5. ENZYMES and METABOLIC REACTIONS

How do reactions occur in cells

•Molecules are in constant motion

•Collisions between molecules allow reactions to occur

Catalysts

allow the body to conserve energy and undergo metabolic reactions

Activation energy

of reactions is too high to be compatible with sustaining life.

Enzymes

•Are proteins that catalyze chemical reactions in our body

•End with the suffix ‘-ase’

•Are specific for substrates

•Can be reused

•Are only needed in small quantities

Enzymatic Reactions

• Catabolic rxn’s

• Anabolic rxn’s

Catabolic reactions

break one substrate into smaller products using hydrolysis

Anabolic reactions

join substrates into a larger product using dehydration synthesis.

active site

Where the substrate joins the enzyme is called the

•‘Lock and Key Model’

The active site of an enzyme is a perfect match to a specific substrate

‘Induced-Fit Model’

•The active site changes shape slightly when the E-S complex join together

•Bringing substrates closer together = increasing substrate concentration

Factors that Affect Enzymes

• Temperature

• pH

• When the concentration of substrate increases the rate of the reaction increases

• When the concentration of the enzyme increases, the rate of the reaction increases

• Inhibitor molecules

How does temperature affects the enzyme

•Reaction rates increase as temperature increases

•Peaks at ~ 37 - 40°C then drops rapidly as enzymes denature

•At low temps, molecules do not have enough energy to undergo reactions.

How does pH affects the enzyme

•Enzymes function within an optimal pH range

•Stomach pH

•Small intestine pH

How does inhibitor molecules affect enzymes

•Inhibitor molecules attach to the enzyme and reduce its ability to bind substrate

•There are two types of inhibitors:

1. Competitive inhibitors

2. Non-competitive inhibitors

Competitive inhibitors

•Attach to enzyme’s active site

•Shape is similar to substrate

•Compete with the substrate

•Often the end product of the reaction

E.g. drugs and poisons

 - CO

 - Methotrexate

Non-competitive inhibitors

•Attach elsewhere on the enzyme (not the active site)

•Attachment changes the 3D shape of enzyme

•Reaction still occurs, but is inhibited

E.g. heavy metals and cyanide

Regulation of Enzyme Activity

• Feedback inhibition

• Precursor activity

Feedback Inhibition

•Turns the path ‘off’

•Prevents accumulation of products

•Final product of pathway interferes with an enzyme by binding with allosteric site and altering the active site

Precursor Activity

•Turns the path ‘on’

•A substrate binds to the allosteric site of the last enzyme in a path

•Speeds up the final product formation