1. Protein Structure and Function
Chapter 3
Protein Structure and Function

2. Key Concepts Most cell functions depend on proteins.
Amino acids are the building blocks of proteins. Amino acids vary in structure and function because their side chains vary in composition

3. Key Concepts, cont’d
Proteins vary widely in structure. The structure of a protein can be analyzed at four levels that form a hierarchy - the amino acid sequence, substructures called alpha helices and beta pleated sheets, interactions between amino acids that dictate a protein’s overall shape, and combinations of individual proteins that make up larger, multi-unit molecules.

4. Key Concepts, cont’d
In cells, most proteins are enzymes that function as catalysts. Chemical reactions occur much faster when they are catalyzed by enzymes. During enzyme catalysis, the reactants bind to an enzyme’s active site in a way that allows the reaction to proceed efficiently.

5. Protein Function Catalysis - enzymes that speed up chemical reactions
Defense - antibodies and complement proteins attack pathogens
Movement - motor and contractile proteins move the cell or molecules within the cell
Signaling - proteins that convey signals between cells
Structure - structural proteins define cell shape and comprise body structures
Transport - carry materials; membrane proteins control molecular movement in and out of the cell.

6. Protein Function

7. The Amino Acid Monomer (basic unit), Polymer (chain)
Amino acid (aka peptide) is the basic unit of a protein (aka polypeptide) chain
Amino acids have R-groups that differ in their size, shape, reactivity, and interactions with water.
Nonpolar R-groups: hydrophobic, no charge
Polar R-groups: hydrophilic, partially (+) or (-) charged
Charged R-groups: completely (+) or (-) charged

8. The Amino Acid

9. Protein Structure
Four-Level Hierarchy Resulting from Hydrogen Bonding:
Primary Structure - chain of amino acids
Secondary Structure - amino acids organized in alpha helices or beta sheets
Tertiary Structure - interactions between alpha helices and/or beta sheets
Quaternary Structure - two or more tertiary structures

10. Protein Structure

11. Enzymes
Catalysts speed up the rate of a reaction by lowering the activation energy
Activation energy - the energy it takes for the reaction to progress
The reactants is the substrate and the site on the enzyme that binds to the substrate is called the active site
substrate (reactant) + enzyme -> product + enzyme

12. Enzymes Activity Effect of conditions on enzymes
pH - work on an optimal pH
temperature - work on an optimal temperature
Enzyme regulation
Cofactors - metals or coenzymes that help the enzyme function
Competitive inhibition - a molecule that binds to the active site that competes for binding
Allosteric inhibition - site other than the active site that inhibits or promotes enzyme activity