U5 - Enzyme Flashcards
metabolism
The sum of all the biochemical reactions inside living cells
metabolic pathways
Reactions are organised into pathways:
a chain of related reactions that sequentially
modify a reactant into the required product
enzyme
- Metabolic pathways are assisted by protein
catalysts called enzymes - Several enzymes are required to perform
each of the required transformations in a
pathway
substrates
the reactants in a reaction
catalysed by an enzyme
Enzyme Functions
- Even though enzymes participate in reactions, they are not consumed by them
- enzyme function depends upon the overall
tertiary shape of the protein: if it loses this
shape it is called denatured
Lock and Key Model
- Enzymes have special areas called “active
sites” - The shape of the active site is unique to its
substrate, acting like a ‘lock’ into which fits
the substrate ‘key’ - Denaturing changes the shape of the ‘lock’
and prevents the ‘key’ from fitting
Induced fit model
- Enzyme structure is flexible
- Binding of the substrate to the active site
causes (induces) a change in the shape of
the enzyme - This change allows the substrate to fit better,
assisting the chemical reaction - Afterwards the products are released and
the enzyme returns to its normal shape
Activation Energy
- the energy needed to make molecules react with each other - enzymes assist (catalyse) reactions by lowering this activation energy enzymes allow reactions to occur within a cell that would otherwise require lethal temperatures
co-enzyme
coenzymes are non-protein organic
molecules that are required for enzymes to
function
Coenzyme function
coenzymes act as messengers, carrying
chemical groups between enzymes
coenzymes bind with their enzymes,
sometimes forming part of the active site for
the substrate
unlike enzymes, they are consumed in
reactions by accepting or donating parts of
their structure
What role does vitamins play in coenzyme composition?
many vitamins are chemical precursors of
important coenzymes, and so are necessary
for metabolism
enzyme activity
substrate concentration increases = increases enzyme activity
-because there are more collisions between
substrate and enzyme
enzyme activity
- A maximum is reached when the enzyme’s
active sites are continually filled - increased enzyme concentration also increases enzyme activity
because there are more active sites available to the substrate
Temperature
- at low temperatures substrates have so little
energy that they will not react, even in the
presence of an enzyme - increase in temperature = increase enzyme activity because
the substrates have more activation energy - most human enzymes function best at around
40°C, increasing temperature above this
point cause denaturing and ↓enzyme activity
pH
- enzymes also have a preferred pH, outside of
this, the enzyme becomes denatured - e.g. pepsin, a stomach enzyme, works best at
pH 2, matching the acidity of the stomach
Control of enzyme activity
- enzyme activity in the cell is controlled in
several ways, most importantly by feedback
inhibition
- competitive – binds to the active site,
competes with substrate - non-competitive – binds outside the
active site, changes enzyme’s shape
competitive inhibitors
- some molecules are similar enough to the
substrate that they can also bind with the
active site - the inhibitor does not react, but temporarily
prevents new substrate from binding
(penicillin is a competitive inhibitor for an
enzyme which strengthens the cell wall of
bacteria, causing the cells to lyse)
non-competitive inhibitors
- some chemicals bind with parts of the
enzyme outside the active site, denaturing
the protein
example: heavy metals (e.g. Hg and Pb) are
non-competitive inhibitors that bind
irreversibly to the polypeptide chain
Thyroid and thyroid
- the thyroid, a large gland in the neck,
secretes the hormone thyroxin - thyroxin regulates metabolism by controlling
the formation of enzymes for cellular
respiration
negative feedback
negative feedback is where the product of a
system suppresses the activity of the system
the more product there is, the less the system
functions, and the levels return to normal
feedback maintains homeostasis: constant
conditions within the body
metabolism of
threonine
negative feedback controls the metabolic
pathway which converts threonine to
isoleucine (amino acids)
a series of enzymes catalyse reactions which
sequentially modify threonine into isoleucine
if the product (isoleucine) becomes too
common it inhibits the first enzyme of the
pathway, decreasing isoleucine production
