hello everybody my name is Iman welcome back to my YouTube channel last time we covered chapter 2 review all about enzymes today we're going to do the MCAT practice problems associated with this chapter now most of the content we've covered will help us answer most of these questions if there's something we haven't covered in the review we'll stop we'll pause we'll review the content in more depth in case we just covered it surface level in the review video and we'll make sure you have all the information you need to be able to handle all of these problems that we do together all right so let's go ahead and get started problem one says consider a chemical reaction a going to B which is catalyzed by a b dehydrogenase which of the following statements is true now we've discussed when we introduced enzymes that enzymes catalyze reactions by lowering the activation energy all right we talked about how if you have some reactant a and you're trying to get to some reactant B the way that you can proceed forward in these reactions by overcoming this energy barrier this activation energy and the way that enzymes make reactions faster is by lowering the activation energy for a reaction now enzymes are not changed or consumed during the course of the reaction even though the activation energy is lowered the only thing that gets affected is the kinetics for the reaction and not the thermodynamics this was a very important point that I attempted to drill um continuously in the review for this chapter enzymes affect kinetics they do not affect thermodynamics what that means is your gips free energy your Delta G remains unchanged in the presence of enzymes and a reaction will continue to occur in the presence or absence of an enzyme it can it'll just simply run slower without the enzyme all right so that's a little quick summary of everything we talked about enzymes in our review now let's look at these answer choices a says that the reaction will proceed until the enzyme concentration decreases so we're trying to find a true statement here all right the reaction will proceed until the enzyme concentration decreases this is not the right way of thinking about this right we said enzymes lower activation energy but they will remain unchanged um they're not changed or consumed during the cons the course of the reaction so the phrasing of this word uh the phrasing of this answer Choice doesn't make it true all right B says the reaction will be most favorable at zero degrees Celsius all right that's not true either all right reactions won't just be favorable at one temperature and that's it and we don't have way enough information to determine this and also remember what does enzyme what do enzymes do they affect the kinetics not the thermodynamics all right C says a component of the enzyme is transferred from A to B all right no again enzymes are not changed or consumed during reactions all right they do not change they do not give a part of themselves to the reaction all right they remain the same after they after their participation or reaction as they did before all right and D says that the free energy of the catalyzed reaction is the same as for the uncatalyzed reaction this is absolutely true Delta G is not affected by the presence or usage of enzymes and so which one of these statements is true the correct answer is d fantastic two says which of the following statements about enzyme kinetics is false all right let's read our answer choices an increase in the substrate concentration leads to proportional increase in the rate of reaction okay we're trying to find a false statement an increase in the substrate concentration leads to a proportional increase in the rate of the reaction that is a quite a bold statement all right an increase in the substance substrate concentration while maintaining a constant enzyme concentration leads to a proportional increase in the rate of the reaction only initially however once most of the active sites are occupied the reaction rates level off remember are Michael Menton curve right it increases but it will reach a plateau where it is constant all right so most of so the reaction rates level off once most of the active sites are occupied regardless of further increase in substrate concentration at high concentrations of substrate the reaction rates approaches its maximal velocity that V Max that we talked about in the review all right and it is no longer Changed by further increase in substrate concentration so right off the bat that first choice that we read a is false we'll read the others most enzymes operating in the human body work at temperatures 37 degrees Celsius correct and enzyme substrate complex can either form a product or dissociate back into the enzyme and substrate also true uh maximal activity of many human enzymes occur occur around a pH of 7.4 also true all right so the answer to 2 is a three says that some enzymes require the presence of non-protein molecules to behave catalytic catalytically and enzyme devoid of this molecule is called a blank all right this is one thing that we didn't Define um but I'm going to Define it now an enzyme devoid of its necessary cofactor is called an APO enzyme all right an April enzyme it is catalytically inactive and so the correct answer here will be B Fantastic Four says which of the following factors determines an enzyme's specificity all right now when we talked about proteins all right and and the reason I'm bringing up proteins is because enzymes are proteins when we talked about proteins in that first chapter we said that a lot of the protein structure a lot of the proteins function is determined by its structure its three-dimensional structure right a protein is determined by its three-dimensional shape now an enzyme specificity is also determined by the three-dimensional shape of its active site all right and so that's really important because regardless of whether we're talking