ENZYMES and METABOLIC REACTIONS

ENZYMES and METABOLIC REACTIONS

• How do reactions occur in cells?– Molecules are in constant motion– Collisions between molecules allow

reactions to occur

ENZYMES and METABOLIC REACTIONS

• How do reactions occur in cells?– Molecules are in constant motion– Collisions between molecules allow

reactions to occur

• How do we speed up reactions in cells?

ENZYMES and METABOLIC REACTIONS

• Enzymes– Are protein catalysts that allow chemical

reactions to take place in our body without increasing the temperature

– End with the suffix ‘-ase’

ENZYMES and METABOLIC REACTIONS

• Enzymes– Are protein catalysts that allow chemical

reactions to take place in our body without increasing the temperature

– End with the suffix ‘-ase’– Examples: urease, amylase, sucrase

Catalysts

• Control the speed of reactions without changing the products formed– By reducing the activation energy

Catalysts

• Control the speed of reactions without changing the products formed– By reducing the activation energy– Tunnel vs. Climbing a mountain

• Remain unchanged and can be used over and over

• Often only needed in small amounts

Enzymes…

• Work on molecules called the substrate– Can be anything

Enzymes…

• Work on molecules called the substrate– Can be anything

• Are substrate-specific

Enzymes…

• Work on molecules called the substrate– Can be anything

• Are substrate-specific

• Alter the substrate in some way

• Examples:

Substrate approaches an enzyme

The enzyme-substrate complex is formed

Reaction is complete. Enzyme remains unchanged.

Products are formed.

Enzymes Models

• Where the substrate joins the enzyme is called the active site

• ‘Lock and Key Model’– The active site of an

enzyme is a perfect match to a specific substrate

Enzymes Models

• ‘Induced-Fit Model’– The active site

changes shape slightly when the E-S complex join together

– Makes a tighter fit

Factors that Affect Enzymes

• What happens at cooler temperatures?

1. Temperature– Reaction rates

increase as temperature increases

– Peaks at ~ 37 - 40°C then drops rapidly

– Why?• E.g. egg frying

Factors that Affect Enzymes2. pH

– Enzymes function within an optimal pH range• Stomach pH• Small intestine pH

Factors that Affect Enzymes

3. Concentration of Substrate Molecules– Reaction rate increases as the substrate

concentration increases up to a point– Animation link– The limiting factor in the reaction may be

the amount of substrate or the amount of enzyme available

Factors that Affect Enzymes4. Inhibitor molecules

– Molecules that attach to the enzyme and reduce its ability to bind substrate

– There are two types of inhibitors:

a. Competitive inhibitors

b. Non-competitive inhibitors

4. Inhibitor molecules

a. 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

- Cyanide

4. Inhibitor molecules

a. 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

Regulation of Enzyme Activity

• Feedback Inhibition– Animation– Turns the path ‘off’– Prevents accumulation of products– Final product of pathway interferes with an

enzyme by binding with allosteric (regulatory) site and altering the active site

Regulation of Enzyme Activity

• Precursor Activity– Animation– Turns the path ‘on’– A substrate binds with the last enzyme in a

path improving the fit of the E-S complex– Binds to the allosteric site– Speeds up the final product formation

Regulation of Enzyme Activity

• Both feedback inhibition and precursor activity are called allosteric activity

• Handout