3.3 & 3.4 Enzymes

LectureRadjewski

Biology, Sixth Edition

Chapter 6, Energy and Metabolism

Enzymes• Enzymes are protein catalysts • enormously speed up reactions. • often have an “-ase” ending to their name.

– e.g., hexokinase, catalase, peptidase, mutase

• Enzymes:1. Lower the activation energy2. Do not add or remove energy3. Do not change the equilibrium 4. Are reused over and over

Biology, Sixth Edition

Chapter 6, Energy and Metabolism

Biology, Sixth Edition

Chapter 6, Energy and Metabolism

Enzymes Lower EA

Activation energy (Ea) is the energy required to break the bonds to begin the reaction

Example: A molecule of sucrose in solution may hydrolyze in about 15 days; with sucrase present, the same reaction occurs in 1 second!

Biology, Sixth Edition

Chapter 6, Energy and Metabolism

A Catalytic Cycle

PEESSE

Active Site

Biology, Sixth Edition

Chapter 6, Energy and Metabolism

Some enzymes require ions or other molecules to function

• Cofactor(s) -- an inorganic component, often an ion such as Ca2+ or Mg2+

• Coenzymes – an organic component – Vitamins– ATP, NADH– Coenzyme A

• Prostethic groups – distinctive non-amino acid atoms that permanently bind to their enzymes – Heme– Flavin– Retinal

Biology, Sixth Edition

Chapter 6, Energy and Metabolism

Enzymes are Very Sensitive

• Each enzyme has an optimal temperature, pH, and ionic strength

• High temperatures rapidly denature enzymes and this is not reversible

• If pH is altered, enzymes become inactive and is not reversible

Biology, Sixth Edition

Chapter 6, Energy and Metabolism

Enzyme Inhibition

• Two types:

• Competitive:• Inhibitor binds at

the active site

• Noncompetitive:• Inhibitor binds at a

site distant from the active site

Biology, Sixth Edition

Chapter 6, Energy and Metabolism

Allosteric Control• If a substance binds to an allosteric site

(another region besides the active site), the conformation of the active site is changed and this affects enzyme activity

Metabolic pathways:

• The first reaction is the commitment step—other reactions then happen in sequence.

• Feedback inhibition (end-product inhibition)—the final product acts as a noncompetitive inhibitor of the first enzyme, which shuts down the pathway.

A B C D F

F might block B C & lower [C] & [D]

E E E E1 2 3 4

Commitment Step