A level Biology 3.1 Mode of action of enzymes – Exam style question – Paper 2

Question [Maximum marks: 11]

Penicillin is an antibiotic that interferes with the synthesis of cell walls in bacteria. Even before
penicillin became widely available in the 1940s, the enzyme penicillinase which breaks down
penicillin had been isolated. This enzyme is now found in many bacteria and gives them
resistance to penicillin.

Fig. 4.1 is a ribbon model of the structure of the enzyme penicillinase. The arrow indicates
the active site of the enzyme.

(a) Explain why the shape of the active site of an enzyme, such as penicillinase, is important.

(b) With reference to Fig. 4.1, identify the aspects of protein structure that are shown and
       those that are not shown.

       Fig. 4.2 shows the changes in energy during the progress of an uncatalysed reaction.

(c)        (i) Draw on Fig. 4.2 a curve to show changes in energy during the progress of the
                   same reaction when catalysed by an enzyme.

(ii) State the term given to the energy level that must be overcome before a reaction                                                                                                                                                            can progress.

(d) Antibiotic resistance is a serious worldwide problem.

       Suggest how antibiotics can be used effectively to avoid the development of widespread
       resistance in bacteria.

Answer/Explanation

Answer:     4(a) this can be answered in the context of penicillinase
                                 1 complementary shape ;
                                 2 substrate, fits into / enters / binds to / with, active site ;
                                     A enzyme-substrate complex / ESC
                                 3 ref. to specificity ;
                                 4 lock and key / induced fit ; A description of induced fit
                                 5 ref. to temporary bonds form with, active site / R groups (of amino acid residues) ; [max 3]

4(b) shown to max 2

          secondary structure ;
          α / alpha, helix ; R ‘helix’ / helical structure unqualified by alpha
          β pleated sheet ;
          tertiary structure / folding ; ignore 3D shape or structure
          globular ;

         not shown to max 2
        amino acids / primary structure / sequence of amino acids ;
        (types of) R groups ;
        bonds / named bonds ; A peptide
        quaternary structure ;
        prosthetic group ; [max 3]

4(c) (i) one lower peak inside line than uncatalysed ;
               start and finish at, dotted lines / same energy levels as uncatalysed ; 

        (ii) activation (energy) / (energy of) activation ; 

4(d) 1 do not prescribe for viral diseases ;
          2 only use when necessary / do not overprescribe ;
          3 only available on prescription / not available ‘over the counter’ ;
          4 people must, complete the course / take as instructed ;
              R take a long course
          5 test to find out which is most appropriate antibiotic to use ;
               A use most, appropriate / effective, antibiotics
               A use narrow-spectrum antibiotics
           6 details of sensitivity test ;
           7 rotate / AW, antibiotics / use in combination ; R use many antibiotics
           8 do not use same antibiotics for humans and animals ; [max 2]

Question

(a) Table 2.1 shows eight ions that are biologically important.

       Choose one ion to match each of the following statements. In each case write one letter from Table 2.1. You may use each letter (A to H) once, more than once or not at all.

       (i) A component of polynucleotides.[1]

       (ii) Ion produced by enzyme activity inside red blood cells.[1]

       (iii) Ion used in the production of all amino acids in chloroplasts.[1]

       (iv) Ion that diffuses through carrier proteins with sucrose into companion cells in phloem tissue.[1]

       (v) Component of haem group in haemoglobin that binds oxygen.[1]

   (b) The enzyme nitrogenase is found in free-living and symbiotic nitrogen-fixing bacteria. Nitrogenase catalyses the reaction:

                                     \(N_{2\left ( g \right )}+ 6 e^{-} + 8H^{+}_{\left ( aq \right )}\rightarrow 2NH{_{4}}^{+} {_{\left ( aq \right )}}\)

          Some nitrogenase enzymes have vanadium ions in their active sites; others have molybdenum ions.

          Explain how the enzyme nitrogenase functions in the fixation of nitrogen.[4]

   (c) Some pea plants were grown with their roots in a solution of mineral ions. The solution was kept aerated for three days.

          The concentrations of five ions in the solution and in the root tissue were determined after the three days. The results are shown in Table 2.2.

