Enzymes for the MCAT: Everything You Need to Know - Shemmassian Academic Consulting (2023)

(Note: This guide is part of ourMCAT BiochemieSerie.)

Part 1: Introduction to Enzymes

Part 2: General enzyme properties

a) Catalytic activity

b) Enzyme-substrate binding

c) Types of enzymes

d) cofactors, coenzymes and vitamins

e) Catalytic amino acids

Part 3: Enzyme Kinetics and Inhibition

a) Michaelis-Menten and Lineweaver-Burk

b) inhibition of competition

c) Non-competitive inhibition

d) Mixed and non-competitive inhibition

Part 4: Regulation of enzyme activity

a) Local environmental conditions

b) Covalent modification

c) Allosterische Regulation

d) Zymogene

e) cooperation

f) feedback control

Part 5: High-yield conditions

Part 6: Enzymes practice passage

Part 7: Enzymes Practice Questions and Answers

Part 1: Introduction to Enzymes

Enzymes are one of the most importantheavily testedScience topics at MCAT and they are an important part of our everyday lives. Every cell in our body performs many of its functions using enzymes, and the dysregulation of these enzymes is responsible for a variety of human diseases such as cancer and hypertension.

Many students struggle with enzymes on the MCAT, often losing valuable points on questions testing subjects from enzymatic inhibition to feedback regulation. In this guide, we'll break down what you need to know - nothing more, nothing less - to study enzymes for the MCAT. All terms in bold throughout the guide are defined in Part 5 of the guide, butWe encourage you to create your own definitions and examples as you move through this resource so they make the most senseyou!

In addition to knowing the content, you also need to know how to analyze the various diagrams, equations and terms related to enzymes that the MCAT presents. At the end of this guide, there is an MCAT-style enzyme practice passage and standalone questions that will both test your knowledge and show you how the AAMC likes to ask questions.

Let's begin!

Part 2: General enzyme properties

a) Catalytic activity

Enzymes are biological catalysts and aCatalystis defined as a substance that accelerates the rate of a chemical reaction without being consumed itself. For example, let's say you need to get from point A to point B, and they are 10 miles apart. You could walk, which would probably take a while. Or you could drive a car and arrive much quicker. In this case, the car serves as our catalyst. Note that the spacing doesn't change, butthe speed at which you get thereIs faster.

Just as a car speeds up our transportation, an enzyme serves to speed up the rate of a biochemical reaction. As a result, an enzyme is classified as onebiological catalyst, which has the following properties (we'll discuss each one of them):

• Lowers the activation energy of a reaction

• Affects the kinetics of the reaction but not the thermodynamics (∆G) or the equilibrium constant

• Regenerates itself

Let us first consider the activation energy. What do we need to expend more energy on: walking or driving 10 miles? If you said go, you're right. (In a car, all we have to do is press the gas pedal!) An enzyme—in our example, the car—reduces the amount ofinput energynecessary to carry out a chemical reaction. By lowering the input energy oractivation energy, a reaction, the reaction can proceed much faster. This brings us to reaction kinetics.

An enzyme affects theKineticof a reaction by accelerating the rate of reaction. Notice thatreaction rateis the rate at which reactants are consumed or the rate at which products are made. Importantly, enzymes do NOT affect the thermodynamics (∆G) or equilibrium constant of a reaction.

Let's look at another example to illustrate this point. We have a machine that turns red balls into red dice. Normally if you give the machine 10 red balls, it will produce 5 red cubes in 1 hour. Now let's say a catalyst acts on the machine - if you give the machine 10 red balls, it will produce 5 red cubes in 1 minute.

The kinetics of the machine will be affected as it can now work faster but the thermodynamics and equilibrium constant remain the same. Every 10 red balls turn into 5 red cubes, no matter how fast the machine can do it.

We have illustrated these effects below in a generically drawn reaction diagram:

Finally, there are enzymesregeneratedduring their catalytic cycles. This is very important because regeneration reduces the amount of protein a cell has to make to carry out biochemical reactions. For example, use cellsa lot ofof ATP, and most of this ATP is made by ATP synthase, an enzyme. (By the way, enzymes often end with-ase!) If each ATP synthase could only synthesize one ATP molecule, the cell would be overrun by the ATP synthase enzymes. The ability of each ATP synthase - and other enzymes - to reset itself after each cycle is amazingabsolutely necessaryto our survival.

