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					ENZYMES

What can you recall from IGCSE?

Define the following terms:
1. Anabolic reactions: Reactions that build up molecules

2.
3.

Catabolic reactions: Reactions that break down molecules
Metabolism:
Combination of anabolic and catabolic reactions
A substance that speeds up reactions without changing the produced substances

4.
5.

Catalyst:

Metabolic pathway: Sequence of enzyme controlled reactions

6.
7.

Specificity:
Substrate:

Only able to catalyse specific reactions
The molecule(s) the enzyme works on Molecule(s) produced by enzymes

8.

Product:

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 Eg. 2.7.3.2

• Classification number

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

Enzymes lower the activation energy of a reaction

Energy levels of molecules

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

Vmax = maximum rate of reaction Rate of Reaction (M)

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?

Initial rate of reaction Rate of Reaction (M)

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
Optimum temperature

Rate of reaction

Rate doubles every 10oC

Enzyme denaturing and losing catalytic abilities

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

pepsin

Rate of Reaction (M)

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.


				
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posted:11/3/2009
language:English
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