Enzyme lab
Concept 8.4 Enzymes speed up metabolic reactions.
• Catalyst
a chemical agent that changes the rate of a reaction without being consumed by the reaction.
• Enzymes – biological catalysts
Enzymes
• Almost all are proteins.
• Names end in “ase ”
Name usually involves cytidine deaminase the substrate with which the enzyme reacts.
• Example: helicase = breaks down helices.
helicase
1 B. Substrate specificity of enzymes
Substrate = reactant that binds to an enzyme .
Fig 8-13 sucrase
Enzyme: sucrase Substrate = sucrose (and only sucrose )
Active site of an enzyme – site at which substrate binds and catalysis occurs
active site
enzyme substrate enzyme-substrate complex
Substrate must “fit” in the active site of the enzyme. Mutations can alter shape of the enzyme or the substrate, reducing or preventing its function.
Enzyme substrate complexes
substrates
2 Substrate binding >> induced fit
Result: chemical groups in position for catalysis .
C. Catalysis in the active site
• Active site R-groups - interact with substrate(s).
• Cofactors may assist in catalysis.
Cofactors
• metal ions e.g., zinc, iron, copper, etc
from trace metals in diet
Coenzymes: small organic molecules
e.g., NAD, FADH
from vitamins in diet
3 Catalytic cycle
Fig 8.17
How does an enzyme catalyze reactions?
• Making bonds: orients substrates for reaction.
Breaking bonds: stresses bonds to be broken
D. Effects of local conditions on enzyme activity
1. Optimal temperature
Increased temperature
rx rate WHY?
Temperature too high
rx rate WHY?
4 Optimal temperature
Fig 8-18
2. Optimal pH
pH 6 - 8 for most enzymes in most organisms
3. Presence or absence of enzyme inhibitors or activators
Small molecules that disable or stimulate enzyme activity
Enzyme inhibitors:
Some are toxins, poisons Some are part of normal enzyme regulation
5 e.g., toxins:
sarin, some snake venoms, malethione
Enzyme: Acetyl cholinesterase
Mode of action of inhibitors: competitive inhibitor • Inhibitor binds to same site as the substrate
Mode of action of inhibitors: noncompetitive inhibitor
• Inhibitor binds somewhere other than active site
6 Concept 8.5 - Regulation of enzyme activity helps control metabolism A. Allosteric regulation of enzymes
Allosteric enzymes: oscillate between 2 conformations.
able to unable to bind bind substrate substrate
Multimeric: many shapes
Conformations stabilized by allosteric activators or inhibitors
non- competitive inhibition Binding site = allosteric site
Metabolic pathways
Substrates can sometimes act as activators.
Products can sometimes act as inhibitors.
E1 E2 E3
A B C D
7 Example: Feedback Inhibition
Pathway regulated by its end product.
8