Biomolecules

What to remember

• Cell contents are separated into an acid-soluble pool (small molecules, the filtrate) and an acid-insoluble fraction (macromolecules: proteins, nucleic acids, polysaccharides; lipids also appear here because membranes break into vesicles).
• Proteins are polymers of amino acids joined by peptide bonds. They have primary, secondary, tertiary and quaternary structure. Polysaccharides are polymers of monosaccharides joined by glycosidic bonds. Nucleic acids are polymers of nucleotides joined by phosphodiester bonds.
• Almost all enzymes are proteins. They bind the substrate at the active site, form an enzyme-substrate (ES) complex, and lower the activation energy. They do not change the equilibrium of the reaction.
• Enzyme activity depends on temperature, pH and substrate concentration. A competitive inhibitor resembles the substrate and blocks the active site, as malonate does for succinic dehydrogenase.

Chemical composition of living tissue

To find out which chemicals are present in living tissue, a small amount of tissue (a vegetable or a piece of liver) is ground in trichloroacetic acid (Cl₃CCOOH) with a mortar and pestle. This gives a thick slurry. The slurry is strained through a cheesecloth or cotton. Two fractions result.

Lipids have molecular weights below about 800 daltons, so they are not true macromolecules. They appear in the acid-insoluble fraction because the cell membrane and other membranes break into vesicles that do not dissolve in acid. Biomacromolecules have molecular weights of more than about 1,000 daltons. They are proteins, nucleic acids and polysaccharides.

Water is the most abundant chemical in living organisms, making up about 70 to 90 per cent of the cell. When a tissue is burnt completely, the carbon compounds are oxidised to gases and the remaining ash contains inorganic elements such as calcium and magnesium.

Primary and secondary metabolites

Thousands of compounds are present in a cell. Those that are common to all cells and essential for basic life processes are primary metabolites. Examples are amino acids, sugars, nucleotides, lipids and proteins.

Secondary metabolites are found in plants, fungi and microbes. They are not directly involved in normal growth, development or reproduction, but they have ecological importance.

Proteins and amino acids

Amino acids are organic compounds with an amino group and an acidic (carboxyl) group on the same carbon, called the α-carbon. The α-carbon also carries a hydrogen and a variable R group. So amino acids are substituted methanes. Twenty types of amino acids occur in proteins. The nature of R decides the properties.

Valine is a neutral amino acid with a hydrocarbon side chain. Amino acids are amphoteric. They have both an ionisable –NH₃⁺ and an ionisable –COO⁻ group. In solution they exist as zwitterions, which carry both positive and negative charges. The structure of the ion depends on the pH of the solution.

A peptide bond is an amide bond between the –COOH of one amino acid and the –NH₂ of the next, formed with loss of a water molecule. A protein is a heteropolymer of amino acids. Amino acids that must be supplied through food are essential amino acids. Others are non-essential.

Levels of protein structure

Examples of protein functions:

Polysaccharides

Polysaccharides are long chains of monosaccharides joined by glycosidic bonds. They form the cell wall of plants and store energy.

In a polysaccharide chain such as glycogen, one end is the reducing end and the other is the non-reducing end. Branched chains have many non-reducing ends. The right end is drawn as the reducing end and the left end as the non-reducing end.

Nucleic acids

Nucleic acids are polymers of nucleotides. A nucleotide has three parts: a nitrogenous base, a pentose sugar and a phosphate group.

• Nitrogenous bases are heterocyclic compounds. Adenine and guanine are purines. Cytosine, uracil and thymine are pyrimidines.
• The sugar is ribose in RNA and 2'-deoxyribose in DNA.
• DNA has thymine. RNA has uracil in place of thymine.

A phosphodiester bond joins the 3' carbon of one nucleotide to the 5' carbon of the next. This forms the sugar-phosphate backbone of the nucleic acid.

Lipids

Lipids are generally water insoluble. They include fatty acids, fats, oils and phospholipids.

• A fatty acid has a carboxyl group attached to an R group. R may have 1 to 19 carbons. Palmitic acid has 16 carbons including the carboxyl carbon. Arachidonic acid has 20 carbons including the carboxyl carbon.
• Fatty acids are saturated (no double bond) or unsaturated (one or more C=C double bonds).
• Glycerol is a simple lipid component, trihydroxy propane.
• Fats and oils are glycerides, esters of fatty acids with glycerol. Oils have a lower melting point, so they remain liquid in winter. Gingelly oil is an example.
• Phospholipids contain phosphorus and a phosphorylated organic compound, for example lecithin. They are the main structural lipids of cell membranes. Some tissues, such as neural tissue, have more complex lipids.

