ENZYMES
[Activation energy]: energy to start a reaction. Requires energy input to
break bonds
Catalyst: substance that stresses chemical bonds & speeds up reaction in
forward and/or reverse directions. Catalysts lower the activation energy
needed for the reaction to occur. Enzymes are not changed or consumed
by the reaction!
Enzymes: Special proteins that act as biological catalysts
a) Globular proteins with specialized 3-D shapes
b) Lower activation energy by;
 bringing two substrates together (greater chance to react)
 stress bonds of a substrate (to break them)
Substrate: The reactant/s that the catalyst or enzyme binds with to in turn
speed up the reaction.
How enzymes work
Substrate specific: each enzyme catalyzes only one reaction (works on only 1-2
substrate/s)
[Active site]: surface shape or cleft on the enzyme to which the substrate must
fit like a lock and key.
The substrate and enzyme bound together produce the enzyme-substrate
complex.
NOTE: Enzymes are NOT reactants! The reaction should still occur without itit just speeds it up.
LOCK and KEY Model
Each enzyme generally catalyzes one reaction, therefore enzymes are specific
to a certain substrate the same way a key is specific to a lock.
This is because the protein (the enzyme) has a specific shape and only certain
substrates will fit into the active site.
Enzyme structure is very important since a less than perfect fit means enzyme
inactivity or dysfunction.
[A Second Model of how an Enzyme Works]
INDUCED FIT MODEL
Induced fit: enzyme changes shape of active site so substrate fits perfectly.
This allows some enzymes to catalyze several different reactions.
This model is the currently favoured model of enzyme activity.
In this model the active site is not as rigid as originally suggested.
The initial interaction between substrate and enzyme is weak. The weak interactions
rapidly lead to a conformational change in the enzyme. In other words, its shape will
adapt to allow several slightly different substrates to fit.
Ex. A glove can fit many hands but not a foot.
http://www.wiley.com/legacy/college/boyer/0470003790/animations/enzyme_binding/enzyme_binding.htm
[Factors affecting enzyme activity]
a) Temperature:
 Higher temperatures cause more collisions among molecules
 Therefore, increased likelihood that a substrate will collide with
an active site and a reaction will take place
 A temperature that is too high disrupts hydrogen bonds and alters
protein shape (denatures)
 Denaturation: the structural change in a protein that causes loss of
properties/functions that is usually permanent
b) pH:
 each enzyme has an optimal pH
 hydrogen ion concentration disrupts ionization of bonds between
amino acids, thus altering the proteins shape
c) Substrate Concentration:
 Increased substrate concentration increases reaction rate until all
enzymes are involved, then reactions level out
 High concentrations cause more collisions between molecules. I.e.
the enzymes are more likely to encounter molecules of reactants.
 The maximum rate of reaction is reached when the active sites of
all the enzymes are continuously filled
d) Enzyme Concentration:
 Increased enzyme concentration increases reaction rate until
all substrate is used up, then reactions decrease.
Enzymes in Action
1. Proteases: Protein-digesting enzymes added to detergent to remove
protein stains from clothes.
2. Pectinases: Enzyme that removes pectin from crushed fruit. Keeps juice
from gelling, keeps it liquid for easier removal. Helps remove solids from
juice.
3. Amylase: the enzyme responsible for catalyzing the breakdown of starch
into smaller sugars (i.e maltose). Secreted in saliva and the small intestine.
4. Lactase: the enzyme responsible for catalyzing the digestion of lactose
 Lactose intolerance- when a person no longer produces lactase. As
a result if such a person ingests food with high lactose the person
will suffer from bloating, cramping, gas production and diarrhea
 Biochemical food industry makes lactose free milk by adding
lactase, which breaks down lactose into glucose and galactose
resulting in a sweeter taste
Note: The scientific name of all enzymes ends in -ase.
[Enzyme inhibitors]: change shape of active site and shut off activity
a) [Competitive inhibitors]: Competitor molecule binds at active site,
prevents substrate from binding.
b) [Non-competitive inhibitors]: End product binds at a allosteric
(different site), changes shape of active site. Substrate can't bind.
c) Allosteric site: region where non-competitive inhibitor binds changing
shape of enzyme.
Enzyme activators: bind to allosteric
sites, keep enzyme in active state,
increase activity of enzyme.
Biochemical Pathways
Metabolic pathways are made up of many chemical reactions and these
reactions are catalysed by enzymes.
Several enzymes are placed side by side on membranes within cells, each
enzyme completing one of the many steps to convert 1 substance into another.
("Assembly Line")
Often, the product of the last reaction in the pathway inhibits the enzyme that
catalyses the first reaction of the pathway.
This is called end-product inhibition
There is a clear advantage in using end-product inhibition for controlling
metabolic pathways. When there is an excess of end-product, the whole
metabolic pathway is shut down as the end product inhibits the first enzyme of
the pathway.
Online Animations:
Amino Acids, Proteins, Dehydration Synthesis & Protein Shapes
Dehydration Synthesis & Hydrolysis (RM Chute) (See examples concerning proteins &
carbos at bottom of that page too!)
Denaturation of a Protein (Cooking an Egg)
Factors Affecting Enzyme Activity
Enzymes (Prentice Hall) - Comprehensive review of enzymes. Click on Web Tutorial
6.2, then on Animation.
How Do Enzymes Work? (McGraw Hill)
Enzymes (Click all but the Allosteric Enzymes link!)
How Do Enzymes Work? (Starch Digestion)
How Temperature Affects Molecular Motion (BEC)
Enzyme Activity
Noncompetitive Enzyme Inhibition (Allosteric Enzymes)
Biochemical Pathways (Several different enzymes in close proximity that help change a
substrate via several steps into its product)