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enzymes
Enzymes

What are enzymes?

- globular proteins that act as biological catalysts
--> catalyst : substance which speeds up a chemical
reaction and lowers activation energy but does not
participate in the reaction itself
- highly specific
--> due to complex primary, secondary, tertiary and
quaternary structure of protein
--> specific action due to presence of active site in enzyme
--- individual
--- specific shape
--- formed by fewer than 10 amino acids
- bonds certain types of substrates

How do enzymes work?
- complementary structure to active site
--> specific
--> any individual enzyme can only catalyse one
particular reaction
- originally thought that the way enzyme and substrate
interacted was through lock and key model
--> each enzyme fitted the substrate exactly
--> no shape change
- later changed to induced fit model
--> active site changes shape when substrate bonds
--> places strain on the substrate bonds which break
--> induced fit brings chemical groups of active site into
positions which enhance their ability to work on substrate
--> as enzymes change shape -> put strain on substrate
-> distorts bonds in molecule
--> lowers activation energy needed to break the bond catalyses chemical reaction

Factors that affect enzymes
- for enzymes to work out
--> come into contact with substrate (collide)
--> have active site which is complementary to substrate
- temperatures (low and high)
- pH
- enzyme concentration
- substrate concentration
- inhibitors (competitive and non-competitive)
- denaturation
--> unfolding protein 3D shape and becomes a linear
amino acid sequence
--> loss of secondary, tertiary or quaternary structure
due to exposure to chemical or physical factors
--> primary structure stays intact -> formed by strong
covalent bonds between amino acids
--> non-covalent bonds, hydrogen bonds and
disulphide bridges forming secondary and tertiary
structure of protein are broken due to high temperatures or
pH outside optimal
--> all biological functions of denatured protein are lost
-> active site no longer fits substrate -> ES complex cannot
form

What is activation energy?
- energy needed to break the bonds in the reactants in order
for products to be formed
- enzymes lower activation energy for reaction
- enzyme destabilises bonds in the substrate by bonding
with the substrate -> acts process of bond breaking -> helps
reaction take place

Enzymes

enzymes

Effect of temperature on enzymes
- all molecules in fluid (gas or liquid) move constantly due to
natural kinetic energy
- random movement -> constant collisions
- good effects on enzymes
--> temperatures increases -> kinetic energy of molecules
increase
--> more movement and more frequent and harder
collisions -> increase likelihood of ES complex formation
--> more products produced
- bad effects on enzymes
--> individual energy of each atom increases -> molecules
vibrate -> put strain on bonds (hydrogen and ionic) holding
enzymes together
--> shape of active site changes (no ES complexes formed)
- rate of reaction increases up to optimum temperature as
kinetic energy of enzyme increases
- temperature increases -> molecular motion and molecular
collisions increase
- more product is formed in given time -> reaction rate
increases
- maximum rate of reaction is reached at optimum
temperature
- beyond optimum -> bonds that stabilise enzyme's tertiary
structure are broken
- enzyme loses shape and active site is altered
- chemical reactions, catalysed or non-catalysed, accelerate
as temperature increases
--> rate of collisions between reactants increase with
temperature
- at low temperatures -> rte of reaction in presence of
enzymes increase
- at optimum temperature -> rate of conversion is at its
greatest
- temperatures higher than optimum reduce enzyme's
activity
--> tertiary structure is changed -> denaturation
occurs

Effect of pH on enzymes
- pH : measure of hydrogen ion concentration
--> acids are sometimes known as proton donors
(excess H+ ions)
--> log10 [H+]
- pH can disrupt bonds in tertiary structure of enzyme
- all enzymes work at different optimum pHs
- effects of pH on rate of enzyme controlled reactions
display bell shaped curves
- changes in pH can affect ionic and hydrogen bonds
responsible for specific tertiary shape of enzymes
- each specific enzyme can only work over a range of pH
- each enzyme has its own optimum pH where rate of
reaction is maximum
- changes in pH changes enzyme protein structure and
activity
- extreme pH values result in irreversible changes in
secondary and tertiary structure
- extreme pH results in denaturation and cessation of
activity
- exact arrangement of active site is fixed by hydrogen
bonds between NH2 (amine group) and COOH (carboxyl
group) of polypeptides making up enzyme
--> too many H+ ions interfere with delicate balance ->
change shape of active site -> denaturation
- H+ ions cluster around side chains of amino acids
(attracted to negatively charged side chains)
--> interfere with binding to substrate -> rate of
reaction declines
- buffer solution could be added to counteract

