E) Organic Catalysts: Enzymes

catalyst: inorganic or organic substance which speeds up the rate of a chemical reaction without entering the reaction itself

enzymes: organic catalysts made of protein

-- most enzyme names end in -ase

-- enzymes lower the energy needed to start a chemical rx. (activation energy)

-- begin to be destroyed above 45 C. (above this temperature all proteins begin to be destroyed)

It is thought that, in order for an enzyme to affect the rate of a reaction, the following events must take place.

1. The enzyme must form a temporary association with the substance or substances whose reaction rate it affects. These substances are known as substrates.

2. The association between enzyme and substrate is thought to form a close physical association between the molecules and is called the enzyme-substrate complex.

3. While the enzyme-substrate complex is formed, enzyme action takes place.

4. Upon completion of the reaction, the enzyme and product(s) separate. The enzyme molecule is now available to form additional complexes.

** Although enzymes may be reused in cells, they eventually are destroyed and new ones must be synthesized.

Active site: the pockets in an enzyme where the reacting molecules (substrate) fit

(Usually enzyme molecules are much larger than the molecules they interact with.)

(Composition of Enzymes)

-- all enzymes are composed of proteins or proteins attached to other molecules

coenzyme -- non-protein part attached to the main enzyme (usually a vitamin or a mineral)

[How do enzymes work?]

substrate: molecules upon which an enzyme acts

the enzyme is shaped so that it can only lock up with a specific substrate molecule

enzyme

substrate -------------> product

"Lock and Key Theory"

-- each enzyme is specific for one and ONLY one substrate (one lock - one key)

-- this theory has many weaknesses, but it explains some basic things about enzyme function

[Factors Influencing Rate of Enzyme Action]

A few enzymes (such as gastric protease) work best at a pH of about 2.0

3. Concentrations of Enzyme and Substrate

** When there is a fixed amount of enzyme and an excess of substrate molecules -- the rate of reaction will increase to a point and then level off.

** This leveling off occurs because all of the enzyme is used up and the excess substrate has nothing to combine with.

-- If more enzyme is available than substrate, a similar rx. rate increase and leveling off will occur. (The excess enzyme will eventually run out of substrate molecules to react with.)