Question 112 marks
Question
(a) During translation, a polypeptide is synthesised when amino acids are added to a growing chain of amino acids. Fig. 1.1 shows part of a growing chain of amino acids and the amino acid cysteine. cysteine SH chain of amino acids CH2 O H O …… C + N C C OH H H OH …… C Fig. 1.1 (i) Complete Fig. 1.1 by showing the formation of the bond between cysteine and the growing chain of amino acids in the process of translation. [3] (ii) State the name of the covalent bond that forms when cysteine is added to the growing chain of amino acids. …… [1] (iii) State the organelle where the reaction shown in Fig. 1.1 takes place. …… [1] (b) Fig. 1.2 is a ribbon diagram showing the three-dimensional structure of a protein from the bacterium Streptococcus. Fig. 1.2 (i) Describe the secondary structure of the protein shown in Fig. 1.2. …… [2] (ii) Explain why the protein shown in Fig. 1.2 has tertiary structure, but not quaternary structure. …… [2] (iii) An analysis of the amino acid composition of the protein in Fig. 1.2 showed that it does not contain any cysteine residues. Explain how the three-dimensional structure of the protein shown in Fig. 1.2 is held in place. …… [3] [Total: 12]
Mark scheme
1(a)(i) C-N bond shown ; 3 =O shown on C and -H shown on N of peptide group ; water shown as a product ; 1(a)(ii) peptide ; 1 A amide R polypeptide bond 1(a)(iii) ribosome ; 1 I 70S / 80S A rough endoplasmic reticulum I rough ER / RER 1(b)(i) any two from: 2 alpha- / α-, helix, helices ; R A or a for alpha beta- / β-, pleated sheet(s) / pleat(s) ; A beta / β, sheet(s) R B or b for beta region of no fixed shape / AW ; A random coil 1(b)(ii) any two from: 2 1 protein is, a single polypeptide / made of one chain of amino acids or quaternary structure is two or more polypeptides / more than one polypeptide ; 2 interactions / bonds, between, R-groups / side chains, (of amino acids) on the same polypeptide or 3 quaternary has, interactions / bonds, between, R-groups / side chains, (of amino acids) on different polypeptides ; 4 AVP ; e.g. no prosthetic group 1(b)(iii) secondary structure 3 1 in alpha-helix each N-H forms hydrogen bond with C=O three or four amino acids apart in same polypeptide ; 2 in beta-pleated sheet each N-H forms hydrogen bond with C=O on adjacent part of the polypeptide ; if mp1 or mp2 not awarded 3 (by) hydrogen bonds (in secondary structure) ; tertiary structure 4 hydrogen bonds between, R-groups with C=O and HN groups / carboxyl and amine groups ; A between polar R groups 5 ionic bonds between R-groups with NH3+ and COO- groups ; A between charged R groups / AW 6 hydrophobic interactions between non-polar R-groups ; if mp4, mp5 or mp6 not awarded 7 (named) interactions / bonds, between R-groups (in tertiary structure) ;