Chapter 3Protein Structure and Function

Key ConceptsMost 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’dProteins 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’dIn 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 reactions2. Defense - antibodies and complement proteins attack

pathogens3. Movement - motor and contractile proteins move the

cell or molecules within the cell 4. Signaling - proteins that convey signals between cells5. Structure - structural proteins define cell shape and

comprise body structures6. Transport - carry materials; membrane proteins control

molecular movement in and out of the cell.

Protein Function

The Amino AcidMonomer (basic unit), Polymer (chain)Amino acid (aka peptide) is the basic unit of a protein

(aka polypeptide) chainAmino acids have R-groups that differ in their size, shape,

reactivity, and interactions with water.Nonpolar R-groups: hydrophobic, no chargePolar R-groups: hydrophilic, partially (+) or (-) chargedCharged R-groups: completely (+) or (-) charged

The Amino Acid

Protein StructureFour-Level Hierarchy Resulting from Hydrogen Bonding:

Primary Structure - chain of amino acidsSecondary Structure - amino acids organized in alpha

helices or beta sheetsTertiary Structure - interactions between alpha helices

and/or beta sheetsQuaternary Structure - two or more tertiary structures

Protein Structure

EnzymesCatalysts speed up the rate of a reaction by

lowering the activation energy Activation energy - the energy it takes for the reaction

to progressThe 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 ActivityEffect of conditions on enzymes

pH - work on an optimal pHtemperature - work on an optimal temperature

Enzyme regulationCofactors - metals or coenzymes that help the

enzyme functionCompetitive inhibition - a molecule that binds to the

active site that competes for bindingAllosteric inhibition - site other than the active site that

inhibits or promotes enzyme activity