1. ENZYMES
A protein with catalytic properties due to its power of specific activation
© 2007 Paul Billiet ODWS

2. Enzymes Catalysts for biological reactions Most are proteins
Lower the activation energy
Increase the rate of reaction
Activity lost if denatured
May be simple proteins
May contain cofactors such as metal ions or organic (vitamins)

3. Name of Enzymes End in –ase Identifies a reacting substance
sucrase – reacts sucrose
lipase - reacts lipid
Describes function of enzyme
oxidase – catalyzes oxidation
hydrolase – catalyzes hydrolysis
Common names of digestion enzymes still use –in
pepsin, trypsin

4. Classification of Enzymes
Class Reactions catalyzed
Oxidoreductoases oxidation-reduction
Transferases transfer group of atoms
Hydrolases hydrolysis
Lyases add/remove atoms to/from a double bond
Isomerases rearrange atoms
Ligases combine molecules
using ATP

5. Examples of Classification of Enzymes
Oxidoreductoases
oxidases - oxidize ,reductases – reduce
Transferases
transaminases – transfer amino groups
kinases – transfer phosphate groups
Hydrolases
proteases - hydrolyze peptide bonds
lipases – hydrolyze lipid ester bonds
Lyases
carboxylases – add CO2
hydrolases – add H2O

6. Learning Check Match the type of reaction with the enzymes:
(1) aminase (2) dehydrogenase
(3) Isomerase (4) synthetase
Converts a cis-fatty acid to trans.
Removes 2 H atoms to form double bond
Combine two molecules using ATP
Adds NH3

7. Chemical reactions
Chemical reactions need an initial input of energy = THE ACTIVATION ENERGY
During this part of the reaction the molecules are said to be in a transition state.
© 2007 Paul Billiet ODWS

8. Reaction pathway
© 2007 Paul Billiet ODWS

9. Making reactions go faster
Increasing the temperature make molecules move faster
Biological systems are very sensitive to temperature changes.
Enzymes can increase the rate of reactions without increasing the temperature.
They do this by lowering the activation energy.
They create a new reaction pathway “a short cut”
© 2007 Paul Billiet ODWS

10. An enzyme controlled pathway
Enzyme controlled reactions proceed 108 to 1011 times faster than corresponding non-enzymic reactions.
© 2007 Paul Billiet ODWS

11. Enzyme structure Enzymes are proteins They have a globular shape
A complex 3-D structure
Human pancreatic amylase
© Dr. Anjuman Begum
© 2007 Paul Billiet ODWS

12. The active site
One part of an enzyme, the active site, is particularly important
The shape and the chemical environment inside the active site permits a chemical reaction to proceed more easily
© H.PELLETIER, M.R.SAWAYA ProNuC Database
© 2007 Paul Billiet ODWS

13. Cofactors
An additional non-protein molecule that is needed by some enzymes to help the reaction
Tightly bound cofactors are called prosthetic groups
Cofactors that are bound and released easily are called coenzymes
Many vitamins are coenzymes
Nitrogenase enzyme with Fe, Mo and ADP cofactors
Jmol from a RCSB PDB file © 2007 Steve Cook H.SCHINDELIN, C.KISKER, J.L.SCHLESSMAN, J.B.HOWARD, D.C.REES
STRUCTURE OF ADP X ALF4(-)-STABILIZED NITROGENASE COMPLEX AND ITS IMPLICATIONS FOR SIGNAL TRANSDUCTION; NATURE 387:370 (1997)
© 2007 Paul Billiet ODWS

14. The substrate
The substrate of an enzyme are the reactants that are activated by the enzyme
Enzymes are specific to their substrates
The specificity is determined by the active site
© 2007 Paul Billiet ODWS

15. The Lock and Key Hypothesis
Enzyme may be used again
Enzyme-substrate complex E S P
Reaction coordinate
© 2007 Paul Billiet ODWS

16. The Induced Fit Hypothesis
Hexokinase (a) without (b) with glucose substrate
This explains the enzymes that can react with a range of substrates of similar types
© 2007 Paul Billiet ODWS

17. Learning Check The active site is (1) the enzyme
(2) a section of the enzyme
(3) the substrate
B. In the induced fit model, the shape of the enzyme when substrate binds
(1) Stays the same
(2) adapts to the shape of the substrate

18. Factors affecting Enzymes
substrate concentration pH
temperature
inhibitors
© 2007 Paul Billiet ODWS

19. Substrate concentration: Non-enzymic reactions
Reaction velocity
Substrate concentration
The increase in velocity is proportional to the substrate concentration
© 2007 Paul Billiet ODWS

20. Substrate concentration: Enzymic reactions
Reaction velocity
Substrate concentration
Vmax
Faster reaction but it reaches a saturation point when all the enzyme molecules are occupied.
If you alter the concentration of the enzyme then Vmax will change too.
© 2007 Paul Billiet ODWS

21. The effect of pH Optimum pH values Enzyme activity Trypsin Pepsin pH 13 5 7 9 11
© 2007 Paul Billiet ODWS

22. Inhibitors
Inhibitors are chemicals that reduce the rate of enzymic reactions.
The are usually specific and they work at low concentrations.
They block the enzyme but they do not usually destroy it.
Many drugs and poisons are inhibitors of enzymes in the nervous system.
© 2007 Paul Billiet ODWS

23. The effect of enzyme inhibition
Irreversible inhibitors: Combine with the functional groups of the amino acids in the active site, irreversibly.
Examples: nerve gases and pesticides, containing organophosphorus, combine with serine residues in the enzyme acetylcholine esterase.
© 2007 Paul Billiet ODWS

24. The effect of enzyme inhibition
Reversible inhibitors: These can be washed out of the solution of enzyme by dialysis.
There are two categories.
© 2007 Paul Billiet ODWS

25. The effect of enzyme inhibition
Competitive: These compete with the substrate molecules for the active site.
The inhibitor’s action is proportional to its concentration.
Resembles the substrate’s structure closely.
Enzyme inhibitor complex
Reversible reaction
E + I EI
© 2007 Paul Billiet ODWS

26. The effect of enzyme inhibition
Non-competitive: These are not influenced by the concentration of the substrate. It inhibits by binding irreversibly to the enzyme but not at the active site.
Examples
Cyanide combines with the Iron in the enzymes cytochrome oxidase.
Heavy metals, Ag or Hg, combine with –SH groups.
These can be removed by using a chelating agent such as EDTA.
© 2007 Paul Billiet ODWS

27. Applications of inhibitors
Negative feedback: end point or end product inhibition
Poisons snake bite, plant alkaloids and nerve gases.
Medicine antibiotics, sulphonamides, sedatives and stimulants
© 2007 Paul Billiet ODWS