Chapter 3Protein 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 Function1. Catalysis - enzymes that speed up chemical reactions

2. Defense - antibodies and complement proteins attack pathogens

3. Movement - motor and contractile proteins move the cell or molecules within the cell

4. Signaling - proteins that convey signals between cells

5. Structure - structural proteins define cell shape and comprise body structures

6. Transport - carry materials; membrane proteins control molecular movement in and out of the cell.

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.o Nonpolar R-groups: hydrophobic; don not form hydrogen bondso Polar R-groups: hydrophilic; form hydrogen bondso Charged R-groups:

Protein StructureFour-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

Enzymes• Catalysts speed up the rate of a reaction by

lowering the activation energy o 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 siteo substrate (reactant) + enzyme -> product + enzyme

Enzymes Activity• Effect of conditions on enzymes

o pH - work on an optimal pHo temperature - work on an optimal temperature

• Enzyme regulationo Cofactors - metals or coenzymes that help the

enzyme functiono Competitive inhibition - a molecule that binds to the

active site that competes for bindingo Allosteric inhibition - site other than the active site

that inhibits or promotes enzyme activity