Learn & Review: Proteins & Amino Acids | Biochemistry

Proteins: The Building Blocks of Function

This summary outlines the structure, formation, and crucial functions of proteins as a type of macromolecule.

Main Idea

Proteins are essential macromolecules composed of amino acids, which fold into complex three-dimensional structures to perform a vast array of functions in the body.

Key Components of Proteins

• Atomic Composition: Like carbohydrates and fats, proteins contain carbon, hydrogen, and oxygen. A key difference is the inclusion of nitrogen.
• Monomer Unit: The basic building block of proteins is the amino acid.
• Polymer Unit: Amino acids link together to form polypeptides, which then fold into functional proteins.

Structure of an Amino Acid

Each amino acid shares a core structure but differs in a specific functional group:

• Amine Group: A nitrogen atom bonded to two hydrogen atoms (-NH₂).
• Carboxyl Group: A carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (-COOH). This is also known as a carboxylic acid group.
• Functional Group (R-group): This variable group attached to the central carbon atom is what distinguishes one amino acid from another.

Types and Properties of Amino Acids

• Number of Types: There are 20 different types of amino acids.
• Essential Amino Acids: A subset of amino acids (about 6-8) that the body cannot synthesize and must be obtained from the diet.
• Functional Group Properties: The R-group dictates the amino acid's characteristics:
  • Hydrophobic: Repel water.
  • Hydrophilic: Attract water.
  • Acidic: Have acidic properties.
  • Basic: Have basic properties.
  • Charged: Can carry a positive or negative charge.

Formation of Polypeptides

• Process: Amino acids link together through a process called dehydration synthesis.
• Mechanism: A water molecule (H₂O) is removed when the hydroxyl group (-OH) from the carboxyl end of one amino acid and a hydrogen atom (H) from the amine end of another amino acid are joined.
• Result: This forms a polypeptide chain, which is a polymer of amino acids.

Protein Folding and Structure

• Linear Polypeptide: Initially, amino acids form a long, linear chain.
• Folding Triggers: The diverse functional groups (R-groups) on the amino acids influence how the polypeptide chain folds upon itself.
  • Charge Interactions: Positive and negative functional groups attract each other, while like charges repel, causing folding.
  • Water Affinity: Hydrophobic groups tend to embed themselves within the protein's core, away from water, while hydrophilic groups are exposed to the aqueous environment.
  • Acidic/Basic Properties: These also contribute to the folding pattern.
• Three-Dimensional Structure: The folding process results in a complex, three-dimensional structure, often referred to as the quaternary structure (though this term can also refer to the arrangement of multiple polypeptide subunits). This folded structure is what defines a functional protein.

Functions of Proteins

Proteins are incredibly versatile and perform numerous vital roles in the body:

• Structural Components: They form the majority of tissues like skin and eyes.
• Enzymes: Proteins act as enzymes, catalyzing biochemical reactions millions of times faster than they would occur naturally.
• Transport: They can shuttle molecules by binding to them and moving them to different locations.
• Receptors: Proteins can act as receptors, binding to other molecules to initiate cellular responses.
• Overall Function: Proteins are responsible for virtually all functions within the body.

Importance of Protein Structure and Mutations

• Functional Significance: The specific three-dimensional folded structure of a protein is directly responsible for its function.
• Impact of Alterations: Even a single amino acid change (e.g., from acidic to basic) can alter how a protein folds, thereby changing its function.
• DNA Mutations: Mutations in DNA can lead to changes in amino acid sequences, potentially affecting protein structure and function.