Enzymes: nature and action

Almost all enzymes are proteins. Some nucleic acids also act as enzymes; they are called ribozymes. An enzyme has a primary structure, and its folding gives a three-dimensional structure with pockets or crevices called active sites. The substrate binds at the active site, and the enzyme converts it to the product.

Chemical reactions are of two kinds. In a physical change, bonds are not broken. In a chemical change, bonds are broken and new bonds form. Enzymes speed up chemical reactions. For example, the reaction CO₂ + H₂O → H₂CO₃ forms about 200 molecules of carbonic acid per hour without an enzyme. With the enzyme carbonic anhydrase, about 600,000 molecules form every second.

How enzymes act

E + S ⇌ ES complex → EP complex → E + P

The substrate must pass through a high-energy transition state before it becomes the product. The difference in energy between the substrate and the transition state is the activation energy. Enzymes lower this energy barrier. They do not change the energy levels of the substrate and the product, so they do not change the equilibrium. The enzyme is unchanged at the end and can act again.

Factors affecting enzyme activity

Km is the substrate concentration at which the reaction rate is half of Vmax. A low Km means high affinity of the enzyme for its substrate.

Competitive inhibition. An inhibitor closely resembles the substrate in molecular structure. It binds to the active site and stops the substrate from binding. The enzyme activity falls. The effect can be reduced by raising the substrate concentration. Malonate inhibits succinic dehydrogenase, which converts succinate to fumarate. Many drugs act as competitive inhibitors.

Classes of enzymes and cofactors

The protein part of an enzyme is the apoenzyme. A non-protein component needed for activity is a cofactor. Apoenzyme plus cofactor gives the holoenzyme. Without the cofactor, the enzyme is inactive.

NEET traps

• 1. Acid-soluble pool is the filtrate. Acid-insoluble fraction is the retentate.
• 2. Lipids are not macromolecules by size, yet they are found in the acid-insoluble fraction.
• 3. Glutamic acid and aspartic acid are acidic. Lysine and arginine are basic.
• 4. A peptide bond joins –COOH to –NH₂ with loss of water. A phosphodiester bond joins nucleotides. A glycosidic bond joins sugars.
• 5. Cellulose does not give the blue colour with iodine. Starch does.
• 6. Inulin is a fructose polymer, not a glucose polymer.
• 7. Nucleoside has no phosphate. Nucleotide has phosphate.
• 8. A competitive inhibitor binds the active site, and its effect is reduced by raising the substrate concentration.
• 9. Enzymes lower the activation energy. They do not change equilibrium or supply energy.
• 10. A co-enzyme is loosely bound. A prosthetic group is tightly bound.

One-liners

• 1. Trichloroacetic acid is used to separate the acid-soluble pool.
• 2. The acid-soluble pool has compounds below about 1,000 daltons.
• 3. Morphine and codeine are alkaloids.
• 4. Rubber is a secondary metabolite.
• 5. Glycine has hydrogen as its R group.
• 6. Amino acids exist as zwitterions in solution.
• 7. Haemoglobin has four subunits, two α and two β.
• 8. Collagen is the most abundant animal protein.
• 9. RuBisCO is the most abundant protein of the biosphere.
• 10. Chitin is a polymer of N-acetyl glucosamine.
• 11. DNA contains thymine and RNA contains uracil.
• 12. Purines are adenine and guanine.
• 13. Malonate inhibits succinic dehydrogenase.
• 14. Apoenzyme plus cofactor is holoenzyme.
• 15. Zinc is a cofactor for carboxypeptidase.

Practice questions

1. Which acid is used to grind tissue when the chemical composition of living tissue is analysed?

   1. Sulphuric acid
   2. Trichloroacetic acid
   3. Acetic acid
   4. Hydrochloric acid
   Answer

   B. Trichloroacetic acid

   Tissue is ground in trichloroacetic acid (Cl₃CCOOH) to give a slurry.

2. Which of the following is a secondary metabolite?

   1. Glucose
   2. Adenylic acid
   3. Alanine
   4. Morphine
   Answer

   D. Morphine

   Morphine is an alkaloid. Sugars, amino acids and nucleotides are primary metabolites.

3. Which of the following is a basic amino acid?

   1. Lysine
   2. Valine
   3. Glutamic acid
   4. Glycine
   Answer

   A. Lysine

   Lysine and arginine are basic. Glutamic acid is acidic. Valine and glycine are neutral.

