Cell Biology:Enzymes

Lesson 2 – Factors Affecting Enzyme Activity(Inquiry into Life pg. 106-109)

Today’s Objectives Analyze the roles of enzymes in

biochemical reactions, including: Apply knowledge of proteins to explain

the effects on enzyme activity of pH, temperature, substrate concentration, enzyme concentration, competitive inhibitors, and non-competitive inhibitors including heavy metals

Factors Affecting Enzyme Activity There are several factors that have an

effect on enzyme activity, such as: 1) pH 2) Temperature 3) Substrate concentration 4) Enzyme concentration 5) Competitive inhibition 6) Non-competitive inhibition (ex: heavy metals)

1) pH Each enzyme operates best at a preferred

pH level Any other pH affects tertiary structure

and slows down reactions Tertiary structure: remember, enzymes are

proteins! Too much of a change in pH stops the

reaction Why? Recall the tertiary structure of proteins:

Tertiary Structure

Bonds between amino acids on the polypeptide help maintain the protein’s shape:Hydrogen bonds + electrostatic interactions = “Salt bridges”

1) pH Since salt bridges depend on ionic charges for their

bonding power, anything which neutralizes such a charge will destroy the salt bridge and make the folded structure of the enzyme (protein) less stable

If an enzyme’s normal shape is destroyed, it is said to be “denatured” and is no longer able to combine efficiently with its substrate

An increase of pH will take an H+ from an NH3+ group and neutralize its charge.

A decrease in pH will put an H+ on a COO- and neutralize its charge

This means that each enzyme has an optimum pH which its folded (active) structure its most stable.

It has its maximum catalytic power at that pH Animation

2) Temperature Cold temperature slows down enzymatic reactions Warm temperatures up to around 40°C will speed up

reactions High temperatures will denature enzymes and cause

reactions to stop If we increase the temperature of the solution the

enzymes are operating in, we will typically see an increase in the reaction rate until a point is reached at which the enzyme starts to unfold

This is a result of breaking hydrophobic bonds and salt bridges between R-groups of the protein as the increase in temperature causes the enzyme’s structure to “wiggle” around

Animation

3) Substrate concentration The more concentration of substrate, the

greater the rate of reaction If we do a series of experiments where we

maintain the same enzyme concentration, but increase the substrate concentration each time, we find that the reaction rate increases as we increase the substrate concentration…..

But eventually a maximum reaction rate will be reached

3) Substrate concentration Eventually, adding substrate will not increase

the reaction rate any further At this point, we say that the enzyme is

saturated To increase the rate again, we would need to

add more enzyme

4) Enzyme concentration Adding more

enzymes will increase the rate of reaction

More catalyst means a faster reaction, so the reaction rate increases

Reaction rate is basically “how much substrate reacts in a particular amount of time” (usually per second)

5) Competitive Inhibition Some molecules are shaped

like a substrate and compete with the substrate for the enzyme’s active site

Example: carbon monoxide (CO) competes with Oxygen (O2)

5) Competitive Inhibition Since some of the enzymes get bonded to the “wrong”

substrate (competitive inhibitor), the amount of “correct” product is reduced

Sometimes these molecules bond temporarily with the enzyme, but sometimes they bond permanently

When this happens, the enzyme is rendered useless If too many important enzymes are inactivated, the

organism may die

5) Competitive Inhibition A molecule which fits into an enzyme’s active

site, but doesn’t react with anything there, is called a competitive inhibitor

If this molecule (an inhibitor) is in the active site, a substrate molecule can’t get in, and that particular enzyme molecule is inactive until the inhibitor falls off

6) Non-competitive inhibition Certain molecules can

bond with parts of enzymes (other than the active site) and cause them to change shape (denature)

This bonding is called non-competitive inhibition

Example: Heavy Metals such as Pb2+ (lead) or Hg2+ (mercury)

6) Non-competitive inhibition A molecule that bonds to an enzyme (other

than in the active site), causing the enzyme to change it’s shape is called a non-competitive inhibitor

Once the enzyme changes shape, it is inactive (no products can form)

When the molecule bonds to the enzyme, the folding of the enzyme changes a little bit, and the active site is distorted in a way which makes it a less effective catalyst

Inhibitor Animation