Protein Structure and Function

Introduction to Proteins

Proteins are essential macromolecules found in all living organisms, performing a vast array of functions necessary for life. Their diversity in structure and function arises from the unique sequence and composition of their building blocks: amino acids.

• Proteins are polymers made from amino acids.

• There are tens of thousands of unique proteins in living organisms.

• Amino acids were discovered in experiments simulating early Earth conditions (e.g., Stanley Miller's experiment) and have also been found in meteorites, suggesting their abundance during the origin of life.

Attributes of Life and the Role of Proteins

For a molecule to be considered a potential spark of life, it must possess three key attributes:

• Information: The ability to store and transmit information.

• Replication: The capacity to make copies of itself.

• Evolution: The potential to undergo changes that can be acted upon by natural selection.

Amino Acids: The Building Blocks of Proteins

Structure of Amino Acids

All proteins are made from just 20 standard amino acids. Each amino acid shares a common structure but differs in its side chain, which determines its properties.

• Each amino acid consists of a central carbon atom (the α-carbon) bonded to:

  • A hydrogen atom (H)

  • An amino group (–NH2)

  • A carboxyl group (–COOH)

  • A variable side chain (R-group)

• The side chain (R-group) is what distinguishes one amino acid from another.

Amino Acids in Water

Amino acids can exist in different forms depending on the pH of their environment, especially in aqueous (water-based) solutions.

• The amino group acts as a base and can accept a proton (H+).

• The carboxyl group acts as an acid and can donate a proton.

• At physiological pH (~7), amino acids typically exist in their zwitterionic (ionized) form, with a positively charged amino group (–NH3+) and a negatively charged carboxyl group (–COO–).

Classification of Amino Acid Side Chains (R-Groups)

The chemical properties of amino acids are determined by their R-groups, which can be grouped into three main types:

• Charged (hydrophilic): Includes both acidic (–) and basic (+) side chains. These amino acids are water-soluble and can form ionic bonds.

• Polar (hydrophilic): Side chains that are uncharged but contain atoms (like oxygen or nitrogen) that can form hydrogen bonds with water.

• Nonpolar (hydrophobic): Side chains that are mostly hydrocarbons and do not interact favorably with water.

Hydrophilic amino acids dissolve in water and often participate in interactions with other charged or polar molecules. Hydrophobic amino acids tend to cluster away from water, influencing protein folding.

Formation of Proteins: Peptide Bonds and Polymers

Polymerization of Amino Acids

Proteins are polymers, meaning they are made by linking together many amino acid monomers. This process is called polymerization.

• Amino acids are joined by peptide bonds through a condensation (dehydration) reaction, which releases a molecule of water.

• The reverse process, hydrolysis, breaks peptide bonds by adding water.

Peptide bond formation equation:

Directionality of Polypeptides

• Polypeptides have an N-terminus (amino end) and a C-terminus (carboxyl end).

• By convention, sequences are written from the N-terminus to the C-terminus (left to right).

Flexibility of the Polypeptide Backbone

• The peptide bond itself is rigid, but the single bonds on either side of the peptide bond can rotate, allowing the polypeptide chain to be flexible and fold into complex shapes.

Terminology

• A short chain of amino acids (usually fewer than 50) is called an oligopeptide or simply a peptide.

• Longer chains are called polypeptides.

• A protein is a complete, functional form of a polypeptide.

Levels of Protein Structure

Primary Structure

The primary structure of a protein is its unique sequence of amino acids. This sequence determines all higher levels of structure and ultimately the protein's function.

• Even a single change in the amino acid sequence can drastically alter protein function (e.g., sickle cell anemia).

Secondary Structure

The secondary structure refers to local folding patterns within a polypeptide, stabilized by hydrogen bonds between backbone atoms.

• Common secondary structures include the α-helix and β-pleated sheet.

• Hydrogen bonds form between the carbonyl oxygen of one amino acid and the amide hydrogen of another.

Tertiary Structure

The tertiary structure is the overall three-dimensional shape of a single polypeptide chain, resulting from interactions among R-groups and between R-groups and the backbone.

• Stabilized by hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, and disulfide bridges (covalent bonds between cysteine residues).

Quaternary Structure

The quaternary structure arises when two or more polypeptide chains (subunits) associate to form a functional protein complex (e.g., hemoglobin).

• Stabilized by the same types of interactions as tertiary structure.

Protein Folding and Function

• Protein folding is crucial for function; misfolded (denatured) proteins are often nonfunctional.

• Some proteins require molecular chaperones to fold correctly.

Functions of Proteins

Proteins perform a wide variety of functions in cells:

• Catalysis: Enzymes speed up chemical reactions.

• Defense: Antibodies attack pathogens.

• Signaling: Proteins transmit signals between cells.

• Structure: Proteins provide support and shape to cells and tissues.

• Transport: Proteins move molecules across cell membranes or throughout the body.

Enzymes are a particularly important class of proteins that catalyze biochemical reactions, often with high specificity and efficiency.

Summary Table: Levels of Protein Structure

Key Learning Objectives

• Describe the basic structure of an amino acid.

• Explain how side chains affect the structure and function of amino acids.

• Understand the synthesis of a peptide chain, including peptide bond formation via dehydration synthesis.

• List and describe the four levels of protein structure.

• Explain the relationship between protein folding and function.

• Provide examples of why proteins are essential to cell function.

• Define what an enzyme is and why enzymes are needed for biological reactions.