enzymes
Enzymes

Effect of substrate concentration on enzymes

- as concentration of substrate increases-> rate of reaction
increases as more ES complexes are formed
- beyond certain point -> rate of reaction no longer increases
--> enzyme concentration becomes limiting factor
- enzymes attach to whatever amount of substrate is available
--> not used up -> few enzyme molecules are needed as long
as there is substrate
- if substrate concentration is slowly increasing
--> rate of reaction will be slow to begin with
--> at low substrate concentrations -> chances of collisions
are low
--- not all active sites are full
- as substrate concentration increases
--> collisions between active sites and enzymes are more
likely
Effect of enzyme concentration on enzymes
--> rate increases
- rate of reaction increases as enzyme concentration
- point will be reached where all active sites are full
increases
--> rate of reaction will stabilise
--> more active sites for substrates to bind to
--> produce is formed at regular rate
- increasing enzyme concentration beyond certain point ->
no effect on rate of reaction
--> more active sites than substrates
--> substrate concentration becomes limiting factor
- rate of reaction is directly proportional to enzyme
concentration
--> enzyme concentration increases -> rate of reaction
increases
- if amount of substrate is limited
--> only so many collisions can occur
--> rate is capped by amount of substrate

Enzyme inhibition

- enzyme inhibitors: any substance or molecule that slows
down the rate of an enzyme reaction by affecting the enzyme
in some way
- gives us information about shape and properties of active
site
- can be used to block particular reactions
--> enable us to construct metabolic pathways
- have important medical and agricultural purposes

Non-competitive inhibitors
- molecules bind with enzyme from active site (allosteric
site)
- causes active site to change shape -> substrate can no
longer bind
- don’t compete
--> inhibitor shape is different to substrate shape
- increasing substrate concentration will have no effect
--> enzyme shape changed
- shape of inhibitor matches allosteric site
- inhibitor binds with site -> alters enzyme tertiary
structure
- substrate can no longer fit into active site
- can be reversible depending on strength of bonding at
allosteric site

Enzymes

enzymes

Competitive inhibitors
- shape of inhibitor matches shape of substrate
- decrease rate of reaction
- due to kinetic energy all molecules move around
--> inhibitor sometimes arrives active site before
substrate does
- inhibitor binds with enzyme's active site to prevent
substrate from binding
- other enzyme's active sites are available to reaction which
can still occur but at a slower rate
- if amount of inhibitors increase
--> further slow rate down
- amount of inhibitors depend on relative concentrations of
substrate and inhibitor
- substrate and inhibitors compete for active site
--> occupy active site preventing substrates from binding
--> fewer ES complexes
- inhibitor is chemically different to substrate
--> cannot be converted into product
- increase concentration of substrate displaces competitive
inhibitor
- will reach same endpoint at the end

Ethylene glycol and alcohol
- ethylene glycol is found in antifreeze
--> if ingested can be broken down by alcohol
dehydrogenase -> form extremely toxic oxalie acid -> die
- ethanol competes with antifreeze to stop ethylene glycol
from interacting with alcohol dehydrogenase
- less oxalic acid is produced
--> harmless ethylene glycol to be excreted

Penicilin and bacteria
- inhibitor of bacterial enzyme forms crosslinks in bacterial
cell in murein (peptidoglycan)
--> stops bacterial cell wall construction
- penicilin is a competitive inhibitor
--> structurally similar to substrate
- cell walls are not built as well
--> fall apart and cannot resist osmotic movement of
water
- resistance
--> enzyme is slightly altered
--> does not bond to penicilin

Intracellular and Extracellular enzymes

- intracellular enzymes
--> catalyse reactions inside cell
--> enzymes are found in cytoplasm or attached to cell
membrane
--> lysosome -> destroys old cells
--> mitochondria -> hydrolysis and respiration
-- chloroplasts -> photosynthesis
- extracellular enzymes
--> catalyse reactions outside cell
--> digestive enzymes
--> spiders digest flies using enzymes they secrete
- inhibitors form a natural part of the way enzymes are controlled
--> saprophytic fungi (mould) secrete enzymes to digest
in cells
food

Feedback Inhibition

--> chemical chaos would result if all of cells metabolic
pathways were open at once
- operation of each metabolic pathway is tightly regulated
--> switched on or off by controlling enzyme activity
- feedback inhibition : the product switches off enzyme in the
process
- prevents cell from wasting chemical resources and energy
- cell processes consist of series of pathways
controlled by enzymes
--> each step is controlled by a different
enzyme
--> possible because of enzyme specificity



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