Introduction
Proteins: Structure and Function
- This summary organizes the information about proteins, protein domains, and protein families into concise sections with clear headings and explanations. It also removes unnecessary details and uses simpler language while maintaining accuracy.
- Functions of Proteins:
- Support: Provide structural support for cells and tissues (e.g., collagen in bones).
- Movement: Enable muscle contraction and cell movement (e.g., actin and myosin in muscles).
- Catalysis: Act as enzymes to speed up chemical reactions in cells (e.g., sucrase breaking down sugar).
- Membrane Transport: Move molecules across cell membranes (e.g., ion channels).
- Recognition and Binding: Allow cells to interact with specific molecules (e.g., antibodies recognizing antigens).
- Homeostasis: Maintain internal balance within the body (e.g., hemoglobin transporting oxygen, albumin regulating blood pressure).
- Gene Expression: Proteins perform various roles in gene expression, including DNA replication, transcription, and translation.
- Protein Composition:
- Polymers made up of 20 different amino acids.
- Amino acids are linked together by peptide bonds to form polypeptide chains.
- Each amino acid has a side chain with unique properties that determines the protein's structure and function.
- Four Levels of Protein Structure:
- Primary Structure: The specific sequence of amino acids in a polypeptide chain. This order is encoded by genes and determines the protein's overall shape and function. Example is the major protein of sperm nucleus, packs DNA
- Secondary Structure: Regular folding patterns within the polypeptide chain, such as alpha helices and beta sheets. These structures form due to hydrogen bonds between backbone atoms. Beta sheets are two types:
Antiparallel:
Parallel:
- the side chains of the amino acid residues dictates which particular polypeptide will form, which can be alpha helix, parallel beta sheet, antiparallel beta sheet, or neither,
- Tertiary Structure: The 3D folding of a single polypeptide chain resulting from interactions between side chains (hydrogen bonds, ionic bonds, disulfide bonds, hydrophobic interactions). Disulfide bonds – S – S – are based on the side chain of the amino acid cysteine:
- The bond is formed by oxidation of two distant cysteines brought in close proximity by the three-dimensional folding:
- Example: Tertiary structure of myoblobin
- Quaternary Structure: The arrangement of multiple polypeptide chains into a functional complex. This level is not present in all proteins.
- hemoglobin is formed by 4 subunits (2 alpha and 2 beta chains), bound non-covalently. Its quaternary structure facilitates binding O2 in lungs and releasing it in other tissues.
- Spatial Structure:
- The 3D arrangement of atoms in a protein molecule.