STPM Biology - chapter 3: Revision essay question 13

Lysozyme is  an enzyme that can destroy certain bacteria by catalyzing the breakdown of polysaccharide chains that form part of their cell wall.

(a)    (i) Name the bond between the adjacent sugar units of a polysaccharide.
        (ii)Explain how the bond may be broken .

(b)   Suggest how the substrate may be held in the active site of an enzyme while the    reaction occurs.

Lysozyme is made up of 129 amino acids. The active site of composed of two amino acids that are at positions 35 and 52 in the amino acid sequence.

(c) Explain how these two amino acids are brought close together to form the active site,

(d)Explain the effect of increasing temperature on enzyme activity.

Answer

(a)(I ) The bond between adjacent sugar units of a polysaccharide is called a 1-4 glycosidic bond .

   (ii) This bond may be broken by hydrolysis (‘water-breakdown’),resulting in the relesde of its individual constituents.

(b)An enzyme is flexible and moulds to fit the substrate molecule. The enzyme initially has a binding configuration,which attracts the substrate. On binding to the enzyme, a temporary stricture called the enzyme –substrate complex is formed at the active site. The substrate disturbs the the shape of the substrate is in turn affected, thus lowering its activation energy . This is referred to as an induced fit of the substrate to the enzyme and allows the substrate to be held while the reaction occurs.

(c) The two amino acids (at positions 35 and 52) that form the active site are brought close together when the polypeptide chain first forms a helical structure(secondary structure),generally called is α-helix , as a result of  hydrogen bonding. The polypeptide helix then bends and twists into a compact tertiary structure. Disulphide, ionic and hydrogen bonds all contribute to the maintenance of tertiary structure allows the formation of the active site,which ontains the two essential amino acids close together .

(d) The rate of chemical reaction is approximately doubled for every 10°c rise in temperature .This is also applies to enzyme-catalysed reactions at low temperature,where an increase in temperature would increase the kinetic energy of the enzyme  molecules and cause the reate of reaction to be faster ,When the temperature increase above 40°c, there is a rapid decrease in the rate of reaction owing to the denaturation of enzyme protein,whereby the enzyme loses its catalytic properties.All enzymes are destroyed at 100°C.

Assignment submitted by C.,L.,Ooi, W.Y.,Ng, K.Q., Beh, Y.H.,Beh, Dhevan  2011/2012 (IBM)