HL 1 Biology

7.6b Enzymes

<<Activation energy and enzymes>>

Activation energy: used to break/weaken bonds in the substrates

  • substrates need to pass through transition state before converted into products
  • some energy is required to reach the transition state
  • When enzyme catalyses reaction; substrate binds to the active site and  altered to reach transition state
      • binding lowers overall energy level of transition state; activation energy of reaction reduced
      • net amount of energy released by reaction unchanged with enzymes
      • activation energy is reduced + rate of reaction greatly increase

Catalysts reduce the activation energy, this is important to living by speeding up the rate at which products are converted. Enzymes reduce the atoms activation energy.

<<The Induced-fit Model>>

Induced-fit Model: when substrates bind, cause structure of active site to change so it fits substrates during enzyme activity

  • since structure + shape of active site and substrates do not exactly fit
  • both substrates and active site undergo conformational changes during binding
      • helps weaken/break bonds in substrate
      • lowers activation energy and catalyze reaction
  • explains how some enzymes can catalyze different reactions
      • different substrates are able to bind to active site
      • e.g. hexokinase: can phosphorylate different hexoses

<<Enzyme inhibition>>

Inhibitors: chemical substances which binds to enzymes and reduces activity of enzyme

  • Competitive:  inhibitor binds reversibly to enzymes active site. While bound, substrates cannot bind
      • e.g. Enzyme: Succinate dehydroenase
      • Inhibitor: malonate
      • e.g. Antibiotics: can take medicine to block enzymes of bacteria.
      • Elevator: When more juniors (inhibitor) ride the elevator, it takes more time for the seniors to wait to ride the elevator.
  • Non-competitive: inhibitor binds some where besides the active site. While bound, active site is rearranged & substrate cannot bind
      • e.g. Enzyme: phosphofructokinase
      • Substrate: fructose-6-phosphate
      • Inhibitor: xylitol-5-phostate
      • e.g. Inhibitor: Mercury+ Substrate: protein + Enzyme: Papain
      • binds to enzyme (papain), mercury in binding to, changes active site.
      • e.g. Minamata Bay: mercury problem
      • Babies were not developed properly because of the enzymes
      • e.g. Alice in Wonderland=
      • Mad Hatter’s hat is cleaned with mercury, he breathed it in = crazy man
      • Enzyme: Nitric Oxide Synthase
      • Inhibitor: morphine

**More inhibitors, slower activity. Balance between substrate and inhibitor determines the rate of reaction.

<<Metabolic pathways>>

Common patterns:

  • most chemical changes happen in sequence of small steps, together forming metabolic pathway
  • most metabolic pathways involve chain of reactions
  • metabolic pathways form a cycle, than chain

~End-product Inhibition of Metabolic Pathways~

Allosteric interactions: enzymes regulated by chemical substances that bind to special sites on enzyme away from active site

Allosteric site: the binding site (switched on when activator binds)

  • Regulated enzyme catalyses one of first reactions in metabolic pathway
  • substance that binds to allosteric site is end-product of pathway
  • end product = inhibitor
  • Pathway rapid in cells when shortage of end-product, can stop when there’s excess end-product
  • when concentration of products increase, a reaction eventually slowdowns and stops. End-product inhibition avoids build=up of intermediate products

Metabolic Pathway: A–(E1)—> B –(E2)—> C

  • A: Substrate
  • E1: Enzyme 1
  • B: Product (Becomes Substrate for Enzyme 2)
  • E2: Enzyme 2
  • C: Product of Enzyme 2
  • Sequence repeated until final product is made

e.g ATP synthesis: phosphofructokinase


The Effectiveness of Enzymes (p.84)

(1) OMP decarboxylase shows the slowest uncatalyzed rate.
(2)OMP decarboxylase has the highest rate when enzymes are present.
(3) Ketosteroid isomerase = 3.8 x 10^11, Nuclease =  5.6 x 10^20,  OMP decarboxylase = 1.4 x 10^24
(4) The OMP decarboxylase is the most effective because of its quickest reaction with a catalyst and slowest reaction without a catalyst.
(5) The substrate binds to the enzyme’s active site which increase reactions by a conformational change in the enzyme. This conformational change in the enzyme will weaken the bonds with the substrates to in order to increase reactions. Also the effectiveness increases in the collision within substrates resulting in more reactions.

Chapter 7 Questions (p.88)

1a (i) Between the temperature of 20˚ to 40˚, no change and effect is placed on the activity of both immobilized and dissolved papain. From 40˚, dissolved papain shows a decline in activity.

(ii) Temperature effects the activity of dissolved papain because the high temperature changes the enzymes structure which would then permit substrates to fit to active site. This change in enzyme structure results to denaturation, and will starts to decline in activity.

b (i) Both the dissolved papain and immobilized papain show a decline in activity as the temperature rises. The activity of dissolved papain starts to decline from temperature of 40˚ whereas immobilized papain only performs a decrease in activity when the temperature is higher than 50˚.

(ii) The difference may be due because of the structure of the papain where the dissolved papain is less stable to heat, causing an earlier decline in activity at a lower temperature compared to the immobilized papain. The enzyme molecule may also be more stable when bound to solid surfaces like those in immobilized papain.

(iii) Maltase can be useful in the small intestines of the human body.

2(a) curve B

b (i) Although activity of enzymes were close to constant, it started to decrease.

(ii) During the experiment the change in enzyme activity occurs due to the reduction in substrate concentration. This is seen when the substrate is converted into a product.

3(a) “Metabolic Pathway” = enzyme catalyzed + form chain or cycle of reactions

b(i) PYR

(ii) FDP

(iii) PEP

4(a) Cells need many different enzymes in their cytoplasm because there are many reactions that are taking place but all of these reactions are catalyzed by enzymes. Reach of these reactions need specific and different enzymes for reaction to be performed, thus allows the control of individual reactions.


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