Enzymes – protein catalysts The active site can lower an EA barrier by orienting substrates correctly straining substrate bonds providing a favorable microenvironment directly participating in the reaction

Induced Fit of Substrate e.g. carboxypeptidase 3D structure and the amino acids present at the binding site determine which substrates bind binding of substrate can induce changes in the structure of the enzyme to enhance binding and the catalytic process – induced fit carboxypeptidase is synthesized in the pancreas and secreted into the small intestine hydrolyzes the peptide bond between the last and second last amino acid in a peptide chain

Substate Active site Enzyme (a) (b) Enzyme- substrate complex hexokinase

Zn2+ - an essential cofactor in carboxypeptidase this shows the amino acids that are specifically involved in binding the zinc cofactor

Mechanism of action mechanism of action – show website mechanism…

(b) Competitive inhibition Competitive inhibitors Bind to the active site of an enzyme, competing with the substrate (b) Competitive inhibition A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme (a) Normal binding

(c) Noncompetitive inhibition Noncompetitive inhibitors Bind to another part of an enzyme, changing the function Figure 8.19 A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its active site no longer functions. Noncompetitive inhibitor (c) Noncompetitive inhibition

Examples of allosteric regulation: -phosphorylation -Ca2+/calmodulin example is a tetramer – cleft between each pair is the regulatory site; then each subunit has an active site tetramer goes back and forth between 2 conformations – active and inactive binding of an activator or an inhibitor stabilizes a particular conformation what will change in a cell is the level of the activator and inhibitor – the regulators other possibility is that binding of the substrate to one active site, stabilizes the active form for the other subunits

Active site available Isoleucine used up by cell Feedback inhibition Isoleucine binds to allosteric site Active site of enzyme 1 no longer binds threonine; pathway is switched off Initial substrate (threonine) Threonine in active site Enzyme 1 (threonine deaminase) Intermediate A Intermediate B Intermediate C Intermediate D Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 End product (isoleucine)

Cellular Respiration C6H12O6 + O2 ↔ 6CO2 + 6H2O forward reaction: DG = -686 kcal/mol reverse reaction: DG = 686 kcal/mol