HL 1 Biology

3.6 & 7.6 Enzymes


Enzymes: globular proteins that work as catalysts, speeds up chemical reactions

  • called biological catalysts because = made by living cells + speeds up biochemical reactions
  • substrates substances enzymes convert into products in reactions
      • substrate —(enzyme)—-> product
  • found in all living cells
  • some are secreted from cells, work outside
  • enzyme-substrate specificity: property of an typical enzyme working on substrates using one reaction
      • observe the active site
      • shape + chemical properties of enzyme’s active site and substrate match (lock and key model)
      • helps only the substrate to bind
      • when substrates are bound to active site, converted into products which gets released and allows the substrates to be freed from the active site, to catalyze another reaction

<<Effects on Enzyme Activity>>

Two factors that have the main effects on enzyme activity: pH and high temperature

~Effect of pH~

pH: the measurement of acidity or alkalinity of a solution

  • higher pH = less acid/ more alkaline
  • lower pH = more acid/ less alkaline & higher hydrogen ion concentration
      • (acidity due to presence of hydrogen ions)
  • pH scale is logarithmic
      • reduction in pH by one unit => solution is 10 times more acidic
      • So…a solution:
      • at pH 7: neutral
      • at pH 6: slightly acidic
      • at pH 5: 10 times more acidic than pH 6
      • at pH 4: 100 times more acidic than pH 6

It’s effect on enzymes…

  • enzymes are sensitive to pH
  • most have optimum pH where activity is highest (But all enzymes have various pH optimum, depends on environment they work at)
      • increase/decrease in pH from optimum = enzyme activity decreases + will stop altogether
      • higher/lower hydrogen ion concentration than natural level enzyme works at = enzyme structure altered, including active site
  • Denaturation: structure of enzyme irreversibly altered when beyond a certain pH

~Effect of High Temperature~

As temperature rises, can effect enzyme activity to increase and decrease:

  • Enzyme activity increase
      • when liquid’s heated; particles inside obtain more kinetic energy
      • so enzymes + substrate molecules move around faster at higher temperature
      • thus, chance of a substrate molecule colliding with active site of enzyme increased
      • therefore, enzyme activity increases
  • Enzyme activity decrease
      • as enzymes are heated; bonds in enzymes vibrate more + chance of bonds breaking increases
      • denaturation as bonds in enzymes break, causing a permanent change in enzyme structure
      • when enzyme molecules denatured, unable to catalyze reactions anymore
      • increase in denatured enzymes in molecules = enzyme activity decrease
      • eventually all stops when enzyme is completely denatured

*When you get a fever, changes the shape of enzymes, cause you to feel sick.

* Eggs, when heated becomes white because they become denatured.

  • The amino acids with hydrogen bonds loose their hydrogen bonds when heated, and they attach to other enzymes. Connect, and become white.

<<Enzymes and Substrate Concentrations>>

Enzymes are unable to catalyze reaction until substrate binds to active site ==> due to random movement of liquid molecules that result in collision between substrate + active site

  • Increase in substrate concentration insolution with fixed concentration of enzymes & substrate:
      1. Rate of enzyme activity increases; as active site collisions will be more frequent
      2. Rate of enzyme activity decreases: more of active sites are occupied at any moment, greater proportion of substrate; active site collisions are blocked


Lactose: sugar that’s naturally present in milk

  • Formula of glucose and galactose conversion by enzyme lactose:
  • lactose —(lactase)—> glucose + galactose

Lactase: an enzyme that catalyzes the hydrolysis of lactose to glucose and galactose.

  • obtained from Kluveromyces lactis
  • Biotechnology companies: culture yeast, extract lactase from yest + purify it to sell to food manufacturing companies
  • Reasons lactase is used in food processing:
      • some are lactose-intolerant; cannot drink more than 250ml of milk per day (unless lactose-reduced)
      • less sweet than galactose/glucose; less sugar needed to be added to sweet foods containing milk (e.g. milk shakes, fruit yogurt)
      • tends to crystallize during production of ice cream, gives texture; glucose/galactose have smoother texture since they are more soluble than lactose + remain dissolved
      • bacteria ferment glucose/galactose more quickly than lactose; production of yogurt and cottage cheese is faster


1. Two different enzymes are needed for the synthesis of glycogen from glucose phosphate because of the enzyme-substrate specificity. Thus, there would need to be one enzyme catalyzing the 1’4 bonds and another enzyme catalyzing the 1″6 bonds formation.

2. Formation of side-branches increase the rate at which glucose phosphate molecules can be linked on to a growing glycogen molecule because the substrate for the enzyme increases as the formation of the 1″6 bond enables the enzyme to produce new chains and 1-4 bonds.

3. Curve A did not show movement because the heat has effected the enzyme activity to decrease. The heat/ high temperature has caused enzymes to denature, thus no enzyme activity is apparent in curve A.

4. (a) Curve B shows an increase in the percent conversion at the first 35 minutes, but the curve gradually starts to level this percent conversion starting form 50 minutes.

(b) As the rate of conversion is increasing at the start of the curve,  it is followed by an rapid increase in the amount of glycogen and substrate molecules. The bonding in glycogen could be producing more glycogen molecules, or adding more glycogen molecules to those that already exists causing the increase in the percent conversion.


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