PJC H2 Chem 2012 Prelim P3 QP

Name: Index No.: CT Group: 11 PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION HIGHER 2 CHEMISTRY 9647/03 Paper 3 Free Response 14 September 2012 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet Cover Page READ THESE INSTRUCTIONS FIRST Write your Centre number, index number and name on all work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough workings. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four out of five questions. State the question number of the questions that you have attempted on the cover page provided. The question numbers should be stated in sequence. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. This document consists of 15 printed pages.

© 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 displays 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. (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 concentrations of substrate. 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. (iv) The side chains of these three am