Proteins: Structure and Function

Overview of Proteins

Proteins are a major class of biomolecules essential for the structure, function, and regulation of the body's tissues and organs. They are polymers made of amino acid monomers linked by covalent bonds known as peptide bonds.

• Proteins: Polymers composed of amino acid monomers.

• Peptide bonds: Covalent bonds that link adjacent amino acids together in a protein chain.

• Proteins have directionality, with a N-terminal (amino end) and a C-terminal (carboxyl end).

Example: Formation of proteins from amino acid monomers involves the joining of amino acids via peptide bonds, resulting in a polypeptide chain with specific directionality.

Amino Acids

Structure and Properties of Amino Acids

Amino acids are the building blocks of proteins. Each amino acid contains a central carbon atom (the alpha carbon) bonded to four different groups: an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group).

• Amino acids: Monomers of proteins.

• Each amino acid has:

  • A central carbon atom (alpha carbon)

  • An amino group (–NH2)

  • A carboxyl group (–COOH)

  • A hydrogen atom

  • A unique R group (side chain) that determines the properties of the amino acid

• Living organisms use 20 different amino acids, each with a distinct R group.

Example: The general structure of an amino acid can be represented as:

• Central carbon atom (C)

• Amino group (–NH2)

• Carboxyl group (–COOH)

• Hydrogen atom (–H)

• R group (side chain, varies among amino acids)

Table: Components of an Amino Acid

Protein-Related Terms

Classification by Chain Length

Proteins and their subunits are classified based on the number of amino acids in the chain. The terminology changes as the chain length increases.

Protein Structure

Levels of Protein Structure

Proteins have a hierarchical structure organized into four levels, each contributing to the protein's final shape and function.

1. Primary structure: The unique sequence of amino acids in a polypeptide chain.

2. Secondary structure: Local folding of the polypeptide chain into structures such as alpha helices and beta sheets, stabilized by hydrogen bonds.

3. Tertiary structure: The overall three-dimensional shape of a single polypeptide chain, determined by interactions among R groups.

4. Quaternary structure: The association of two or more polypeptide chains to form a functional protein complex.

Example: Hemoglobin is a protein with quaternary structure, composed of multiple polypeptide subunits.

Denatured Proteins & Chaperones

Protein Denaturation and Chaperone Proteins

The structure and shape of a protein are critical for its function. Denaturation is the process by which a protein loses its native shape due to changes in environmental conditions, such as pH, temperature, or salt concentration. Chaperone proteins assist in the proper folding and refolding of proteins.

• Denatured protein: A protein that has lost its functional shape.

• Denaturation can result from environmental changes (e.g., heat, pH, salt concentration).

• Chaperone proteins: Specialized proteins that help other proteins fold correctly or refold after denaturation.

Example: Heat can denature enzymes, causing them to lose their catalytic activity. Chaperone proteins can sometimes help restore the correct folding.

Practice Questions (Examples)

• Which two functional groups are always found in amino acids? (Answer: Amino group and carboxyl group)

• What is the term for an amino acid chain with more than 50 covalently linked amino acids? (Answer: Polypeptide)

• Which of the following is true of protein structure? (Sample correct answer: Peptide bonds join the amino group on one amino acid with the carboxyl group of another amino acid.)

• What is the role of a chaperone protein? (Answer: Assist in protein folding and refolding.)

Additional info: The notes include practice questions to reinforce understanding of protein structure and function. The content is suitable for introductory college-level General Biology courses.