Download enzymes - charlestonbiology

Section 6 – Metabolism & Enzymes
Unit 1 – Human Cells
Cell Metabolism
 The sum of all the biochemical reactions that occur











in a living cell
- most of these are connected within a complex
network
- most reactions are catalysed by enzymes
Metabolic Pathways:
2 types
Catabolic:
- the breakdown of complex molecules into simpler
ones
- involves the release of energy
Anabolic:
- the biosynthesis of complex molecules from
simpler ones
- requires the input of energy
These two pathways often depend on one another
Two types of pathway
 Aerobic respiration is a type of
catabolic pathway
 - releases energy (ATP)
 This energy can then be used for the
synthesis of protein from amino acids
 - an anabolic pathway
 - requires energy
Reversible and Irreversible steps
 Metabolic pathways have both reversible and












irreversible stages
Glycolysis converts glucose to pyruvic acid
- involves many intermediates along the way
Glucose is converted to intermediate 1 by
enzyme 1
- this is irreversible
Ensures levels of glucose stay low within a
cell
- allows more glucose to diffuse into the cell
Intermediate 1 converting to intermediate 2
is reversible
- aided by enzyme 2
- if the cell has an excess of intermediate 2, it
can be reversed to intermediate 1
- this can then be used in an alternative
pathway
Intermediate 2, to intermediate 3 is
irreversible
- glycolysis MUST continue
Alternative Routes
 Allow for steps in a pathway to be
bypassed
 In glycolysis, glucose can be
converted into sorbitol
 - this bypasses 3 intermediates, and
rejoins the pathway at a later stage
 Used when there is a plentiful
supply of sugar
Activation energy & enzyme action
 Chemical reactions require





chemical bonds to be broken in the
reactants
The energy needed to break these
bonds is the activation energy
Bonds break when reactants absorb
energy to make them unstable
- now in a transition state
- reaction can proceed
Energy input is often in the form
of heat
Properties & Functions of a catalyst
 Catalysts:
 - lower the activation energy for a






chemical reaction
- speed up the rate of a chemical
reaction
- take part in the reaction, but
remain unchanged
Living cells can’t withstand high
temperatures
- require a catalyst to help reactions
proceed at low temperatures
Biological catalysts are called
enzymes
Biochemical reactions couldn’t
proceed without enzymes
Enzyme action
 Enzymes are made of protein
 On their surface they have an





active site
The shape of this site is determined
by the chemical structure and
bonding of amino acids within the
polypeptide chains
Enzymes are specific
- only act on one substrate
Chemical structure of substrate
complements the active site
- shows an affinity (chemical
attraction)
Induced fit and orientation
 The active site is flexible and







dynamic
It’s shape changes slightly to
incorporate the substrate
- an induced fit
If 2 substrates are involved, the
active site determines their
orientation
The active site weakens
chemical bonds in the substrates
- lowering the activation energy
Once complete, products have a
low affinity for the active site
- products separate
Direction of enzyme action
 Metabolic pathways involve many reactions
 - each involving an enzyme
 E.g. enzyme 2 only works when substrate B is available
 - a product of one reaction (intermediate metabolite) is the substrate for




the next
Most metabolic pathways are reversible
Enzymes can go into reverse, and convert intermediates back
E.g. If C was in excess, enzyme 2 could convert it back to B
Enzymes often form part of a multi-enzyme complex
Factors affecting enzyme action
 Enzymes require a suitable







temperature, pH and a supply of
substrate to be able to function
Substrate concentration:
At low concentrations, the
reaction rate is low
- too few substrates to fill the
active sites
Increasing concentration, more
active sites filled
- increasing rate of reaction
Eventually all active sites are filled
(saturation)
- reaction rate levels off
Control of metabolic pathways
 Metabolic pathways involve




several stages
Involve metabolites
(intermediates) being
converted by an enzyme
Each enzyme is coded for by a
gene (or genes)
If an enzyme is absent, the
pathways stop
Therefore, gene expression can
regulate enzyme action
Enzyme control – switching genes
on/off
 Some metabolic pathways are only required to operate at






certain times
Enzymes are therefore only produced when needed
- helps conserve cell resources
- e.g. ATP, amino acids
Genes for producing enzymes can therefore be switched “on”
or “off ” when required
- enzyme induction
E.g. In E.Coli, enzymes only active if lactose is present
Gene action in E.coli
 E.coli can only obtain glucose by breaking down lactose
 Lactose is digested by the enzyme B-galactosidase
 The enzyme is only produced when lactose is present
 This is gene expression being regulated by a signal
molecule
Lac Operon – absence of lactose
 An operon consists of:
 structural genes
 - contain the DNA code for an enzyme
 operator gene
 - which controls the structural gene
 The operator gene is switched off by a repressor molecule
 - this is coded for by a regulator gene
Lac operon – presence of lactose
 If lactose is present, it binds to the repressor
 Therefore the operator gene is now free
 The structural gene switches on
 - B-galactosidase production starts
 Once all lactose has been digested, the repressor is freed
 It binds again to the operator gene – no more enzyme
Control by regulation of enzyme action
 Signal molecules:
 e.g. Epinephrine
 – a hormone released by the




adrenal glands
- binds to membrane receptors
on liver cells
- activates the cells to produce
an enzyme
- this converts glycogen stores
into glucose
Signal molecules can be
intracellular or
extracellular
Control by regulation of enzyme action
 Inhibitors:
 - a substance that can decrease the






rate of an enzyme controlled
reaction
COMPETITIVE
- compete with the substrate for
the enzyme’s active site
- has a structure similar to the
substrate
- as active sites get blocked,
reaction rate decreases
- increasing substrate
concentration can help reaction
rate to increase again
- substrates outnumber the
inhibitor
Control by regulation of enzyme action
 Inhibitors:
 NON-COMPETITIVE
 - don’t combine with the enzyme’s







active site
- attach to a non-active (allosteric) site
- this changes the overall enzyme shape
- active site is therefore affected, and no
longer binds with it’s substrate
More enzymes affected, the lower the
reaction rate
Allosteric sites can also be bound to by
activators
These can change the active site to help
activate enzymes
Inhibitors and activators can therefore
be regulatory molecules
Control by regulation of enzyme action
 End product inhibition:
 Occurs as the concentration of an end








product starts to accumulate
Some of the end product can bind to an
enzyme earlier in the pathway
E.g. enzyme 1
This stops any further conversion of
substrates by enzyme 1
Once end product concentration goes
back to normal, it will no longer bind to
enzyme 1
- pathway can resume
Process is known as negative feedback
- very finely tuned
Feedback inhibition animation