4. In the amino acid alanine, the R group is

   1. CH₂OH
   2. H
   3. CH₃
   4. COOH
   Answer

   C. CH₃

   R is H in glycine, CH₃ in alanine and CH₂OH in serine.

5. In solution, an amino acid exists as a zwitterion because it has

   1. both an ionisable –NH₃⁺ and an ionisable –COO⁻ group
   2. only a basic group
   3. only an acidic group
   4. no ionisable group
   Answer

   A. both an ionisable –NH₃⁺ and an ionisable –COO⁻ group

   A zwitterion carries both positive and negative charges.

6. Adult human haemoglobin, made of two α and two β subunits, is an example of which level of protein structure?

   1. Secondary
   2. Primary
   3. Tertiary
   4. Quaternary
   Answer

   D. Quaternary

   Quaternary structure is the arrangement of more than one polypeptide chain.

7. The most abundant protein in the whole biosphere is

   1. trypsin
   2. RuBisCO
   3. collagen
   4. insulin
   Answer

   B. RuBisCO

   RuBisCO is the most abundant protein in the biosphere. Collagen is the most abundant in the animal world.

8. Which polysaccharide is a polymer of fructose?

   1. Glycogen
   2. Cellulose
   3. Inulin
   4. Starch
   Answer

   C. Inulin

   Inulin is a fructose polymer. The others are glucose polymers.

9. Nucleotides in a nucleic acid are joined to one another by

   1. glycosidic bonds
   2. peptide bonds
   3. phosphodiester bonds
   4. hydrogen bonds
   Answer

   C. phosphodiester bonds

   A phosphodiester bond joins the 3' carbon of one nucleotide to the 5' carbon of the next.

10. Which nitrogenous base is a pyrimidine found in RNA but not in DNA?

    1. Uracil
    2. Thymine
    3. Guanine
    4. Adenine
    Answer

    A. Uracil

    Uracil replaces thymine in RNA. Adenine and guanine are purines.

11. The class of enzymes that join two compounds using energy from ATP is

    1. lyases
    2. isomerases
    3. hydrolases
    4. ligases
    Answer

    D. ligases

    Ligases link two compounds, using ATP energy.

12. Enzymes that cleave bonds by means other than hydrolysis, leaving double bonds in the products, are

    1. oxidoreductases
    2. lyases
    3. hydrolases
    4. transferases
    Answer

    B. lyases

    Lyases remove groups from substrates without hydrolysis, leaving double bonds.

13. Haem in peroxidase and catalase is an example of a

    1. co-enzyme
    2. apoenzyme
    3. metal ion cofactor
    4. prosthetic group
    Answer

    D. prosthetic group

    A prosthetic group is an organic cofactor tightly bound to the apoenzyme.

14. The co-enzymes NAD and NADP contain the vitamin

    1. vitamin C
    2. niacin
    3. thiamine
    4. biotin
    Answer

    B. niacin

    NAD and NADP contain niacin.

15. The Km of an enzyme is the substrate concentration at which

    1. the enzyme is denatured
    2. the reaction rate is equal to Vmax
    3. the reaction rate is half of Vmax
    4. the product concentration is highest
    Answer

    C. the reaction rate is half of Vmax

    Km is the substrate concentration giving half the maximum velocity.

16. Which of the following is a nucleoside?

    1. Adenosine
    2. Adenylic acid
    3. Deoxyribose
    4. Thymidylic acid
    Answer

    A. Adenosine

    A nucleoside is a base plus a sugar. Adenylic acid and thymidylic acid are nucleotides.

17. Given below are two statements: Statement I: Lipids are true macromolecules with molecular weight above about 1,000 daltons. Statement II: Lipids appear in the acid-insoluble fraction because membranes break into vesicles that do not dissolve in acid. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    Lipids have molecular weights below about 800 daltons, so they are not true macromolecules.

18. Given below are two statements: Statement I: Rubber and gums are secondary metabolites. Statement II: Secondary metabolites are directly needed for the basic growth and reproduction of the organism. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Secondary metabolites are not directly involved in growth, development or reproduction.

19. Given below are two statements: Statement I: Amino acids are amphoteric. Statement II: The structure of the zwitterion of an amino acid depends on the pH of the solution. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both are correct. Amino acids have both acidic and basic ionisable groups.

20. Given below are two statements: Statement I: Starch holds iodine in its helical structure and gives a blue colour. Statement II: Cellulose also gives a blue colour with iodine. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Cellulose gives no blue colour with iodine.

