PJC H2 Chem 2012 Prelim P3 Soln

© PJC 2012 9647/03/JC2 Prelims/2012 2 Answer any four questions 1 Enzymes are biological molecules that catalyse chemical reactions . Almost all chemical reactions in a biological cell need enzymes in order to occur at rates sufficient for life. (a) Acquired Immuno -Deficiency Syndrome (AIDS) is a disease of the human immune system caused by the Human Immunodeficiency Virus (HIV). HIV- 1 Protease is an enzyme which speeds up the reproduction of HIV. In the development of anti -HIV drugs, scientists often study the amino acid sequence of the HIV -1 Protease to understand how this enzyme dis plays its biological properties in the human body. (i) A segment of the polypeptide structure of HIV -1 Protease, containing 10 amino acids, was digested using an enzyme and the following fragments were obtained. leu – asn – phe ile – gly – cys – thr – leu leu – thr – glu – ile – gly Deduce the primary structure of this segment of HIV-1 Protease. leu – thr – glu – ile – gly – cys – thr – leu – asn – phe (ii) A segment of the HIV strand acts as a substrate , which binds to the active site of the HIV -1 Protease which speeds up the reproduction of the HIV virus. The rate of this enzyme- catalysed reaction is investigated and can be represented by the following graph. Explain the difference in the rate of reaction at high and low co ncentrations of substrate. At low substrate concentration, § Rate of reaction increases linearly / reaction is first order wrt the substrate concentration as active sites of the enzyme are not fully occupied. At high substrate concentration, § Rat e of reaction is constant / rate is independent of substrate concentration / reaction is zero order wrt the substrate concentration as all active sites occupied. rate substrate concentration

© PJC 2012 9647/03/JC2 Prelims/2012 3 (iii) The structural formulae of the three most common amino acids present in the protein chain in HIV-1 Protease are shown below. CH2 C H NH2 COOH CH2 COOH CH C H NH2 COOH CH2 CH3 CH3 SH C H NH2 COOH CH2 glutamic acid (glu) leucine (leu) cysteine (cys) In how many different ways can these three amino acids be coupled by peptide bonds to form a tripeptide? Draw the structural formula of one such peptide. 3! = 6 ways CH2 C H2N C CH2 COOH O CH C H N C CH2 CH3 CH3 O SH C H N COOH CH2 H H H **Accept other correct tripeptides (iv) The side chains of these three amino acids are responsible for maintaining the tertiary structure of HIV-1 Protease. State and draw the type of side-chain interaction between two cysteine (cys) residues. State the type of reaction that is involved in the formation of the side- chain interaction.

© PJC 2012 9647/03/JC2 Prelims/2012 4 Type of reaction: oxidation (v) Since its discovery in 1981, scientists have studied the protein structure of HIV in order to prevent