Enzymes
- Most chemical reactions need an input of energy to start called
activation energy.
- Most chemical reactions in a cell need so much activation
energy that it would harm the cell if it had the energy available
or the reactions are too slow for cell to be useful.
- Cells solution to this problem is to use a chemical called a
catalyst.
- A catalyst is a chemical that will alter the rate of a chemical
reaction, without being modified, by lowering the activation
energy needed.
- Cells generally use enzymes which are just protein catalysts.
Two chemical reactions speeds
Energy
given off
Energy
needed
Activation energy
w/out enzyme
Activation energy
w/ enzyme
reactants
products
Rate of reaction
- Reactants an enzyme works on are called substrates.
- Location where substrate attaches to enzyme is called the active
site.
- Enzymes (proteins) have a very specific 3-dimensional shape
that is critical to its function.
- Each enzyme can only work on one type of substrate.
Ex: Enzyme sucrase works on substrate sucrose.
- Current model for how an enzyme most likely works is called
“Lock and Key” model:
product(s) – glucose & fructose
substrate(s) - sucrose
active site
enzyme
(sucrase)
enzyme
(sucrase)
Sucrose
C
enzyme
(sucrase)
enzyme
(sucrase)
Glucose + Fructose
- Enzymes are not affected by the reaction and can be used over
and over again.
- When an enzyme loses its shape it is no longer able to function.
An enzyme that has lost its functional shape is called denatured.
- Two environmental influences can denature an enzyme:
a) Temperature – each enzyme has a very specific temperature
that it can function in. Change the
temperature and it will denature the enzyme.
Ex: Siamese cats have an enzyme that
produces the dark colors at it’s tips
(nose, ears, paws, & tail). The enzyme
is found all over the body but is
denatured everywhere but at the tips
where it is cooler.
b) pH
– each enzyme has a very specific pH that it
can function in. Change the pH and it will
denature the enzyme.
Ex: An enzyme that works to break down
starch in your mouth (pH 7) will not
work in your stomach (pH 2) because it
denatures and vice-versa.
- Besides losing its shape, the amount of enzyme or substrate will
effect its functionality. More substrate or more enzyme will
increase how quickly the enzyme works and forms products.