In the figure below we show you how regeneration would make a big difference in the number of ATP synthases needed for a cell if it could not regenerate itself over several catalytic cycles.

b) Enzyme-substrate binding

We've talked about the general properties of enzymes, and now we're going to dip our toes into more down-to-earth details. We often talk about chemical reactions by writing down reaction schemes like A → B → C. For enzymes, we can draw a reaction scheme that can help us better understand what exactly is going on, and it looks something like this:

E + S → ES → E + P

E = Enzym

S = Substrate

P = product

We'll come back to the overall reaction scheme when we look at enzyme inhibition in the next section, but let's zoom in on the ES, or enzyme-substrate complex.

There are two models for describing the enzyme-substrate complex: 1) theLock and key modeland 2) theinduced adaptation model. The key-lock model describes the substrate as the "key" and the enzyme as the "lock". Without changing conformations, the key should fit snugly in the lock for the two to bind.

The induced fit model (which is generally the more accepted model) states that enzyme and substrate conformations need not be as rigid as the lock-and-key model suggests. Instead, upon substrate binding to the enzyme, both undergo slight conformational changes to enhance their binding to one another.

c) Types of enzymes

There are six known types of enzymes that the MCAT wants you to know:

If the cofactor or the coenzyme is extremely tightly bound to the enzyme, it is called aprosthetic group. Enzymes are referred to by their cofactors or coenzymesHoloenzyme, and without cofactors or coenzymes are called enzymesApoenzyme.

e) Catalytic amino acids

You should know themOne letter code,Three letter codeand structures for all 20 amino acids. If you don't, now is a good time to learn them, as they are almost guaranteed to show up in some form on the MCAT.

Now that you know the amino acids, we have a question for you: is an arginine or a valine more of a catalytically important amino acid based on what you know about its structure and chemical properties?

If you said arginine, you're right. Since you know all of the above properties of each of the 20 amino acids, you know exactly what chemical properties each amino acid has. In general, charged, polar, or nitrogenous amino acids are more likely to be involved in the catalytic steps of an enzyme. Why? These amino acids have interesting properties that allow them to perform chemistry with the substrate. For example, a lone pair of electrons on a terminal nitrogen in lysine can serve as a nucleophile. (For more information on electrophiles and nucleophiles, seeour guide to the basics of organic chemistry.)

Nonpolar and hydrophobic amino acids are less likely to be catalytically important amino acids because they cannot directly participate in interesting chemistry. Valine has no atoms that are highly electrophilic or want to serve as nucleophiles. As a general rule of thumb, nonpolar and hydrophobic amino acids are more important in protein-protein interactions and form the core of a protein, not in conducting biochemical reactions.

Part 3: Enzyme Kinetics and Inhibition

a) Michaelis-Menten and Lineweaver-Burk

Michaelis Menten

You must be familiar with the analysis of two large diagrams when it comes to enzymes. The first is the Michaelis-Menten plot. To create oneMichaelis MentenPlot, the experimenters use two very specific conditions:

• The enzyme concentration is kept constant

• The substrate concentration is increased

The researchers then draw thereaction speed, or how fast the reaction proceeds versus increasing substrate concentration. So if we have 100 enzymes, researchers could measure reaction rate in the presence of 10-500 substrates, increasing by 10 substrates for each reaction rate measurement. Let's look at a Michaelis-Menten diagram:

The two big values ​​to keep an eye on are v_maxand k_m, and these become especially important when we start talking about inhibition in the next section.

As the substrate concentration increases along the x-axis, you may find that the curve levels off as it approaches v_max. We define V_maxas the maximum rate at which the reaction can proceed given the current enzyme concentration and the rate at which the enzymes are working. Think back to our specific conditions - we determine the amount of enzyme we use in the experiment and keep it constant throughout.

We can state exactly the same concept in terms of an equation, and you should be familiar with the following equation for the MCAT:

The equation shows that v_maxis simply the maximum rate of all enzymes combined and is calculated by multiplying the concentration (or number) of enzymes by the rate of each enzyme.

