ENZYMES - PROBLEMS



Week # 10 - Enzymes

Lecutres - 26, 27, 28

Concepts:

Classification of enzymes - "-ase" in name, by name of reaction

Parts list for an enzyme and how it works.

Inhibitors - reversible competitive and noncompetitive, irreversible, allosteric, feddback

Applications

Ques. 1: Consult a list of classes of enzymes. Which two types have we studied reactions in the earlier part of the course?

Ques. 2. a. Explain the difference between the cofactor and coenzyme.

b. Describe the relationships of the following parts of a typical enzyme: cofactor, enzyme, coenzyme, vitamin, protein.

Ques. 3: a. Explain what is meant by the lock and key theory of enzyme action.

Include: active site, substrate, enzyme.

b. Expalin the Induced Fit Model of enzyme action and contrast with the lock and key model.

Ques. 4. How does an enzyme distinguish between the correct and incorrect

substrate in the active site?

Ques. 5: Write a paragraph giving a complete summary of all parts of an enzyme and how it works.

Ques. 6: a. List four factors which may effect how well an enzyme works.

b. From the above factors, list two non-specific enzyme inhibitors. Explain how they inhibit enzyme action.

c. What happens to the active site and the molecular geometry of the enzyme as it is denatured?

d. Excess of either acid or base causes denaturing of protein. What type of bonds are disrupted by this action?

e. Name some other denaturing methods to inhibit or destroy enzyme activity.

f. If an enzyme has been denatured, is it likely that its enzyme activity can be restored? Is the inhibition reversible or irreversible? Explain.

Ques 7. Briefly define and explain the actions of the following enzyme inhibitors. Give some examples:

a. reversible competitive inhibitor

b. reversible noncompetitive inhibitor

c. irreversible inhibitor

d. allosteric inhibitor

e. feedback inhibitor

QUES. 8: If the concentration of inhibitor is less than that of the substrate and the substrate has a higher affinity for the active site, is the enzyme inhibited a lot, or a little?

Ques. 9. MSG (monosodium glutamate) is used for a meat flavoring agent. Only the L optical isomer is effective. After drawing the mirror image, explain why the D isomer apparently is unable to react with taste receptor sites on the tongue.

Ethanol is metabolized in the body by oxidation to acetaldehyde, which is in turn further oxidized to acetic acid by aldehyde oxidase enzymes. Normally, the second reaction is rapid so that acetaldehyde does not accumulate in the body.

Ques. 10: CH3CH2OH + (0) ( + (0) (

A drug, disulfiram (Antabuse) inhibits the aldehyde oxidase which causes the accumulation of acetaldehyde with subsequent unpleasant side-effects of nausea and vomiting. This drug is sometimes used to help people overcome the drinking habit.

QUES. 11: Which part of the disulfiram molecule probably interacts with the active site of aldehyde oxidase.? Hint: Look for something similar to the -C=O group.

Methanol poisoning occurs because methanol is oxidized to formaldehyde and formic acid which attack the optic nerve causing blindness. Ethanol is given as an antidote for methanol poisoning because ethanol competitively inhibits the oxidation of methanol. Ethanol is oxidized in preference to methanol and consequently, the oxidation of methanol is slowed down so that the toxic by-products do not have a chance to accumulate.

QUES. 12: Ethylene glycol, if ingested, can be poisonous. Ethylene glycol is oxidized by the same enzymes used in the previous examples by ethanol and methanol. Describe how ethanol can be used as an antidote.

Ques. 13. In the digestion of food, what do the following enzymes do? Give the substance acted upon and the final products in words. (Need to look in the index or chap 23 and 25)

amylase,

maltase

lactase

sucrase

lipase

pepsinogen

pepsin

trysin

chymotrypsin

Ques. 14. Enzymes are used for diagnostic testing of health conditions: List 5 enzymes and the conditions that are measured if elevated.

Penicillin Mode of Action - Enzyme Inhibition:



All penicillin derivatives produce their bacteriocidal effects by inhibition of bacterial cell wall synthesis. Specifically, the cross linking of peptides on the mucosaccharide chains is prevented. If cell walls are improperly made cell walls allow water to flow into the cell causing it to burst.

Resemblances between a segment of penicillin structure and the backbone of a peptide chain have been used to explain the mechanism of action of beta-lactam antibiotics. The structures of a beta-lactam antibiotic and a peptide are shown on the left for comparison. Follow the trace of the red oxygens and blue nitrogen atoms.

The bacterial cell wall synthesis is completed when a cross link between two peptide chains attached to polysaccharide backbones is formed. The cross linking is catalyzed by the enzyme transpeptidase. First the terminal alanine from each peptide is hydrolyzed and secondly one alanine is joined to lysine through an amide bond.

Penicillin binds at the active site of the transpeptidase enzyme that cross-links the peptidoglycan strands. It does this by mimicking the D-alanyl-D-alanine residues that would normally bind to this site. Penicillin irreversibly inhibits the enzyme transpeptidase by reacting with a serine residue in the transpeptidase. This reaction is irreversible and so the growth of the bacterial cell wall is inhibited.

Since mammal cells do not have the same type of cell walls, penicillin specifically inhibits only bacterial cell wall synthesis.

Ques. 15: In your own words explain how the penicillin drug works including enzyme inhibition, cell wall synthesis, peptide chains, beta lactam structure.

Sulfa Drugs - Mechanism for Action:



Normally folic acid is synthesized in two steps in bacteria by the top reaction on the left. If a sulfa drug is used, the first enzyme is not to specific and can use the sulfonamide in the first reaction. This reaction produces the product containing pteridine and the sulfa drug. The next and final step is the reaction PABA + with glutamic acid to make folic acid. If the sulfa drug has been substituted for the PABA, then the final enzyme is inhibited and no folic acid is produced.

Recent studies indicate that substituents on the N(1) nitrogen may play the role of competing for a site on the enzyme surface reserved for the glutamate residue in p-aminobenzoic acid-glutamate through one of the following two ways:

a) Direct competition in the linking of PABA-glutamate with the pteridine derivative.

b) Indirect interference with the coupling of glutamate to dihydropteroic acid.

The basic structure of sulfonamide cannot be modified if it is to be an effective competitive "mimic" for p-aminobenzoic acid. Essential structural features are the benzene ring with two substituents para to each other; an amino group in the fourth position; and the singly substituted 1-sulfonamido group.

Ques. 16: In your own words explain how the sulfa drug works including enzyme inhibition, folic acid, and antimetabolite.

Ques. 17. Match the name of the vitamin with its letter designation, give use, and/or deficiency disease.

Vitamin Letter Use Deficiency

___Thiamine B6

___Ascorbic Acid D

___Riboflavin B2

___Pyridoxine B1

___Cobalamin A

___Tocopherol E

___Retinol C

___Niacin B3

___Pantothenic Acid B5

___Cholecalciferol B12

Ques. 18. Which vitamins should not be taken in large doses since they are toxic?

Ques. 19. Which vitamins are likely to be lost by cooking in water? Why?

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