Overview
This lecture covers protein structure, including amino acid composition, the four levels of protein organization, and how sequence and bonds determine protein function.
Amino Acids and Protein Formation
- Proteins are polymers made of amino acids, their monomers.
- Each amino acid has a central carbon bonded to an amino group (NHâ‚‚), carboxyl group (COOH), hydrogen, and a variable R group.
- There are 20 different amino acids, each with a different R group.
- Amino acids join via condensation reactions, forming peptide bonds and releasing water.
- The water is removed from the carboxyl group (OH) of one amino acid and the amino group (H) of another.
Levels of Protein Structure
- Primary structure: the specific sequence of amino acids in a polypeptide chain.
- Secondary structure: polypeptide chain forms alpha helices or beta pleated sheets, held by hydrogen bonds.
- Hydrogen bonds form between the oxygen of the carboxyl group (C=O) and the hydrogen of the amino group (N-H).
- Tertiary structure: further folding into a unique 3D shape, stabilized by hydrogen, ionic, and disulfide bonds between R groups.
- Disulfide bonds form only between sulfur-containing amino acids.
- Quaternary structure: protein structure with two or more polypeptide chains (e.g., hemoglobin).
Protein Function and Denaturation
- The specific sequence of amino acids determines bond locations and the unique 3D shape.
- The shape of a protein determines its function (e.g., enzyme active site or carrier protein specificity).
- Changes in temperature or pH can break hydrogen and ionic bonds, leading to loss of structure (denaturation).
- Denaturation disrupts protein function due to loss of the specific 3D shape.
Mutations and Protein Structure
- Mutations are changes in DNA that can alter the amino acid sequence.
- A change in sequence changes bond locations, the 3D shape, and potentially the function of the protein.
Key Terms & Definitions
- Amino acid — the monomer unit of proteins, containing a central carbon, amino group, carboxyl group, R group, and hydrogen.
- Peptide bond — bond formed between amino acids during condensation reactions.
- Primary structure — the sequence of amino acids in a protein.
- Secondary structure — local folding into alpha helices or beta sheets via hydrogen bonds.
- Tertiary structure — overall 3D shape formed by further folding, held by various bonds.
- Quaternary structure — protein structure with more than one polypeptide chain.
- Denaturation — loss of protein structure and function due to bond disruption.
- Mutation — change in DNA sequence that can alter protein structure.
Action Items / Next Steps
- Review the diagrams of protein structures and practice drawing amino acids.
- Learn the types and locations of bonds at each structural level.
- Watch the recommended biochemical tests video to learn how to test for protein presence.