport protein composed of multiple subunits, illustrating quaternary structure.

Summary Table: Levels of Protein Structure

Additional info: The notes have been expanded to include definitions, examples, and academic context for clarity and completeness.