21. Given below are two statements: Statement I: Glycogen is a polymer of fructose. Statement II: Inulin is a polymer of glucose. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    B. Both Statement I and Statement II are incorrect

    Glycogen is a glucose polymer and inulin is a fructose polymer.

22. Which of the following statements are correct? 1. A nucleotide contains a phosphate group. 2. A nucleoside is made of a nitrogenous base and a sugar. 3. Thymine is present in RNA.

    1. 1 and 2 only
    2. 2 and 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Answer

    A. 1 and 2 only

    Thymine is found in DNA. RNA has uracil.

23. Which of the following statements are correct? 1. Oils have a lower melting point than fats. 2. Palmitic acid has 16 carbon atoms including the carboxyl carbon. 3. Phospholipids contain no phosphorus.

    1. 1 only
    2. 1 and 2 only
    3. 2 and 3 only
    4. 1, 2 and 3
    Answer

    B. 1 and 2 only

    Phospholipids contain phosphorus. Statements 1 and 2 are correct.

24. Given below are two statements: Statement I: Enzymes lower the activation energy of a reaction. Statement II: Enzymes shift the equilibrium of a reaction towards the products. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Enzymes do not change the equilibrium. They only speed up reaching it.

25. Given below are two statements: Statement I: A competitive inhibitor closely resembles the substrate in structure. Statement II: Malonate inhibits the enzyme succinic dehydrogenase. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both are correct. Malonate resembles succinate and binds the active site.

26. Given below are two statements: Statement I: Low temperature permanently destroys enzymes. Statement II: Enzymes from organisms living in hot springs remain active at about 80 to 90°C. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    Low temperature only keeps the enzyme in a temporarily inactive state.

27. Assertion (A): Enzymes increase the rate of chemical reactions. Reason (R): Enzymes lower the activation energy of the reaction. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    Lower activation energy lets more substrate molecules reach the transition state.

28. Assertion (A): Starch gives a blue colour with iodine. Reason (R): Starch forms helical structures that hold iodine molecules. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    The helical structure holds iodine and gives the blue colour.

29. Assertion (A): Increasing the substrate concentration without limit keeps increasing the reaction rate of an enzyme. Reason (R): Enzyme molecules become saturated at high substrate concentration. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    D. A is false but R is true

    The rate reaches Vmax because the enzyme is saturated.

30. Assertion (A): Lipids are found in the acid-insoluble fraction. Reason (R): Lipids are macromolecules with molecular weight above 1,000 daltons. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    C. A is true but R is false

    Lipids are below about 800 daltons. They are insoluble because membranes form vesicles.

31. Assertion (A): Malonate inhibits succinic dehydrogenase. Reason (R): Succinic dehydrogenase converts succinate to fumarate. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    B. Both A and R are true but R is not the correct explanation of A

    Both are true. Malonate inhibits because it resembles succinate, not because of the product formed.

32. Assertion (A): A peptide bond is formed by the addition of a water molecule. Reason (R): A peptide bond is an amide bond between the –COOH of one amino acid and the –NH₂ of the next. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    D. A is false but R is true

    A peptide bond forms with the loss of a water molecule.

33. Match List I with List II. List I: a. Oxidoreductases b. Transferases c. Hydrolases d. Ligases List II: i. DNA ligase ii. Alcohol dehydrogenase iii. Transaminase iv. Lipase Choose the correct answer from the options given below.

    1. a-ii, b-iv, c-i, d-iii
    2. a-ii, b-iii, c-iv, d-i
    3. a-iv, b-iii, c-ii, d-i
    4. a-iv, b-i, c-ii, d-iii
    Answer

    B. a-ii, b-iii, c-iv, d-i

    Each class is matched to a standard example.

34. Match List I with List II. List I: a. Cellulose b. Glycogen c. Inulin d. Chitin List II: i. Exoskeleton of arthropods ii. Polymer of fructose iii. Plant cell wall iv. Energy store in animals Choose the correct answer from the options given below.

    1. a-iii, b-i, c-iv, d-ii
    2. a-iv, b-ii, c-i, d-iii
    3. a-iii, b-iv, c-ii, d-i
    4. a-iii, b-ii, c-iv, d-i
    Answer

    C. a-iii, b-iv, c-ii, d-i

    Each polysaccharide is matched to its role or monomer.

