REACTIONS & ENZYMES
Diagram showing how high teperatures alter enzyme structures Y axis: enzyme activity. X axis: temperature, centigrade. Plotted line climbs A graph showing the . Describe how pH, temperature, and the concentration of an enzyme and its The activity of an enzyme can be measured by monitoring either the rate at which a. Table of Contents. Endergonic Enzymes: Organic Catalysts | Learning Objectives | Links . Plot of enzyme activity as a function of pH for several enzymes.
In our laboratory exercise, we will use this much finer method for finding the best-fit parameters for the untransformed Michaelis-Menten relationship.
Enzyme catalyzed reactions can be inhibited competitively Enzymes can be altered in various ways. One way, is by competition between the substrate and a molecular analog called a competitive inhibitor. The competitive inhibitor is so similar to the substrate that it binds at the active site and accomplishes some kind of induced fit with the enzyme. However, the competitive inhibitor is different enough that it cannot react in the same way as the substrate.
So no chemical reaction occurs, no product is formed, the inhibitor is released unchanged, as is the enzyme. This relationship can be depicted as: The enzyme-inhibitor complex falls apart without forming a product. This competitive inhibition does not occur in isolation.
Generally the reaction system looks more like this: Obviously the frequency that the substrate occupies the site will be determined by how abundant the substrate is compared to the inhibitor.
If the substrate is present in relative abundance then the rate of the reaction will be the same as if the inhibitor were absent. In other words, the presence of a competitive inhibitor does not change the Vmax. It takes more substrate to reach half of Vmax. Again, if one measures the rates of reaction systems with various substrate concentrations and repeats those in the presence of the inhibitor, then a plot like the one above can be made.
If the Vmax is the same in the presence and absence of the inhibitor, and the Km increases in the presence of the inhibitor, then you have great evidence for a competitive inhibitor. An example of an enzyme regulated by competitive inhibition in plants perhaps to the benefit of people is 5-enolpyruvylshikimatephosphate EPSP synthase. This enzyme is critical in the synthesis of aromatic amino acids phenylalanine and tyrosine.
Without these amino acids, a plant cannot translate complete proteins. Severe symptoms of yellowing, wilting, and death follow. Thus glyphosate can be used as an herbicide vegetation killer.
Because humans do not have EPSP synthase, it is "safe" to use compared to most other herbicides.
Biologists have inserted extra copies of EPSP synthase genes into the genome of crop plants behind a constitutive promoter. This causes the transformed plants to over-produce EPSP synthase, swamping out the effect of any glyphosate sprayed on them.
This way the engineered crop plants can be grown in a weed-free field using a relatively safe herbicide. Of course if the crop plant has wild relatives nearby, the genes for this herbicide resistance could be passed to the wild relatives producing herbicide resistant weeds.
Enzyme catalyzed reactions can be inhibited non-competitively Another way to inhibit a enzyme-catalyzed reaction is by use of a non-competitive inhibitor. This kind of molecule binds to the enzyme somewhere besides at the active site. This binding therefore does not interfere with the affinity and therefore Km of the enzyme for the substrate. What inhibitor binding does is to alter the effectiveness of the other functional groups of the enzyme in catalyzing the reaction through poorer induced fit.
This will, of course, reduce the Vmax possible. This kind of relationship is depicted below. Enzymes that are allosterically regulated often work in this non-competitive way. Once an enzyme has been "deactivated" this way, no amount of substrate will increase the rate of reaction back to normal. The only solution is to metabolize the inhibitor, thereby restoring normal conformation of the enzyme and normal rates of reaction.
Enzyme activity is pH sensitive As a protein, made of some 20 different amino acids in primary structure that end up producing the secondary, tertiary and quaternary structure, its conformation is pH dependent. Some of the amino acids have R groups with imidazole hiscarboxyl asp, glu or amino lys, arg. This change in charge will, very likely, alter the conformation and thereby activity.
This is demonstrated above.
The equilibrium is shifted to the left when acids are increased low pH It is shifted to the right when acids are decreased high pH Thus conformational changes induced by pH will of course change the reaction rate as exemplefied below.
