Protein Structure and Enzyme Function How Protein Activity Depends on Proper Folding

Four Levels of Protein Structure Primary (Sequence) Tertiary (Folding by R-group interactions) Depending on the sequence of amino acids, the polypeptide chain can either form a spiral like the tube on the left, or a pleated sheet like on the right. Some proteins have a spiral section followed by a sheet section, etc. Fig 2.26, p35. Quaternary (Two or more chains associating) Secondary (Coiling by Hydrogen Bonding)

The “R” Group Differs for Each Amino Acid Proteins: Structure Primary structure = chain of amino acids Amino acids have common features Carboxylic Acid Group Amino Group     See p 34 R Central Carbon The “R” Group Differs for Each Amino Acid

Proteins: Structure Forming the Protein Chain Phenylalanine Leucine Here we position two amino acids close together so that the amine group of one is close to the carboxyl group of the other. Note that a -H is hanging out from one, and a -OH from the other. These will be broken off (by an enzyme) to form water. The remaining bonds of the two amino acids will be connected to form a dipeptide (two amino acids joined by a peptide linkage). Dehydration Synthesis between COOH & NH2

Proteins: Structure Forming the Protein Chain The Peptide Bond Water Phenylalanine-Leucine Dipeptide

Four Levels of Protein Structure Secondary Structure: Folded structure due to hydrogen bonds between the amino and acid groups of amino acids N C N C H O H or O O H O H C N C N

Four Levels of Protein Structure Tertiary Structure: Three dimensional folded structure due to attractions and repulsions between R groups Can involve covalent bonding hydrogen bonding ionic interactions hydrophilic and hydrophobic interactions

Four Levels of Protein Structure Quaternary Structure: Association of two or more protein chains eg. Hemoglobin is composed of 4 protein chains 2 are called alpha hemoglobin 2 are called beta hemoglobin

Applying Your Knowledge Primary Secondary Tertiary Quaternary Which structure results from hydrogen bonding? Which structure involves an association of two or more protein chains? Which structure describes the linear sequence of amino acids? Which structure depends upon interactions between the R groups of the amino acids?

Enzymes Assist in Biological Reactions Enzymes are biological catalysts. biological: composed of protein or, rarely, RNA catalyst: speeds up a reaction without being changed by the reaction

Properties of Enzymes Most enzymes are proteins. All enzymes are SPECIFIC for the reactants (substrates) in the reactions that they catalyze. Enzymes speed up biological reactions by lowering the activation energy for the reaction.

Enzyme-Substrate Interactions 1 Substrates enter active site Active Site 2 Shape change promotes reaction Enzyme Product released; enzyme ready again

Activation Energy: Controls Rate of Reaction Energy content of molecules high low Activation energy without catalyst Activation energy with catalyst Progress of reaction

Properties of Enzymes Three dimensional structure of an enzyme preserves its ACTIVE SITE active site: region on enzyme where reactants bind Conditions that can affect three dimensional structure include: heat, pH (acid/base balance) and other chemicals (salt, charged ions)

Effects of Temperature and pH on Enzymatic Activity fewer collisions between enzyme and substrate enzyme unfolds enzyme unfolds

Applying Your Knowledge Active Site Activation Energy Enzyme Catalyst A biological catalyst is a(n) __________ . The region on an enzyme where reactants bind is the _________. Enzymes (raise or lower) the (1, 2, 3 or 4) of a reaction.