35. Match List I with List II. List I: a. Prosthetic group b. Co-enzyme c. Metal ion cofactor d. Holoenzyme List II: i. NAD ii. Apoenzyme plus cofactor iii. Haem in catalase iv. Zinc in carboxypeptidase Choose the correct answer from the options given below.

    1. a-i, b-iii, c-ii, d-iv
    2. a-iii, b-iv, c-ii, d-i
    3. a-ii, b-iv, c-i, d-iii
    4. a-iii, b-i, c-iv, d-ii
    Answer

    D. a-iii, b-i, c-iv, d-ii

    Each term is matched to its example or description.

36. Match List I with List II. List I: a. Glutamic acid b. Lysine c. Valine d. Tyrosine List II: i. Aromatic amino acid ii. Neutral amino acid iii. Acidic amino acid iv. Basic amino acid Choose the correct answer from the options given below.

    1. a-iii, b-ii, c-iv, d-i
    2. a-iii, b-iv, c-ii, d-i
    3. a-iv, b-i, c-iii, d-ii
    4. a-iii, b-i, c-ii, d-iv
    Answer

    B. a-iii, b-iv, c-ii, d-i

    Each amino acid is matched to its class.

37. Match List I with List II. List I: a. Collagen b. Insulin c. Trypsin d. GLUT-4 List II: i. Enzyme ii. Enables glucose transport into cells iii. Hormone iv. Intercellular ground substance Choose the correct answer from the options given below.

    1. a-iv, b-iii, c-i, d-ii
    2. a-iv, b-i, c-iii, d-ii
    3. a-iv, b-iii, c-ii, d-i
    4. a-i, b-iv, c-ii, d-iii
    Answer

    A. a-iv, b-iii, c-i, d-ii

    Each protein is matched to its function.

38. Match List I with List II. List I: a. Morphine b. Anthocyanin c. Ricin d. Concanavalin A List II: i. Toxin ii. Alkaloid iii. Lectin iv. Pigment Choose the correct answer from the options given below.

    1. a-iii, b-ii, c-i, d-iv
    2. a-i, b-ii, c-iv, d-iii
    3. a-ii, b-iv, c-i, d-iii
    4. a-iii, b-ii, c-iv, d-i
    Answer

    C. a-ii, b-iv, c-i, d-iii

    Each secondary metabolite is matched to its group.

39. An enzyme reaches half of its maximum velocity at a substrate concentration of 5 mM. The Km of this enzyme is

    1. 20 mM
    2. 5 mM
    3. 2.5 mM
    4. 10 mM
    Answer

    B. 5 mM

    Km is the substrate concentration at half of Vmax.

40. A mixture contains succinic dehydrogenase, succinate and malonate, and the rate of fumarate formation is low. Adding more succinate will

    1. have no effect, since malonate binds permanently
    2. decrease the rate further
    3. destroy the enzyme
    4. increase the rate because succinate competes better with malonate
    Answer

    D. increase the rate because succinate competes better with malonate

    Malonate is a competitive inhibitor, so extra substrate reduces its effect.

41. A tissue is ground in trichloroacetic acid and filtered. A protein in the tissue will be found in the

    1. retentate (acid-insoluble fraction)
    2. filtrate (acid-soluble pool)
    3. both fractions equally
    4. neither fraction
    Answer

    A. retentate (acid-insoluble fraction)

    Proteins are macromolecules and stay in the acid-insoluble fraction.

42. How many peptide bonds are present in a tripeptide?

    1. 3
    2. 2
    3. 1
    4. 4
    Answer

    B. 2

    A chain of n amino acids has n minus 1 peptide bonds.

43. A highly branched polysaccharide such as glycogen has

    1. many reducing ends and one non-reducing end
    2. only reducing ends
    3. one reducing end and many non-reducing ends
    4. only non-reducing ends
    Answer

    C. one reducing end and many non-reducing ends

    Branched chains have many non-reducing ends and one reducing end.

44. An enzyme loses its activity when a loosely bound organic cofactor is removed by dialysis. Activity returns when NAD is added. The cofactor is a

    1. metal ion
    2. apoenzyme
    3. prosthetic group
    4. co-enzyme
    Answer

    D. co-enzyme

    A loosely bound organic cofactor is a co-enzyme. A prosthetic group is tightly bound.

45. A single strand of nucleic acid has 100 nucleotides joined in a chain. How many phosphodiester bonds does the chain contain?

    1. 98
    2. 99
    3. 100
    4. 101
    Answer

    B. 99

    n nucleotides in one chain are joined by n minus 1 phosphodiester bonds.