Enzymes (p.36-40)

1.3 Biochemical Reactions

Chemical reaction

Activation energy:

certain amount of energy required to begin any

chemical reaction

Enzyme

name usually ends in –ase (eg. Amylase)

protein catalyst

speeds up a chemical reaction without being

consumed

reduces the activation energy

Activation Energy is reduced by:

Bring two substrates together in correct position

stretch/bend bonds

Transfer electrons

Add/remove hydrogen ion

destabilize

substrate

Enzyme Structure

very specific

Specific 3-D shape allows binding to one specific substrate

Specific function for specific biochemical reaction

Enzyme + Substrate Enzyme-substrate complex

Induced-Fit Model

1. substrate binds to the active siteof enzyme

2. functional groups come close

3. enzyme protein change its shape better grip

4. Forms enzyme-substrate complex

5. reaction occurs

6. products less affinity for enzyme released

7. enzyme returns to original shape recycled

Factors affecting Enzyme activity

1. Temperature

2. pH

3. Enzyme concentration

4. Substrate concentration

1. Temperature

Too low: bonds not flexible enough for substrate to

fit properly

Too high: enzyme denatures

2. pH

Bonds sensitive to H+ concentration

3. Enzyme concentration

More enzymes

more likely to

collide with substrates

increased rate of

reaction

Can saturate

4. Substrate Concentration

More substrates

increased rate of

reaction

Until Point of saturation

(all enzymes of occupied)

Enzyme - Regulation Mechanisms

Cofactors/Coenzymes

Competitive inhibitors

Non-Competitive

inhibitors

Allosteric Changes

Feedback Inhibition

Cofactors/Coenzymes

Cofactors:

nonprotein

inorganic substances

Zn2+, Mn2+

Coenzyme:

organic substances

NAD+

derivative of Vitamin B3

electron carrier in cellular respiration

NADP+

electron carrier in photosynthesis

Cofactors/Coenzymes

placed within the

active site of the

enzyme

helps to draw

electrons from the

substrate molecules.

Competitive Inhibitors

similar to substrate

block active site

reversible

overcome by

increasing [substrate]

Noncompetitive Inhibitors

not compete for active site

bind to another site on enzyme (allosteric site)

change shape

prevents substrate from binding

DDT, poison, inhibits enzymes of nervous system

Video: Competitive and non-competitive inhibition.mp4.mp4

Allosteric Changes

Allosteric site:

receptor site some distance away from the active site

Allosteric activator:

stabilizes active form of enzyme

Allosteric inhibitor:

stabilizes inactive form of enzyme

Feedback Inhibition

for a series of sequential reactions

each step has specific enzymes

a product in later steps inhibits enzyme for earlier step

binds to allosteric site of enzyme using weak bonds

Feedback Inhibition

Cells use this process to

prevent overproduction

of a certain product