IB Must Know’s Mrs. Selimovic-Milo
3.6.1 Define enzyme and active site
Enzymes
Metro High School
Enzyme: A globular protein that increases the rate of a biochemical reaction by lowering the activation energy threshold
(i.e. a biological catalyst)
Active Site: The site on the surface of an enzyme which binds to the substrate molecule
Activation Energy Threshold – Energy needed to be reached to start of the chemical reaction.
3.6.2 Explain enzyme-substrate specificity
Active site and substrate complement each other in terms of both shape and chemical properties (e.g. opposite charges)
Binding to the active site brings the substrate into close physical proximity, creating an enzyme-substrate complex
The enzyme catalyses the conversion of the substrate into a product (or products), creating an enzyme-product complex
As the enzyme is not consumed in the reaction, it can continue to work once the product dissociates (hence only low
concentrations are needed)
Enzyme-Substrate Specificity
Lock and Key Model
Enzymes and substrates share specificity (a given enzyme will only interact with a small number of specific substrates
that complement the active site)
This explanation of enzyme-substrate interaction is described as the 'lock and key' model (a lock only opens in response
to a specific key)

Compare with Induced Fit Model (7.6.2)
3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity
Temperature
 Low temperatures result in insufficient thermal energy for the activation of a given enzyme-catalysed reaction to
be achieved
 Increasing the temperature will increase the speed and motion of both enzyme and substrate, resulting in higher
enzyme activity
 This is because a higher kinetic energy will result in more frequent collisions between enzyme and substrate
 At an optimal temperature (may differ for different enzymes), the rate of enzyme activity will be at its peak
 Higher temperatures will cause enzyme stability to decrease, as the thermal energy disrupts the hydrogen bonds
holding the enzyme together
 This causes the enzyme (particularly the active site) to lose its shape, resulting in a loss of enzyme activity
(denaturation)
IB Must Know’s Mrs. Selimovic-Milo
pH
Enzymes
Metro High School
 Changing the pH will alter the charge of the enzyme, which in turn will protein solubility and may change the
shape of the molecule
 Changing the shape or charge of the active site will diminish its ability to bind to the substrate, abrogating enzyme
function
 Enzymes have an optimum pH (may differ between enzymes) and moving outside of this range will always result
in a diminished rate of reaction
Substrate Concentration
 Increasing substrate concentration will increase the activity of a particular enzyme
 More substrate means there is an increased likelihood of enzyme and substrate colliding and reacting, such that
more reactions will occur and more products will be formed in a given time period
 After a certain point, the rate of reaction will cease to rise regardless of further increases to substrate
concentration, as the environment has become saturated with substrate and all enzymes are bound and
reacting (Vmax)
Factors Affecting Enzyme Activity
3.6.4 Define denaturation
Denaturation is a structural change in a protein that results in the loss (usually permanent) of its biological properties
 Heat and pH are two agents which may cause denaturation of an enzyme
Denaturation
3.6.5 Explain the use of lactase in the production of lactose-free milk
IB Must Know’s Mrs. Selimovic-Milo
Enzymes
Metro High School
Lactose is a disaccharide of glucose and galactose which can be broken down by the enzyme lactase
Historically, mammals exhibit a marked decrease in lactase production after weaning - leading to lactose intolerance
(incidence is particularly high in Asian / African / Native American / Aboriginal populations)
Lactose-free milk can be produced by purifying lactase (e.g. from yeast or bacteria) and binding it to an inert substance
(such as alginate beads)
Milk passed over this immobilised enzyme will become lactose-free
The generation of lactose-free milk can be used in a number of ways:
 As a source of milk for lactose-intolerant individuals
 As a means to increase the sweetness of milk (glucose and galactose are sweeter in flavour), thus negating the
need for artificial sweeteners
 As a way of reducing the crystallisation of ice-creams (glucose and galactose are more soluble than lactose)
 As a means of shortening the production time for yogurts or cheese (bacteria ferment glucose and galactose
more readily than lactose)
7.6.1 State that metabolic pathways consist of chains and cycles of enzyme-catalysed reactions
 Most chemical changes in a cell results from chains and cycles of reactions, with each step controlled by a
separate specific enzyme
 This allows for a far greater level of control and regulation of metabolic pathways (such as photosynthesis and cell
respiration)
7.6.2 Describe the induced fit model
 When enzymes and substrates bind, the active site is not completely rigid and may undergo a conformational
change in shape to better fit the substrate
 This conformational change may increase the reactivity of the substrate and be necessary for the enzyme's
catalytic activity
 The induced fit model explains how an enzyme may be able to bind to, and catalyse, several different substrates
(broad specificity)
The Induced Fit Model
IB Must Know’s Mrs. Selimovic-Milo
Enzymes
Metro High School
7.6.3 Explain that enzymes lower the activation energy of the chemical reactions that they catalyse
 Every reaction requires a certain amount of energy to proceed - this is the activation energy (Ea)
 Enzymes speed up the rate of a biochemical reaction by lowering the activation energy
 If more energy is in the products than the reactants, energy is lost from the system (endergonic)
 These reactions are usually anabolic (building things up), as the energy is being used up in bond
formation between two substrate molecules
 If more energy is in the reactants than the products, excess energy is released into the system (exergonic)
 These reactions are usually catabolic (breaking things down), as the energy is released from the broken
bonds within molecules
Reaction Pathway of a Typical Exergonic / Exothermic Reaction
7.6.4 Explain the difference between competitive and non-competitive inhibition, with reference to one example of
each
Competitive Inhibition
 A molecule (inhibitor) which is structurally / chemically similar to the substrate and binds to the active site of the
enzyme
 This serves to block the active site and thus prevent substrate binding (competes for the active site)
 Its effect can be reduced by increasing substrate concentration
Example: Relenza is a competitive inhibitor of neuraminidase (influenza virus enzyme), preventing the release of virions
from infected cells
Non-competitive Inhibition
 A molecule (inhibitor) which is not structurally or chemically similar to the substrate and binds to a site other than
the active site (allosteric site)
 This causes a conformational change in the active site, meaning the substrate cannot bind
 Its effect cannot be reduced by increasing substrate concentration as it is not competing for the active site
IB Must Know’s Mrs. Selimovic-Milo
Enzymes
Metro High School
Example: Cyanide (CN-) inhibits enzymes (cytochrome oxidase) in the electron transport chain by breaking disulphide
bonds within the enzyme
Competitive versus Non-competitive Inhibition
7.6.5 Explain the control of metabolic pathways by end-product inhibition, including the role of allosteric sites
End-product inhibition is a form of negative feedback in which increased levels of product decrease the rate of product
formation
 Because metabolic pathways usually consist of chains (e.g. glycolysis) or cycles (e.g. Krebs cycle), the product
can regulate the rate of its own production by inhibiting an earlier enzyme in the metabolic pathway
 The product binds to an allosteric site of an enzyme, causing a conformational change in the active site (noncompetitive inhibition)
 As the enzyme can not currently function, the rate of product formation will decrease (and with less product there
is less enzyme inhibition)
End-Product Inhibition
An example of end-product inhibition is the regulation of ATP formation by phosphofructokinase (an enzyme in glycolysis)
 ATP inhibits phosphofructokinase, so that when ATP levels are high, glucose is not broken down (but instead can
be stored as glycogen)
 When ATP levels are low, phosphofructokinase is activated and glucose is broken down to make more ATP