Why now v_maxappear at all? At a certain point, the amount of substrate is so large that the enzymes are already working as fast as possible. Once we reach that saturation point, adding a new substrate will make no difference in how fast the enzyme can work.

For example, suppose a machine can make 10 new pens from 10 pieces of wood every minute. We run an experiment where we add pieces of wood and measure the power after a minute. If we add 1 piece of wood, the machine can quickly take care of it and make a single pencil. If we add 5 pieces of wood, the machine will produce all 5 pins in one minute. If we add 10 pieces of wood, the machine will work as fast as it can, producing 10 pencils in a minute. If we add 2,000 pieces of wood, the machine cannot make all 2,000 pins in one minute. In fact, it will only be possible to produce 10 pencils per minute. This saturation principle works the same with enzymes; Once you've reached saturation point by adding too much substrate (wood in our example), the enzymes can't increase production any further since they're already working at their maximum rate.

Finally, consider K_mon the Michaelis-Menten plot defined as the substrate concentration at ½ v_max. We can think of K_mB. the binding affinity of the substrate for the enzyme. Of course, if the substrate can bind the enzyme with a higher affinity, it is more likely to be "captured" by the enzyme and converted into the product. A small K_mindicates high substrate affinity and large K_mindicates low substrate affinity.

Now we know K_m, we define a final term: catalytic efficiency. Catalytic efficiency describes how effective an enzyme is in converting substrate to product and is defined as:

Remember k_Catdescribes the speed of a single enzyme, so the higher the speed, the better the efficiency. K_mdescribes the affinity of the enzyme for the substrate, i.e. the lower the K_m, the better the efficiency.

For each type of inhibition, you should be able to answer these three questions:

1) What happens to the V_max?

2) What happens to the K_m?

3) How does the Lineweaver-Burk diagram look like?

Ininhibition of competition, an inhibitor binds to the active site of the enzyme, preventing binding of the substrate. However, as the amount of substrate increases, the effect of the inhibitor decreases and v_maxcan be reached. For example, let's say we have 1 inhibitor and 1 substrate competing for a binding site on an enzyme. There will be a fight and we don't know who will win. Now suppose we have 1 inhibitor and 10 substrates competing for a binding site on an enzyme. The substrates are more likely to encounter the enzyme's active site first because there are more of them. If we further increase the concentration of these substrates, the enzymes do not realize that the inhibitor is present and v_maxremain the same since the enzymes are allowed to work at their maximum rate (answer to question 1).

As we discussed in the last section, K_mis a measure of the binding affinity for the substrate and the enzyme. If the inhibitor is possibly blocking the binding site, the substrate cannot bind as often. Therefore, the binding affinity becomes worse, so that K_mwill increase (answer to question 2).

Finally, the Lineweaver-Burk plot in the presence of a competitive inhibitor looks like this (answer to question 3):

Innon-competitive inhibitionthe inhibitor binds selectively to the enzyme-substrate (ES) complex. Now even if we add more substrate, the enzyme cannot work as fast because the enzyme-substrate complex has been bound by the inhibitor. As a result v_maxdecreases (answer to question 1).

When inhibitors bind the enzyme-substrate complex, the amount of uninhibited enzyme-substrate complex decreases. This decrease in the enzyme-substrate complex causes an increase in the affinity between the substrate and the enzyme, since they now have to replenish the lost enzyme-substrate complex (according to Le Chatelier's principle)! Thus, the increase in affinity means that K_mdecreases (answer to question 2).

The Lineweaver-Burk plot in the presence of a non-competitive inhibitor looks like this (answer to question 3):

Inmixed inhibition, the inhibitor binds to an allosteric site or a non-active site regulatory pocket on both the free enzyme and the enzyme-substrate complex. However, often this bond is biased towards one or the other. For example, 70% of the inhibitors could bind the enzyme alone and 30% could bind the enzyme-substrate complex. As a result, a mixed inhibition can be quite complicated and is not often tested on the MCAT.

