Monday, 10/3Monday, 10/3Topic 7 TEST WEDNESDAY!Topic 7 TEST WEDNESDAY!

• Explain the secondary and tertiary levels of protein structure.

• 4 marks

secondary structure: [2 max]• folding / pleating of polypeptides to form b - pleated sheets / coiling of polypeptides to

form a -helix;• held in place by hydrogen bonds;• make structure stable;• contributes to strength of fibrous proteins;• provide structural role in organisms;• eg a -helix is keratin / b-sheet is silk;

tertiary structure: [2 max]• 3-D shape;• due to bonding between amino R-groups / residues;• hydrogen bonds / disulphide bridges / sulphur bonds / ionic bonds;• form globular proteins;• which are soluble;• eg lysozyme / enzymes;

7.6Enzymes

7.6.1 State that metabolic pathways consist of enzyme-catalysed reactions in:

• Chemical rxns usually multiple steps

• Each step has own enzyme

Chains

(glycolysis)

Cycles

(Krebs, Calvin Cycles)

7.6.2 Describe the induced-fit model. (1958)• Extension of lock-and-key model

(1894)• Accounts for ability of some

enzymes to bind to several substrates w/similar rxns

• Active site may not be as rigid as originally thought

• Shape adapts somewhat perfect fit

• Allows slightly diff substrates to fit

• Analogy: glove fits on several diff hands...but not on a foot

7.6.3 Explain that enzymes lower the activation energy of the chemical reactions that they catalyse.

• Speed up rxn, but don’t change enzyme & don’t get “used up”

• Rxns require activation energy

• Active site facilitates change– Lowers activation energy

• Collisions: higher act energy = need faster collision speed – More molecules able to react,

rate of rxn increases

Exothermic Rxn

7.6.4 Explain the difference between competitive and non-competitive inhibition, with reference to one example of each.

• Competitive inhibition – inhibiting molecule is structurally similar

to substrate molecule ... COMPETES!– binds to active site, prevents substrate binding– Add more substrate, decrease effect of inhibitor

7.6.4 Explain the difference between competitive and non-competitive inhibition, with reference to one example of each.

• EX: Prontosil (antibacterial drug) inhibits synthesis of folic acid (vit B) in bacteria drug binds to enzyme that makes folic acid

need folic acid for nucleic acid synthesis

Prontosil bacterial cell dies b/c can’t make DNA/RNA

Animal cells not affected b/c don’t MAKE folic acid, get it from food. (no enzyme, so no effect)

7.6.4 Explain the difference between competitive and non-competitive inhibition, with reference to one example of each.

Non-competitive inhibition– inhibitor binds to an enzyme (not to its

active site) – causes conformational change in active site

decrease in activity– Adding more substrate: no effect

7.6.4 Explain the difference between competitive and non-competitive inhibition, with reference to one example of each.

Non-competitive inhibition• Ex: CYANIDE

attaches to –SH groups in cytochrome c oxidase, destroys disulfide bridges, changes tertiary structure, no function

(normally catalyzes reduction of oxygen to water, so without it cell respiration stops, organism dies)

7.6.5 Explain the control of metabolic pathways by end-product inhibition, including the role of allosteric sites.

• Allostery = type of non-compet. inhibition

• “allosteric site” binding – Reversible change in shape of AS– Allosteric activators –

• speed up rxn

– Allosteric inhibitors – • slow down rxn• end product of pathway can be

allosteric inhibitor

7.6.5 Explain the control of metabolic pathways by end-product inhibition, including the role of allosteric sites.

• Another example: Glycolysis– [product] increases, fits in allosteric site of

previous enzyme in pathway, changes active site, reduces/stops rxns

– Level of product reduced...enzyme releases allosteric molecule, active site changes back to normal, rxns proceed