Transcript ENZYMES - ClickBiology

ENZYMES
What can you recall from IGCSE?
Define the following terms:
1.
Anabolic reactions: Reactions that build up molecules
2.
Catabolic reactions: Reactions that break down molecules
3.
Metabolism:
Combination of anabolic and catabolic
reactions
4.
Catalyst:
A substance that speeds up reactions
without changing the produced substances
5.
Metabolic pathway: Sequence of enzyme controlled reactions
6.
Specificity:
Only able to catalyse specific reactions
7.
Substrate:
The molecule(s) the enzyme works on
8.
Product:
Molecule(s) produced by enzymes
Naming enzymes:
• Intracellular enzymes
Work inside cells eg.DNA polymerase
• Extracellular enzymes Secreted by cells and work outside cells eg.
pepsin, amylase
• Recommended names Short name, often ending in ‘ase’ eg.
creatine kinase
• Systematic name
Describes the type of reaction being
catalysed eg.
ATP:creatine phosphotransferase
• Classification number
Eg. 2.7.3.2
Timeline of enzyme discovery
1835:
Breakdown of starch to sugar by malt
1877:
Name enzyme coined to describe chemicals in yeast that ferment sugars
1897:
Eduard Buchner extracted enzyme from yeast and showed it could work outside cells
1905:
Otto Rohm exyracted pancreatic proteases to supply enzymes for tanning
1926:
James B Sumner produced first pure crystalline enzyme (urease)
and showed enzymes were proteins
1930-1936:
Protein nature of enzymes finally established when digestive enzymes
crystallised by John H Northrop
1946: Sumner finally awarded Nobel prize
Energy levels of molecules
Enzymes lower the activation energy of a reaction
Initial energy state
of substrates
Activation energy
of enzyme catalysed
reaction
Activation energy
of uncatalysed
reactions
Final energy state of
products
Progress of reaction (time)
Enzymes lower activation energy by forming an
enzyme/substrate complex
Substrate + Enzyme
Enzyme/substrate complex
Enzyme/product complex
Product + Enzyme
In anabolic reactions
enzymes bring the substrate
molecules together.
In catabolic reactions the
enzyme active site affects the
bonds in substrates so they
are easier to break
Lock-and-key hypothesis assumes the active site
of an enzyme is rigid in its shape
How ever crystallographic studies indicate proteins are flexible.
The Induced-fit hypothesis suggests the active site is
flexible and only assumes its catalytic conformation after
the substrate molecules bind to the site.
When the product leaves
the enzyme the active site
reverts to its inactive state.
Enzymes are globular proteins
• Active site has a specific
shape due to tertiary
structure of protein.
• A change in shape of the
protein affects shape of
active site and the function
of the enzyme.
Click to link to jmol interactive representation courtesy of
University of Arizona
Characteristics of enzymes
• Only change the rate of reaction. They do not change
the equilibrium or end products.
• Specific to one particular reaction
• Present in very small amounts due to high molecular
activity:
Turnover number = number of substrate molecules
transformed per minute by one enzyme molecule
Catalase turnover number = 6 x106/min
How would you measure the effect of an enzyme?
• Compare uncatalysed rate with catalysed.
• Enzymes can increase rate by a factor of
between 108 to 1026
Characteristics of enzymes
• Rate of enzyme action is dependent on number of
substrate molecules present
Rate of Reaction (M)
Vmax = maximum rate of reaction
Vmax approached as all
active sites become
filled
Some active sites free
at lower substrate
concentrations
Substrate concentration
Why do scientists measure the initial rate of
reaction of enzyme-catalysed reactions?
Rate of Reaction (M)
Initial rate of reaction
They measure rate
at start of reaction
before any factors,
eg. substrate
concentration, have
had time to change.
Independent variable
Rate of enzyme –catalysed reactions are affected
by temperature.
Temperature coefficient Q10:
rate of reaction at (x + 10) oC
Q10 = ----------------------------------------rate of reaction at x oC
Q10
for between 0 - 40 oC is 2
Enzymes denature at 60oC
Rate of reaction
Optimum temperature
Enzyme denaturing and
losing catalytic abilities
Rate doubles
every 10oC
Temperature
Some thermophilic bacteria have enzymes with optimum
temperatures of 85oC
pH affects the formation of hydrogen bonds and
sulphur bridges in proteins and so affects shape.
trypsin
cholinesterase
Rate of Reaction (M)
pepsin
2
4
6
pH
8
10
Enzymes in medicine
Glucose oxidase + peroxidase + blue dye on dipsticks to
detect glucose in urine:
Glucose
Glucose oxidase
Hydrogen peroxide
peroxidase
Dye:
Blue---Green---Brown
Dye changes according to
amount of glucose
Enzyme-linked immunosorbent assays (ELISAs)
detect antibodies to infections.
Now answer the exam questions
• The first question you will complete and then swap with
your partner. You will then mark each others work using
the provided mark-scheme. You must agree each others
marking.
• The second question you will complete and mark your
own and then I will mark it and see if I agree with your
marking.
• The third question you will complete and I will mark it.
You will then check my marking and we will agree a
score.
• The total score for all three questions will be recorded.