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
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