However, there is a special case where 50% of the inhibitors bind the enzyme alone and 50% bind the enzyme-substrate complex and this is termed non-competitive inhibition. Again, as with all mixed inhibitors, the inhibitor attaches an allosteric site to the enzyme.

Since the enzyme-substrate complex is bound by the inhibitor, v_maxmust decrease, as we have shown previously for non-competitive inhibition (answer to question 1). Interestingly, both the enzyme alone and the enzyme-substrate complex are bound by the inhibitor K with the same affinity_mremain the same (answer to question 2).

The Lineweaver-Burk plot for non-competitive inhibition looks like this (answer to question 3):

Part 4: Regulation of enzyme activity

Given the importance of enzymes in our bodies, there are several regulatory mechanisms that we will detail in this section.

a) Local environmental conditions

The two primary local environmental conditions affecting enzyme activity are temperature and pH.

Temperature

Enzymatic activity has a love-hate relationship with temperature - we'll explain. Up to a certain point, an increase in temperature leads to an increase in the kinetic energy of enzymes and substrates. In other words, the molecules bounce around more quickly, meaning an enzyme and substrate are more likely to meet in the right orientation. But once the temperature istoo highthe enzyme becomes denatured and loses its ability to catalyze the reaction.

PH value

Earlier in this guide we talked about which amino acids are commonly found in the active site of enzymes. These amino acids include charged amino acids such as glutamic acid, aspartic acid, lysine, arginine, and histidine. Each of these amino acids has a specific pKa for their side chain functional groups anddepending on the pH of the solutionthese functional groups can be protonated or deprotonated. The protonation status is important as it can determine whether or not the enzyme can perform its catalytic function.

Therefore, pH is extremely important for enzymes in the body since their active site residues must have the correct protonation status. Most enzymes are functional at a physiological pH of 7.4, but gastric enzymes are most active at a pH of 2 because the stomach must be acidic to perform its function.

b) Covalent modification

Many enzymes are regulated byPost-translational modifications (PTMs)the proteins are added. These PTMs include phosphorylation, acetylation, glycosylation, and methylation, among others. For example, phosphorylation is important for the MAP kinase signaling pathway because every protein is activated by phosphorylation.

c) Allosterische Regulation

In addition to the active center have many enzymesallosteric sites. These are sites remote from the active site that can bindallosteric activatorsorAllosteric Inhibitorsto switch the enzyme "on" or "off".

d) Zymogene

Some enzymes are designed for highly specialized functions and are dangerous if not tightly controlled.Zymogeneare inactive forms of enzymes that must be cleaved to become activated. An example is trypsinogen (zymogen), which must be broken down into trypsin to become active. Trypsin digests proteins, so if it were expressed in the wrong place, it would destroy entire cells and tissues.

e) feedback control

Most enzymes are involved in signaling pathways that require careful control.Feedback PolicyThis is how cells achieve this careful control, since the production of intermediates in the metabolic pathway can regulate the activity of the enzyme. The most common form of feedback regulation isnegative regulation. When negatively regulated, the product of the enzyme or a product further down the pathway can inhibit the enzyme and reduce flux through the pathway. A less common form of feedback regulation isFeed-Forward (Positive) Regulationin which the substrate of an enzyme or a product upstream in the enzyme pathway can activate the enzyme to increase flux through the pathway.

f) cooperation

Many enzymes have multiple binding sites or form multidomain proteins. As a result, they can bind more than one substrate, and this multiplicity can affect the binding affinity of the enzyme for multiple substrates.

cooperativeis best understood with an example. Here we will talk about hemoglobin - an oxygen-binding protein present in red blood cells. Each hemoglobin molecule contains four identical subunits, each of which binds an oxygen molecule. Interestingly, after the first hemoglobin subunit binds an oxygen substrate, the next hemoglobin subunit ismuch more likelyto bind another oxygen substrate, and the pattern continues for the third and fourth hemoglobin subunits. As a result, hemoglobin is very efficient at picking up oxygen!

