U5 - Enzyme

Question 1

metabolism

Answer 1

The sum of all the biochemical reactions inside living cells

Question 2

metabolic pathways

Answer 2

Reactions are organised into pathways:
a chain of related reactions that sequentially
modify a reactant into the required product

Question 3

enzyme

Answer 3

• Metabolic pathways are assisted by protein
  catalysts called enzymes
• Several enzymes are required to perform
  each of the required transformations in a
  pathway

Question 4

substrates

Answer 4

the reactants in a reaction

catalysed by an enzyme

Question 5

Enzyme Functions

Answer 5

• 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

Question 6

Lock and Key Model

Answer 6

• 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

Question 7

Induced fit model

Answer 7

• 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

Question 8

Activation Energy

Answer 8

- 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

Question 9

co-enzyme

Answer 9

coenzymes are non-protein organic
molecules that are required for enzymes to
function

Question 10

Coenzyme function

Answer 10

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

Question 11

What role does vitamins play in coenzyme composition?

Answer 11

many vitamins are chemical precursors of
important coenzymes, and so are necessary
for metabolism

Question 12

enzyme activity

Answer 12

substrate concentration increases = increases enzyme activity
-because there are more collisions between
substrate and enzyme

Question 13

enzyme activity

Answer 13

• 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

Question 14

Temperature

Answer 14

• 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

Question 15

pH

Answer 15

• 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

Question 16

Control of enzyme activity

Answer 16

• enzyme activity in the cell is controlled in
  several ways, most importantly by feedback
  inhibition

1. competitive – binds to the active site,
   competes with substrate
2. non-competitive – binds outside the
   active site, changes enzyme’s shape

Question 17

competitive inhibitors

Answer 17

• 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)

Question 18

non-competitive inhibitors

Answer 18

• 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

Question 19

Thyroid and thyroid

Answer 19

• the thyroid, a large gland in the neck,
  secretes the hormone thyroxin
• thyroxin regulates metabolism by controlling
  the formation of enzymes for cellular
  respiration

Question 20

negative feedback

Answer 20

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

Question 21

metabolism of

threonine

Answer 21

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