Enzymes Flashcards
Describe how an enzyme, such as pepsin, breaks down a substrate. (5mks) F212 2015 q1b
substrate / protein , shapeis (nearly) complementary
to active site; ora
substrate / protein , enters / fits into , active site
(on enzyme) ; [ binds to / holds / bonds to]
induced fit / description of induced fit ;
(forms) enzyme-substrate complex / ESC ;
destabilising / straining / AW , of bonds (in substrate) ;
then (forms) enzyme-product complex ;
product(s) / amino acids , leave (active site) ;
[ “substrate binds to the active site which is
complementary to the substrate shape” = 2 marks, ]
Another student suggested that he should repeat the investigation at least twice. How would this have improved the investigation? F212 2015 q1civ
improve reliability; [identify , anomalous results / outliers]
assess, variability / spread of results; [eference to statistical test; standard deviation / t-test / Mann-Whitney]
allows calculation of mean [improves accuracy of mean]
Pepstatin is a competitive inhibitor of pepsin.
On Fig. 1.1, draw a line to represent the effect of adding a fixed concentration of pepstatin on the rate of pepsin activity over the whole range of substrate concentrations F212 2015 q1di
line drawn below line on graph ;
line from origin that does not peak or plateau ; [lines touching at right hand end; plateau if it joins the original line plateau below original line if it starts 4 small ]
squares (or fewer) from the end
Pepstatin acts as a competitive inhibitor of pepsin.
What can you conclude about the structure of pepstatin? F212 2015 q1dii
similar shape to , substrate / (part of) albumin / protein;
complementary (shape) to (part of) active site; [ same shape as part of substrate/ structure/ tertiary structure]
Amylase is an extracellular enzyme that catalyses the breakdown of the polysaccharide starch (amylose) in the digestive system of many animals.
(a) Why is the enzyme amylase described as being extracellular?
[F212/01 June 14 q2]
(works) outside cells; [secreted from cells works; in named extracellular environment e.g. digestive tract]
A student investigated the effect of changing the concentration of starch on the rate of starch
breakdown by amylase. Graph of starch concentration vs rate of product formation, almost linear positive gradient till plateau.
(i) To calculate the rate of starch breakdown, the student measured the concentration of the
breakdown product.
State the other variable the student needed to know in order to calculate the rate of this
reaction.
[F212/01 June 14 q2]
time / time taken ; [how long it took]
A student investigated the effect of changing the concentration of starch on the rate of starch
breakdown by amylase. Graph of starch concentration vs rate of product formation, almost linear positive gradient till plateau.
(ii) Explain the shape of the graph shown in Fig. 2.1.
5mks [F212/01 June 14 q2]
linear part of the graph means:
1) MORE (successful) collisions with (amylase) active site (at increasing starch concentration); [few(er) active sites occupied at low starch concentrations]
2) MORE ESC (at increasing starch concentration) ; [ESC formed more easily]
3) so more product formation IN A GIVEN TIME (at increasing starch concentration) ; ora
curve / plateau , means…
4) all / most , active sites (of amylase) are occupied ; [all active sites are full of substrate]
5) enzyme / amylase , working , at / near, maximum rate / Vmax ; [enzyme at full capacity]
6) (so) further increase in starch concentration
has no effect (on rate) ; [Must link to 4 or 5]
7) enzyme CONCentration , is / becomes , LIMITing factor ; [the increasing part of the graph is because starch concentration is the limiting factor]
A student investigated the effect of changing the concentration of starch on the rate of starch
breakdown by amylase. Graph of starch concentration vs rate of product formation, almost linear positive gradient till plateau.
(iii) The student kept the pH of the solution constant during the experiment.
Explain why it is important that the pH was kept constant. (3mks) (f212 June 14 q2biii)
1) (so) charges in active site do not change;
2) (so) hydrogen / ionic , bonds unaffected;
3) (so) tertiary structure/ 3D shape/ active site, unaltered; [tertiary structure affected]
4) (so) enzyme/ tertiary structure, does not denature;
5) (so) substrate, fits/ is complementary shape to, active site;
6) so the results are valid/ as the rate (of reaction) will vary if pH varies/ so that only one (independent) variable is changed;
[The mark points refer to a constant pH preventing damage to the enzyme; throughout the appropriate marking point for an answer that describes what would happen if the pH changed.]
A student investigated the effect of changing the concentration of starch on the rate of starch
breakdown by amylase. Graph of starch concentration vs rate of product formation, almost linear positive gradient till plateau.
(iv) Suggest two other variables the student should have kept constant during the experiment. (F212 June 14 q2biv)
1) temperature (of the reaction mixture);
2) enzyme/ amylase , concentration;
3) (total) volume of (reaction) solution; [volume of enzyme solution]
4) concentration of, cofactors/ chloride ions; [concentration of coenzymes]
Many enzymes are associated with non-protein molecules known as cofactors. Some cofactors are small inorganic ions.
Rennin is an enzyme that is involved in the digestion of milk. It converts soluble caseinogen in milk into insoluble casein. The cofactor Ca2+ is associated with this reaction.
A student wished to investigate the effect of Ca2+
on the action of rennin.
Describe how the student could carry out this investigation and produce valid results. (5mks) F212 jan13 3b
1 carry out with and without , Ca2+/ cofactor; [use a control with no calcium’]
2 using at least three concentrations (of Ca2+)
(other than zero);
3 keep, concentration / volume of, enzyme/ rennin, constant;
4 keep, concentration/ volume of, caseinogen/ substrate/ milk, constant;
5 keep, temperature / pH , constant;
6 measure appearance of , product / casein
or measure disappearance of , substrate / caseinogens
or assess cloudiness (of solution);
7 over time intervals/ after fixed time/ end point time;
8 replicates/ repeats; [must imply minimum of 3 in total, i.e. original plus two]
Enzyme cofactors are often derived from vitamins and minerals in the diet.
Proteins are required in large amounts in the diet whereas vitamins and minerals are required
only in small amounts.
Suggest why. F212 jan13 3c
idea of cofactors / minerals , being,
recycled / used again ;
idea that in enzyme action total mass of , cofactor /
coenzyme ,
very small compared to total mass of protein;
idea that proteins are used for purposes other than
enzymes;
proteins are not stored in the body but vitamins and
minerals are;
some enzymes don’t need cofactors;
