RI Nov 16 H2 P2

2016 H2 Biology P2 9744/02 1 (a) Explain why, in the reaction with the enzyme only, as substrate concentration increases: (i) the rate of reaction increases at first [2] 1. As substrate concentration increases, frequency of effective collisions * between enzyme and substrate molecules increases; 2. Rate of enzyme-substrate complex * formation increases and rate of reaction increases as active sites * of enzymes are readily available and substrate concentration is limiting; (ii) the rate of reaction becomes constant. [2] 1. All active sites* of enzymes are saturated with substrate at any one point in time; 2. Concentration of substrate is no longer limiting and enzyme concentration is limiting, rate of reaction will remain constant (graph plateaus). (b) Explain why in Fig 1.1, the addition of a non-competitive inhibitor causes the reaction to become constant at a lower rate. [2] 1. Inhibitor binds to site other than active site and changes conformation* of active site*; 2. Hence inhibitor effectively decreases availability of enzymes as it forms an enzyme-inhibitor complex; 3. Effects of inhibition cannot be overcome by increasing substrate concentration; (c) Draw, on Fig. 1.1, the approximate shape of t he curve if a competitive inhibitor were added to the enzyme instead of a non-competitive inhibitor. [2] 1. Correct shape and labelled; 2. approaches maximum rate of reaction; (d) Suggest why the penicillin molecule is an effective inhibitor of transpeptidase. [2] 1. Cell wall peptide and penicillin are similar in conformation and hence both are complementary in shape and charge to the active site; 2. Penicillin can act as a competitive inhibitor*for the active site *of transpeptidase; 3. Hence, cell wall peptides cannot bind to acti ve site and cross-links of cell wall peptides cannot form. 2 (a) Identify the molecules labelled A and B on Fig. 2.1. [2] A: messenger RNA B: RNA polymerase (R: general transcription factor as the molecule is unzipping and separating the double helix DNA) (b) Explain how the control elements shown in Fig. 2.1 influence transcription. [4] Promoter (c) Competitive inhibitor

1. serve as recognition site for the binding of general transcription factors * and RNA polymerase*; 2. to initiate transcription*; 3. has critical elements, TATA box* that determines the precise location of transcription start site; 4. has critical elements, CAAT and GC boxes * to improve efficiency of promoter by recruiting general transcription factors* and RNA polymerase* to promoter. Enhancer 5. when bound with specific transcription factors* known as activators*, promotes assembly of transcription initiation complex* 6. when bound with specific transcription factors* known as activators*, may recruit histone acetyltransferase* and chromatin remodeling complexes * to decondense chromatin (increase accessibility of promote