Protein Structure and Function Chapter 3 Protein Structure and Function

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

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.

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.

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.

Protein Function

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

The Amino Acid

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

Protein Structure

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

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