Cooperativity is often described by theHill coefficient, which is interpreted as follows:

Part 5: High-yield conditions

Catalyst:a substance that accelerates a reaction without being consumed itself

Biological Catalyst:a molecule (can be a protein or a nucleic acid) that increases the rate of a biochemical reaction without being consumed itself

activation energy:the input energy required to start a chemical reaction

Kinetic:how fast a reaction converts reactants into products

Response rate:the rate at which reactants are consumed OR the rate at which products are formed

Regeneration:the ability of enzymes to reform and participate in multiple catalytic cycles

Lock-and-Key-Modell:an enzyme–substrate binding theory that presents the substrate as a “key” that fits perfectly into the enzymatic “lock” without changing conformation

Induced fit model:an enzyme-substrate binding theory that states that the substrate and enzyme undergo slight conformational changes upon binding to each other

One-letter amino acid code:a one-letter code defining an amino acid (e.g. K is lysine)

Three letter amino acid code:a three-letter code that defines an amino acid (e.g. Lys is lysine)

Michaelis-Menten:a graph measuring reaction rate versus increasing substrate concentration while enzyme concentration is held constant

Response speed:the rate at which a given concentration of enzymes converts substrate to product

v_max:the maximum rate at which a given concentration of enzyme can convert substrate to product

K_m:the substrate concentration at ½ v_maxand also corresponds to the affinity between enzyme and substrate

Saturation:the point at which additional substrate does not increase the reaction rate because the enzymes are already working as fast as possible

Catalytic Efficiency:describes the efficiency of an enzyme by including how fast the enzyme works (k_Cat) and how well the enzyme can bind substrate (K_m) using the following expression: k_Cat/K_m

Line Weaver Burk:a double reciprocal of the Michaelis-Menten plot in which (1/reaction rate) is plotted against 1/[S].

inhibition of competition:An inhibitor competes with substrate binding to the active site, resulting in an unmodified v_maxand a raised K_m

Non-competitive inhibition:an inhibitor binds to the enzyme-substrate complex, resulting in decreased v_maxand a flat K_m

Mixed inhibition:an inhibitor binds to both the enzyme alone and the enzyme-substrate complex, resulting in decreased v_maxand a raised or lowered K_m

Non-competitive inhibition:a subclass of mixed inhibition in which an inhibitor binds 50% of the time to the enzyme alone and 50% of the time to the enzyme-substrate complex, resulting in decreased v_maxand an unchanged K_m

Cofactor/Coenzyme:binds to the active center of the enzyme and supports the catalysis of the reaction

Prosthetic group:a cofactor or coenzyme that binds extremely tightly to the enzyme

Holoenzym:an enzyme with its cofactors and/or coenzymes

Apoenzyme:an enzyme without its cofactors and/or coenzymes

Post-translational modification:the covalent addition of a functional group or atom to an enzyme

Allosteric sites:a site remote from the active site that can be used as a regulation point

Allosteric Activators:binds to the allosteric site of an enzyme and activates the enzyme

Allosteric Inhibitors:binds to the allosteric site of an enzyme and inhibits the enzyme

Zymogene:inactive forms of enzymes that must be cleaved to become activated

feedback control:the control of flow through biological pathways using positive and negative regulation

negative rule:The enzyme's product or a product further down the pathway can inhibit the enzyme and reduce flux through the pathway

Feed-forward (positive) Regelung:An enzyme's substrate or a product upstream in the enzyme pathway can activate the enzyme to increase flux through the pathway

Cooperation:the binding of a substrate to a multisubunit protein affects the binding of subsequent substrates

Hill coefficient:a term that describes whether a multi-subunit protein is non-cooperative (=1), positively cooperative (>1), or negatively cooperative (<1).

Sigmoidal:an S-shaped curve characteristic of cooperative behavior

Part 6: Enzymes practice passage

Angiotensin converting enzyme (ACE) inhibitors are commonly used to treat high blood pressure and heart failure. By inhibiting ACE, the production of angiotensin II decreases and blood vessels dilate to lower blood pressure and strain the vascular system. However, inhibition of ACE via synthetic inhibitors produces various undesirable side effects. Researchers therefore want to develop a natural ACE inhibitor.

