Classification and Nomenclature of Enzymes and Co-factors
This quiz closes the Biomolecules course by testing two connected ideas: how enzymes are classified and named, and what non-protein helpers they need to work.
- The six classes of enzymes and the reaction pattern each one performs.
- The four-digit number that gives every enzyme a precise identity.
- Co-factors: prosthetic groups, co-enzymes and metal ions, plus the meaning of apoenzyme.
- Key examples: haem in catalase, NAD and NADP with niacin, and zinc in carboxypeptidase.
Work carefully through each reaction scheme and each definition — these are the details most often confused in examinations.
Q1. On what basis have most discovered enzymes been grouped into different classes?
Enzymes are classified according to the type of reactions they catalyse, not by the substrate they happen to act upon.
Q2. Into how many classes are enzymes divided, and how is each enzyme named?
Enzymes are divided into 6 classes, each with 4-13 subclasses, and each enzyme is named accordingly by a four-digit number.
Q3. Which class of enzymes catalyses the reaction ##\ce{S_{reduced} + S'_{oxidised} -> S_{oxidised} + S'_{reduced}}##?
Oxidoreductases, also called dehydrogenases, catalyse oxidoreduction between two substrates S and S'.
Q4. Transferases catalyse the transfer of a group G between two substrates. Which group is excluded from this transfer?
Transferases catalyse transfer of a group G other than hydrogen, following the pattern ##\ce{S-G + S' -> S + S'-G}##.
Q5. An enzyme catalyses the hydrolysis of a glycosidic bond. To which class does it belong?
Hydrolases catalyse hydrolysis of ester, ether, peptide, glycosidic, C-C, C-halide or P-N bonds.
Q6. Which statement correctly describes lyases?
Lyases catalyse removal of groups from substrates by mechanisms other than hydrolysis, leaving double bonds.
Q7. Enzymes that catalyse the inter-conversion of optical, geometric or positional isomers belong to which class?
Isomerases include all enzymes catalysing inter-conversion of optical, geometric or positional isomers.
Q8. Ligases catalyse the linking together of two compounds. Which bond types are given as examples of such joining?
Ligases catalyse joining of C-O, C-S, C-N, P-O etc. bonds.
Q9. In an enzyme that requires a non-protein constituent for activity, what is the protein portion called?
When a non-protein co-factor is bound to make the enzyme catalytically active, the protein portion is called the apoenzyme.
Q10. Which of the following correctly lists the three kinds of co-factors?
Three kinds of cofactors may be identified: prosthetic groups, co-enzymes and metal ions.
Q11. What distinguishes a prosthetic group from other co-factors?
Prosthetic groups are organic compounds distinguished from other cofactors in that they are tightly bound to the apoenzyme.
Q12. In peroxidase and catalase, which catalyse the breakdown of hydrogen peroxide to water and oxygen, what is the prosthetic group?
In peroxidase and catalase, haem is the prosthetic group and it is a part of the active site of the enzyme.
Q13. Which statement best describes co-enzymes?
Co-enzymes are organic compounds whose association with the apoenzyme is only transient, usually occurring during the course of catalysis.
Q14. The coenzymes NAD and NADP contain which vitamin as their essential chemical component?
Coenzyme nicotinamide adenine dinucleotide (NAD) and NADP contain the vitamin niacin.
Q15. Zinc acts as a cofactor for the proteolytic enzyme carboxypeptidase. What happens to catalytic activity when the co-factor is removed from an enzyme?
Catalytic activity is lost when the co-factor is removed from the enzyme, which testifies that co-factors play a crucial role in catalytic activity.