The inflection points on each side of the curve represent the pKa of the critical R-groups in the enzyme.
The pH that gives the maximum rate at the peak of the curve is called the optimum pH. Enzyme activity is temperature-sensitive As with all proteins, the conformation is sensitive to temperature. As with all chemical reactions, the enzymem catalyzed reactions are temperature sensitive. This relationship is shown below. This bell-shaped curve is superficially similar in overall shape to that induced by changes in pH.
- Animal organisation - digestion - AQA
- Enzyme Kinetics
- Investigating the effect of pH on amylase activity
Close examination of the left side of the curve shows that it is exponential. Higher temperatures accelerate the catalytic effect. This relationship continues up to the optimum temperature. The activity at these super-optimal temperatures falls precipitously to 0.
Enzyme Concentration (Introduction to Enzymes)
Substrate concentration is not a likely regulatory factor in cells In most plant cells, it is impossible to control the reaction rate by regulating the concentration of the substrate.
It takes an fold increase in the substrate concentration to change the rate of an enzyme-catalyzed reaction from 0.
I will try to explain that here. If you do some algebra on the Michaelis-Menten function you get: You can then multiply both sides by [S'] and simplify: This kind of range is not expected physiologically for very many natural substrates.
Introduction to Enzymes
Obviously enzyme-catalyzed reactions are not sensitive to small changes in substrate concentration. Most enzyme systems are allosterically regulated Typical enzymes then must be regulated by the presence and absence of allosteric regulators that either activate or inhibit the reaction. In the case of an activator, the binding of the activator increases the chance of binding of the substrate. In the case of an inhibitor, the binding of the inhibitor decreases the chance of binding of the substrate.
Great examples of allosteric regulation are described throughout plant physiology. In other words, the enzyme molecules are saturated with substrate. The excess substrate molecules cannot react until the substrate already bound to the enzymes has reacted and been released or been released without reacting. Ten taxis enzyme molecules are waiting at a taxi stand to take people substrate on a minute trip to a concert hall, one passenger at a time.
If only 5 people are present at the stand, the rate of their arrival at the concert hall is 5 people in 10 minutes. If the number of people at the stand is increased to 10, the rate increases to 10 arrivals in 10 minutes. With 20 people at the stand, the rate would still be 10 arrivals in 10 minutes.
The rate would simply be higher 20 or 30 people in 10 minutes before it leveled off. Concentration of Enzyme When the concentration of the enzyme is significantly lower than the concentration of the substrate as when the number of taxis is far lower than the number of waiting passengersthe rate of an enzyme-catalyzed reaction is directly dependent on the enzyme concentration part b of Figure This is true for any catalyst; the reaction rate increases as the concentration of the catalyst is increased.
To some extent, this rule holds for all enzymatic reactions. After a certain point, however, an increase in temperature causes a decrease in the reaction rate, due to denaturation of the protein structure and disruption of the active site part a of Figure This fact has several practical applications.
We sterilize objects by placing them in boiling water, which denatures the enzymes of any bacteria that may be in or on them. We preserve our food by refrigerating or freezing it, which slows enzyme activity. Hydrogen Ion Concentration pH Because most enzymes are proteins, they are sensitive to changes in the hydrogen ion concentration or pH.
Ionizable side groups located in the active site must have a certain charge for the enzyme to bind its substrate. An enzyme exhibits maximum activity over the narrow pH range in which a molecule exists in its properly charged form.
The median value of this pH range is called the optimum pH The pH at which a particular enzyme exhibits maximum activity. With the notable exception of gastric juice the fluids secreted in the stomachmost body fluids have pH values between 6 and 8. Not surprisingly, most enzymes exhibit optimal activity in this pH range. However, a few enzymes have optimum pH values outside this range. For example, the optimum pH for pepsin, an enzyme that is active in the stomach, is 2.
Concept Review Exercises The concentration of substrate X is low. What happens to the rate of the enzyme-catalyzed reaction if the concentration of X is doubled?