In a new study, scientists aim to purify and introduce natural ACE inhibitors extracted from itZiziphus jujubaFruit. First proteins outZiziphus jujubawere lysed by trypsin, papain and a combination of the two. Then, acquired peptide fragments from the digest were purified by chromatography and assayed for ACE inhibitory activity. Peptide fractions containing inhibitory activity were sequenced using tandem mass spectrometry. Finally, to elucidate the mode of peptide binding to ACE, homology modeling, molecular docking, and molecular dynamics simulations were performed.

The amino acid sequences of the three amino acid F2 and F4 peptides, which were the most active hydrolysates, were determined as IER and IGK with IC50 values ​​of 0.060 and 0.072 mg/ml, respectively. Peptides F84 and F72 were determined to be AAA and III, respectively. Of the 234 peptides screened, only F2 and F4 showed an inhibitory effect on ACE, so the researchers decided to study the kinetics of inhibition.

The F2 and F4 peptides were added in increasing concentrations to a constant concentration of ACE enzyme. ACE activity was assayed and the results plotted as a Lineweaver-Burk plot (Figure 1).

Question 1: Which of the following peptides is most likely to bind to a phosphate-coated column?

A) F2

B) F4

C) F72

D) F84

Question 2: Which of the following statements best describes how trypsin activity is regulated?

A) It is sequestered in specific organelles.

B) It is secreted as an inactive zymogen before cleavage to the active product.

C) It is only expressed for short periods during the S phase of the cell cycle.

D) It is bound by two allosteric inhibitors that prevent it from becoming active at the wrong sites.

Question 3: According to the passage, the researchers found that the IER peptide binds near the zinc binding site near the active site of the enzyme. Which of the following statements best describes the role of zinc in the enzyme?

A) Coenzyme

B) Apoenzym

C) Cofactor

D) Catalytic residue

Question 4: The researchers are using a new peptide that is known to be a non-competitive inhibitor. Which of the following Lineweaver-Burk plots are researchers most likely to observe?

Question 5: Researchers are testing the kinetics of a novel enzyme inhibitor. After evaluating the Michaelis-Menten kinetics, they conclude that the inhibitor binds to both the enzyme alone and the enzyme-substrate complex, although it binds to the enzyme alone with a slightly higher affinity. This is an example of:

A) Inhibition of competition

B) Noncompetitive inhibition

C) Non-competitive inhibition

D) Mixed inhibition

Answer key for passage-based questions

1. Answer option B is correct.According to the passage, peptide F4 has the sequence IGK. Hence the charge is 0 + 0 + 1, and the positive charge would likely bind the negatively charged phosphate on the column. Peptide F2 has the sequence IER carrying the charge 0-1+1 for a net charge of 0 (choice A is wrong). Peptide F72 has sequence III and peptide F84 has sequence AAA, both of which are neutrally charged since they are 0+0+0 (choices C and D are false).

2. Answer option B is correct.Trypsin is a zymogen, meaning its inactive form (trypsinogen) must be cleaved to produce the active enzyme (trypsin). Trypsin is not sequestered in specific organelles (choice A is wrong), is only expressed during S phase (choice C is wrong), or is bound by allosteric inhibitors (choice D is wrong).

3. Answer option C is correct.Zinc is a cofactor or ion that aids in catalysis and is commonly found in enzymes. A coenzyme assists an enzyme in catalysis, but it is a protein (choice A is wrong). An apoenzyme is a protein without its cofactors or coenzymes (choice B is wrong). A catalytic residue is an amino acid, not a zinc ion (choice D is incorrect).

4. Answer option D is correct.Non-competitive inhibitors decrease both the v_maxand the k_m, whereby the y-intercept (1/v_max) to become more positive and the x-intercept (-1/K_m) to become more negative. The easier way to spot a non-competitive inhibitor is that the Lineweaver-Burk plot is parallel and shifted up.

5. Answer option D is correct.Mixed inhibition occurs when an inhibitor binds to both the enzyme alone and the enzyme-substrate complex. In this case, the inhibitor can bind the enzyme alone 60% of the time while binding the enzyme-substrate complex 40% of the time. Competitive inhibition occurs when an inhibitor binds to the active site of an enzyme (choice A is wrong). Non-competitive inhibition is a subclass of mixed inhibition that describes an inhibitor that binds an allosteric site, and this type of inhibitor binds the enzyme alone and the enzyme-substrate complex with equal affinity (choice B is wrong). Non-competitive inhibition occurs when an inhibitor only binds the enzyme-substrate complex (choice C is incorrect).

