Enzyme Notes

Enzymes:

A type of protein that acts as a catalyst to speed up chemical reactions by lowering activation energy
Enzymes can perform their functions repeatedly without being used up, also called catalytic cycle
The temperatures needed so sustain life would be too high that the heat used to speed up the reactions would denature the proteins, causing the enzymes to be ineffective-why fevers are dangerous

Activation Energy & Enzymes:

Activation energy is the energy required to initiate a chemical reaction
If the activation energy is too large, the reaction will start but be too slow for biological systems

How Enzymes Work:

Enzymes are specific for one particular reaction
Acceleration of reaction does not affect net energy released or absorbed ΔG
Lower Ea by increasing the number of effective collisions OR put stress on existing bonds
Although the activation energy is lower in an enzyme-catalyzed reaction than in an uncatalyzed reaction, the energy released is the same with or without catalysis.
In other words, Ea is lower, but ΔG is unchanged.

Naming Conventions:

Names end in -ase
Named for the compound they break down
- Enzymes used in hydrolysis are hydrolases
- Ex lactase breaks down lactose

Enzymes are Substrate-Specific:

Substrate is the substance or reactant that the enzyme acts on
Active site is the location on an enzyme that substrates bind to
- Each active site only fits on kind of substrate molecule, therefore enzymes are substrate-specific
Enzymes are characterized by specificity, which is a function of their active sites.
Shape & function of active site determined by the sequence of amino acids.
Here chemical bonds can be broken down or bonds b/w substrates can be formed
Each enzyme typically speeds up only one or a few chemical reactions

Lock and Key Model:

People with lactose intolerance don't make enough of the enzyme lactase, which is needed to digest lactose.
During enzyme catalyzed reaction, substrate & enzyme join to form the enzyme-substrate complex.
Chemical bonds within a substrate is broken down or bonds between substrate can be formed (enzymes can work both directions)
Substrate-specific enzymes do not bind to isomers
\n

Induced Fit Model:

Contrary to the lock & key model of specificity, there is NOT an exact perfect fit
As substrate attaches to active site, it changes shape to fit substrate
- Strains existing bonds in substrate, lower Ea needed to break bonds
Binding of a substrate with an enzyme causes a change in enzyme shape & reduce the activation energy of the reaction.
The substrate fit closely into the active site
This causes active site to change shape slightly to accommodate substrate
Once reaction occurs, products of reaction are released from enzymes

Enzyme Action & Hydrolysis of Sucrose:

Enzyme sucrase is involved in process of hydrolysis to split sucrose (disaccharide) into glucose and fructose (monosaccharides)

Factors that Affect Enzyme Function:

Rate of reaction depends upon factors such as:
- Temperature
- pH
- Substrate concentration
Any change in condition that alters the enzymes 3D shape
Temp. too low, bonds that determine shape of enzyme not flexible enough to enable substrates to fit properly; when too hot, bonds too weak to maintain enzyme shape

Acidity and pH:

Substrate Concentration:

As substrate [ ] increases, so does the reaction rate to a point, then it levels off
Maximum rate of reaction reached when enzyme is saturated, as all active sites are filled

Enzyme Helpers:

Some enzymes require presence of additional molecules or ions (non-protein) to catalyze reactions and assist in the functioning of the enzyme
There are two types:
- Coenzymes
- Cofactors
Both are non-protein
Some enzymes require either non-protein cofactors, such as inorganic substances or organic coenzymes, before they can work properly. These may bind to the active site with covalent bonds, or they may bind weakly with the substrate

Coenzyme:

Cofactors:

Enzyme Activity is Regulated by Other Mol:

Inhibitors are molecules that interfere with substrate binding and reduce enzyme activity
- Toxins, pesticides, antibiotics
Two types:
- Competitive
- Non competitive

Competitive Inhibitors:

Bind to enzyme’s active site, competing with substrate
- Penicillin- blocks enzyme that bacteria uses to build their cell walls
When inhibitor present in high [ ] it out competes subtract
Overcome by increasing substrate [ ]

Non-Competitive Inhibitors:

Bind to site other than active site
- Allosteric inhibitors bind to allosteric site causing enzyme to change shape and no longer function (inactive)
Can't be overcome by increasing substrate

Allosteric Regulation:

Conformational changes by regulatory molecules
- Allosteric inhibitors: keeps enzyme in inactive form
- Allosteric activators: keeps enzyme in active form
When activator binds, enzyme remains in active form with active sites available for binding of substrates, thus increasing enzyme activity

Irreversible Inhibition:

Feedback Inhibition:

Each reaction in a metabolic pathway requires specific enzymes
In feedback inhibition, the end product of a metabolic pathway shuts down the pathway
- End product binds with allosteric site
Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed