Protein Structure and Function

Introduction

Proteins are essential macromolecules in all living organisms, performing a vast array of functions necessary for life. They are polymers made from amino acids, and their structure determines their function. This guide covers the structure of amino acids and polypeptides, the levels of protein structure, and the role of proteins as enzymes.

Amino Acids and Polypeptides

Structure of Amino Acids

Amino acids are the building blocks of proteins. Each amino acid has a central carbon atom (the alpha carbon) bonded to four groups:

• Amino group (-NH2)

• Carboxyl group (-COOH)

• Hydrogen atom

• Side chain (R group) – this varies among different amino acids and determines their unique properties

Example: Glycine is the simplest amino acid, with a hydrogen as its side chain. Arginine is more complex, with a larger side chain containing several nitrogen atoms.

Formation of Polypeptides

Polypeptides are polymers of amino acids linked by peptide bonds. A peptide bond forms between the carboxyl group of one amino acid and the amino group of another, releasing a molecule of water (a condensation reaction):

• Peptide bond: The covalent bond joining two amino acids in a protein chain.

• Polypeptide backbone: The repeating sequence of atoms along the core of the polypeptide chain, excluding the side chains.

Equation for peptide bond formation:

Levels of Protein Structure

Primary Structure

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

• Example: The protein titin has over 34,000 amino acids in its primary structure.

Secondary Structure

The secondary structure refers to local folding patterns within a polypeptide, stabilized by hydrogen bonds between backbone atoms. The two most common types are:

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

• Beta pleated sheet (β-sheet): A sheet-like arrangement formed by hydrogen bonds between parallel or antiparallel strands.

Tertiary Structure

The tertiary structure is the overall three-dimensional shape of a single polypeptide chain, determined by interactions among side chains (R groups). These interactions include:

• Hydrogen bonds

• Hydrophobic interactions

• Van der Waals forces

• Disulfide bonds (covalent bonds between cysteine residues)

• Ionic bonds

Additional info: The folding and conformation of the protein are critical for its biological activity.

Quaternary Structure

Some proteins consist of more than one polypeptide chain. The quaternary structure describes the arrangement and interaction of these multiple subunits.

• Example: Hemoglobin is composed of four subunits (two alpha and two beta chains).

Summary Table: Levels of Protein Structure

Protein Function

Overview of Protein Functions

Proteins perform a wide variety of functions in cells, including:

• Cellular transport: Channels and co-transporters move substances across membranes.

• Immune defense: Immunoglobulins (antibodies), T cell receptors.

• Storage: Ferritin (iron storage), calmodulin (calcium binding).

• Hormones: Renin, leptin.

• Gene regulation: Transcription factors.

• Osmotic balance: Serum albumin.

• Structural support: Collagen.

• Fuel: Some proteins can be metabolized for energy.

• Enzymes: Catalyze biochemical reactions (approx. 75,000 types).

Enzymes: Protein Catalysts

Definition and Function

Enzymes are proteins that act as biological catalysts, speeding up chemical reactions without being consumed in the process. They are highly specific for their substrates.

• Substrate: The molecule upon which an enzyme acts.

• Active site: The region of the enzyme where the substrate binds and the reaction occurs.

General reaction:

Enzyme Specificity and Efficiency

• Enzymes are reaction-specific; each enzyme typically catalyzes only one type of reaction.

• Turnover number: The number of substrate molecules converted to product per enzyme molecule per second when the enzyme is fully saturated with substrate.

• Rate enhancement: Enzymes can increase reaction rates by factors of to compared to uncatalyzed reactions.

Enzyme Kinetics

• Michaelis constant (): Indicates the affinity of the enzyme for its substrate; lower means higher affinity.

• Enzyme concentration and substrate concentration both affect the rate of reaction.

Michaelis-Menten equation:

where is the reaction rate, is the maximum rate, is the substrate concentration, and is the Michaelis constant.

Regulation of Enzyme Activity

• Cells regulate enzyme activity to control metabolic pathways.

• Enzymes can exist in active and inactive forms, often regulated by other molecules or covalent modifications.

Key Points to Remember

• Proteins are chains of amino acids; the sequence determines the protein's identity and function.

• Protein structure is hierarchical: primary, secondary, tertiary, and quaternary levels.

• The 3D shape of a protein is crucial for its function; altering the shape can disrupt function.

• Enzymes are proteins that catalyze biochemical reactions with high specificity and efficiency.