Part 7: Practice questions on enzymes

Question 1: Researchers mutate the active site aspartate of an enzyme into four different amino acids. The mutated enzymes have the following properties:

Which mutant has the highest catalytic efficiency?

A) D78A

B) D78G

C) D78R

D) D78E

Question 2: What kind of enzyme adds a phosphate group to a protein substrate?

A) Dehydrogenase

B) Lyase

C) Transferase

D) Carboxylase

Question 3: Compound X is a non-competitive inhibitor of enzyme A. Assuming Michaelis-Menten kinetics, what effects would the addition of compound X have on the kinetics of enzyme A?

Which of the following statements best explains the low catalytic efficiency of the D78R mutant?

A) Disturbance of the hydrophobic effect

B) Change of direction of an important residue of the active site

C) Disulfidbrückenbruch

D) Reduction in the size of an active site residue

Question 5: Which of the following types of enzymatic inhibition is a subclass of mixed inhibition?

A) Competitive

B) Uncompetitive

C) Uncompetitive

D) Traditional

Question 6: Researchers mutate the active site aspartate of an enzyme into four different amino acids. The mutated enzymes have the following properties:

Which mutant has the lowest catalytic efficiency?

A) D78A

B) D78G

C) D78R

D) D78E

Question 7: Where is an enzyme with an observed maximum activity at pH 2 most likely to be found?

a brain

B) kidney

C) Magen

D) Leber

Answer key for stand-alone practice questions

1. Answer option D is correct.The catalytic efficiency is defined by k_Cat(the catalytic activity of an enzyme) divided by K_m. WT has the highest catalytic efficiency because there is no active site mutation. The mutant with the highest catalytic efficiency would have the highest k_Catand the lowest K_m. This combination is satisfied by the D78E mutation (choice D is correct). This mutation also makes sense because it has the least effect on enzyme activity due to the maintenance of a negative charge in the active site by the aspartate to glutamate mutation.

2. Answer option C is correct.Kinases add phosphate groups to a protein substrate, and kinases are a subclass of transferases. A dehydrogenase removes hydrogen atoms (choice A is wrong), a lyase cleaves a biomolecule without water (choice B is wrong), and a carboxylase adds a carboxyl group to a biomolecule (choice D is wrong).

3. Answer option D is correct.Non-competitive inhibitors bind the E-S (enzyme-substrate complex), thereby interfering with the activity of the enzyme and leading to a decrease in V_max. (Choice C is wrong). Based on Le Chatelier's principle, the amount of E-S complex seems to decrease, forcing the reaction scheme E + S → ES to the right. S decreases and K_mis a measure of the substrate concentration. Therefore k_mdecreases (choice D is correct; choices A and B are wrong).

4. Answer option B is correct.Changing the charge of an important active site residue is likely to change the mechanism of the enzyme and thereby lead to a reduction in catalytic efficiency.

5. Answer option C is correct.Non-competitive inhibition is a subclass of mixed inhibition, in which the inhibition binds the enzyme alone and the enzyme-substrate complex with equal affinity (choice C is correct). Competitive and non-competitive inhibition are in classes by themselves (choices A and B are wrong). Traditional escapement is made up (choice D is wrong).

6. Answer option C is correct.The catalytic efficiency is defined by k_Cat(the catalytic activity of an enzyme) divided by K_m. WT has the highest catalytic efficiency because there is no active site mutation. The mutant with the highest catalytic efficiency would have a high k_Catand a low K_m. Therefore, the mutation with the lowest catalytic efficiency would have a low k_Catand a high k_m. D78R meets this requirement. Also, it makes sense that changing an active site residue from negatively to positively charged would have a negative impact on enzymatic efficiency!

7. Answer option C is correct.An enzyme with maximum activity at pH 2 is most likely to be found in the stomach because the stomach uses an acidic environment to help break down food. Enzymes found in the brain, kidneys, and liver tend to function around the normal physiological pH of about 7.4 (choices A, B, and D are incorrect).