          With reference to Table 2.2, suggest how cell surface membranes of root cells are responsible for the concentrations of ions in the roots compared to the surrounding solution.[5] [Total: 14]

Answer/Explanation

Ans:

2 (a) (i) G ;

          (ii) B/C ;

          (iii) A/F ;

          (iv) B ; 

          (v) D ; 

   (b) 1 nitrogen and hydrogen/ substrates, bind to/AW, active site ;
          2 enzyme-substrate complex (forms) ;
          3 ref. lock and key / induced fit, mechanism ;
          4 activation energy of reaction is lowered ;

          5 example of how activation energy lowered ;
                  e.g. strain on (triple) bond of, N2 / (di)nitrogen
                          A bond broken between nitrogen (atoms)
                  nitrogen and hydrogen ions held close together for bond formation
                  transfer of electrons
                  alternative pathway

          6 product/NH4+, leaves active site ;
          7 ATP, required/ used/provided from respiration ;
          8 ref. anaerobic conditions for enzyme action ;

          9 suggestion as to use of, vanadium/ molybdenum, in active site ;
                  e.g. act as cofactor/ coenzyme
                  transfer of, electrons /protons

   (c) 1 concentration of all the ions is greater in the root tissue than in the solution ; ora A roots
          2 comparative data quote ;

          according to these data
          3 (so) ions will not diffuse into the root tissue ;
                      A if (facilitated) diffusion only, initially / till equilibrium reached

          4 (so) active transport ; A active, uptake/ pumping I facilitated diffusion

          5 use ATP ; A energy
                               R ATP energy

          6 move ions, against concentration gradient/ from low to high concentration;
                  A diffusion gradient

          7 use, membrane/ integral/ intrinsic / transmembrane/ transport/ carrier, proteins ;
                  R channel proteins

          8 are specific / have specific binding sites ;
          9 involve, conformational/ shape, change ;

          10 comparative data quote to suggest that some ions are pumped more than others ;
                  e.g. steepest gradients for K+ and SO 4

          11 phospholipid bilayer/ hydrophobic core (of cell surface membrane) is impermeable to ions ;
          12 so ions cannot diffuse out/(membrane) proteins only allow inward flow of ions ;
          13 AVP ; e.g. suggestion of differing numbers of specific membrane proteins to explain observation of mp 10

Question

Starch is composed of two polysaccharides, amylose and amylopectin.

   Fig. 3.1 shows a molecule of α-glucose before being added to the end of a molecule of amylose.

   (a) (i) Complete Fig. 3.1 to show how a molecule of α-glucose is added to the amylose. [3]

         (ii) Name the bond that forms between glucose molecules in polysaccharides, such as amylose.[1]

   (b) Glycogen and cellulose are two other polysaccharides.

         Complete Table 3.1 to compare glycogen and cellulose with amylose.[3]

(c) Type 2 diabetes (insulin-independent diabetes) is a non-infectious disease.

         If not treated, this disease is characterised by large fluctuations in the concentration of glucose in the blood.

         Maltase is an enzyme that completes the digestion of starch in humans. Molecules of maltase are bound to the microvilli of epithelial cells in the small intestine.

         Ascorbase is a drug used in the treatment of type 2 diabetes. Molecules of ascorbase have a very similar shape to that of the substrate for maltase.

         (i) Explain how ascorbase acts to inhibit these membrane-bound enzymes. [3]

         (ii) Suggest why ascorbase can be used to treat people who have type 2 diabetes. [2] [Total: 12]

Answer/Explanation

Ans:

3 (a) (i) –H and –OH indicated ; A –OH on end of amylose and –H on alpha glucose
                water eliminated/ condensation ; A dehydration
                oxygen bridge/glycosidic bond, drawn between C1 and C4 ;

                If the whole glucose molecule and/or the end of the amylose molecule has not been drawn, then only award mp3 if C1 and C4 are indicated in some way, e.g. by numbering them or putting in the hydrogens. 
         (ii) (1,4/1,6) glycosidic ; A glucosidic A phonetic spelling of glycosidic

    (b)

    (c) (i) maltase and maltose must be correctly referenced
                    ignore references to reversible/irreversible

                (ascorbase) binds to/ fits into/ enters active site ;
                complementary (shape) to active site ;
                so substrate/ maltose, cannot enter/ cannot bind ;
                      A no/few, ES complex
                      A prevents formation of ES complexes
                      A ascorbase forms enzyme inhibitor complex

                competes with substrate / competitive inhibition ;
                slows the (rate of), digestion/ hydrolysis / breakdown, of maltose ;
                      R ‘stops the reaction’
                      R if in context of starch

                alternative answer if candidates assume ascorbase is an enzyme:
                ascorbase, breaks down/ digests / hydrolyses, maltase ;
                       A ascorbase destroys the active site of maltase
                so no enzymes to digests maltose ;
                slows / stops, reaction/digestion/hydrolysis / breakdown, of maltose ;

           (ii) inhibits / slows down/ prevents, breakdown/(catalysing) hydrolysis / digestion, of maltose (to glucose) ; I starch
                  less glucose is absorbed/ passes across membranes /enters blood ;

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