Factors affecting Enzymes

Specification

Enzymes & pH

Each specific enzyme can only work over a particular range of pH Each enzyme has its own optimum pH where the rate of reaction is maximum The effects of pH on the rate of enzyme controlled reactions display characteristically bell shaped curves ABC Enzyme A = amylase optimum pH = 7.2 Enzyme B = pepsin optimum pH = 2.0 Enzyme C = lipase optimum pH = 9.0 Changes in pH can affect the ionic and hydrogen bonds responsible for the specific tertiary shape of enzymes. Extremes of pH break these bonds and denature the enzyme Enzymes and pH

Denaturation: Hydrogen ions interfere with the bonds that hold the 3D, tertiary structure, this changes the shape of the active site so no enzyme substrate complexes can be formed

Summary Low pH = lots of H + ions H + ions have a positive charge Either extreme of H + ion concentration can interfere with the hydrogen and ionic bonds holding the tertiary structure together. If the pH change affects the charge on the amino acids at the active site, then the properties of the active site change and the substrate can no longer bind At high pH values, the –COOH group will dissociate to become a charged –COO - group

PPQ A teacher asked two students to state the optimum pH for this enzyme. Student A gave the answer ‘pH 3.25’ but student B gave the answer ‘somewhere between pH 3.0 and pH 4.3’. The teacher said that student B had given the better answer. 1.Explain why student B’s answer was better. [2] 2.Explain why this enzyme is not active at pH 7. [2]

1. Markscheme [2] not enough points plotted / experiment not carried out at enough (different) pH values; only 1 point between / no points between ; don’t know / uncertainty of, rate between those points / where peak should be / where optimum is; 3.25 reading might be anomalous; cannot draw, curve / line of best fit; rises to, 3 / 3.25, and falls after 4.3;

2. Markscheme [2] note ~ enzyme is completely inactive at pH 7 loss of tertiary structure / loss of 3D structure / (enzyme) denatured; (change in pH/[H+]) alters charge distribution on (enzyme) molecule; hydrogen / ionic, bonds affected; changes (shape of) active site; enzyme substrate complex cannot be formed / substrate not attracted to active site / substrate cannot bind to active site / AW;

Enzymes & Temperature

Enzymes and Temperature Increasing rate: more kinetic energy means more collisions, which means more enzyme substrate complexes form

Enzymes and Temperature Denaturation: vibrations break bonds holding the 3D shape, the tertiary structure in place, this changes the shape of the active site, so no enzyme substrate complexes can be formed

PPQ [3]

Mark Scheme

Enzymes & Enzyme Concentration

The Effect Of Enzyme Concentration On The Rate Of Enzyme - Catalysed Reactions Rate of reaction Increasing concentration of enzyme The rate of reaction is directly proportional to the enzyme concentration As enzyme concentration increases, the rate of reaction increases In living cells, enzyme concentrations are usually much lower than substrate concentrations Substrate concentration is rarely a limiting factor

Enzymes and Enzyme Concentration As EC increases, more active sites become available so more enzyme substrate complexes form With a fixed concentration of substrate: A point will be reached when all the substrates are occupying active sites

Enzyme & Substrate Concentration

The Effect Of Substrate Concentration On The Rate Of Enzyme - Catalysed Reactions Low Substrate Concentration Low product concentration per unit time Increased Substrate Concentration More product formation; increased reaction rate

Further increase in substrate concentration Excess substrate concentration Maximum product formation; maximum rate of reaction No further increase in product formation; maximum reaction rate maintained Enzyme concentration is the LIMITING FACTOR The Effect Of Substrate Concentration On The Rate Of Enzyme - Catalysed Reactions

Enzymes and Substrate Concentration As SC increases there are more collisions and more enzyme substrate complexes form Point of saturation: further SC has no effect, as all the active sites are occupied, forming enzyme substrate complexes as fast as possible

PPQ Explain the effects of enzyme concentration, substrate concentration and competitive inhibitors on the rate of an enzyme-controlled reaction. [9]

Markscheme Enzyme concentration (3 max) 1reaction (rate) increases with increased enzyme; A high / low 2more active sites available; 3in excess substrate / as long as enough substrate (molecules available to occupy active site); 4(as reaction progresses) the rate will decrease as substrate, used up / becomes limiting; R plateau

Markscheme Substrate concentration (3 max) 1reaction (rate) increases with increased substrate; A high / low 2more, molecules available to enter active site / ESC formed; A more successful collisions 3reaches point where all active sites occupied; 4no further increase in rate / reaches Vmax; A plateau / levels off 5enzyme conc. becomes limiting / unless add more enzyme;

Markscheme Competitive inhibitor (5 max) 1inhibitor has similar shape to substrate; 2can, fit / occupy, active site; 3for short time / temporary / reversible; 4prevents / blocks, substrate from entering active site; 5rate determined by relative concentrations; 6little inhibition / rate little reduced, if substrate conc. > inhibitor conc.; ora 7ref to chance of, substrate / inhibitor, entering active site; 8effects can be reversed by increasing substrate conc. General points 10drawing a suitable graph to illustrate point made with labelled axes; 11ref to optimum (rate);

Temperature coefficient Q 10