about the lock and key model or the induced fit model the active site determines Which substrate the enzyme will react with and so when we're talking about which of the following factors determinate enzymes specificity the three-dimensional shape of its active site is definitely super huge all right the prosthetic group on the enzyme that's not correct the type of cofactor required for an enzyme to be active is also not the main factor that determines specificity and neither does the Michaelis constant and so the correct answer here is a all right 5 says enzymes increase the rate of the reaction by blank decreasing the activation energy all right that is definitely true enzymes increase the rate of reaction by decreasing the activation energy let's read B decreasing the overall free energy absolutely not all right enzymes do not affect thermodynamic values like Gibbs free energy or enthalpy or things like that all right it definitely doesn't increase I'm sorry it definitely doesn't increase activation energy quite the contrary enzymes decrease activation energy all right and then the last answer choices increases the overall free energy again has no effect on thermodynamic quantities so the correct answer here is a enzymes increase the rate of the reaction by decreasing the activation energy phenomenal all right six in the equation below substrate C is an allosteric inhibitor to enzyme one which of the following is another mechanism necessarily caused by substrate C all right this is a really cool Problem by limiting the activity of enzyme one the rest of the pathway is going to be slowed down this is the definition of negative feedback where the product you create inhibits a function that results in the slowing down or stopping of your pathway all right so this is negative feedback this is the definition of negative feedback all right so if we're dealing with negative feedback all right what do we what kind of things are we talking about we can have reversible or irreversible um inhibition all right so now let's look at these answer choices we have a competitive inhibition okay that follows under negative feedback but it's incorrect because there's actually no competition for the active site with allosteric with allosteric Inhibitors all right with allosteric interactions and while many products do indeed competitive competitively inhibit an enzyme in the pathway that creates them this is an example of an allosterically inhibited enzyme so they're not competing for the active site instead what C does to the enzyme is it doesn't affect the active site it binds somewhere else allosterically okay and so competitive inhibition is not the correct answer here all right now B says irreversible inhibition which is just a very broad category um and it's not specific enough and there's just not enough information for Choice B to be correct because we aren't told whether the inhibition is reversible or irreversible so we're going to cross B off all right now in general allosteric interactions are temporary all right they're temporary and choice C here is incorrect because it is the opposite of what occurs when enzyme one is reduced all right when enzyme one is reduced we have negative feedback all right the opposite would be feedback enhancement so Choice C is not correct all right and then the correct answer is obviously what we said in the beginning is negative feedback fantastic all right 7 here says the activity of an enzyme do I have my answer choices yes there in the next page okay the activity of an enzyme is measured at several different substrate concentrations and the data is shown here below all right here we have substrate concentration here we have velocity millimole per second now we talked about a couple equations all right but what's more important here is to make this even simpler especially because I believe you don't have calculators on the MCAT is relationships understanding relationships is really what's going to make this easier to answer all right now we can see that as the concentration of the substrate increases significantly what's happening to the rate all right the rate is only changing at a small Pace cons compared to the substrate concentration all right there's only a small change in the rate all right and this occurs as V Max this occurs as we approach Vmax right because the V Max is near 100 millimole per m per second all right V Max over 2 equals 50 millimole per second and the substrate concentration giving this rate is only 0.5 um millimolar and it corresponds to the km all right and so the km for this enzyme is just approximately 0.5 all right so that's an important thing to keep in mind now eight eight says consider a reaction catalyzed by enzyme a with a km value of five times ten five times is that five times ten to the minus 6 molar and our V Max is 20 millimolar per minute all right consider a reaction catalyzed by enzyme a with a km value of this and a view max value of this at a concentration of 5 times 10 to the minus 6 molar substrate the rate of the reaction will be blank all right so as with the last question relationships here are very important add a concentration of 5 times 10 to the minus 6 molar enzyme a is working at one half of its V Max because the concentration is equal to the km of the enzyme therefore one-half of 20 millimolar per minute is 10 millimolar per minute and that's going to be our answer Choice a all right now nine says at a concentration of 5 times 10 to the minus 4 molar substrate the rate of the reaction will be blank all right now at this concentration there is a hundred times more substrate than present at half maximal velocity all right at high values significantly larger than the value of KM the enzyme is at or near its V Max here all right because here the concentration is so much greater it's nearing the V Max all right and so um the rate of the reaction is going to be pretty close to V Max so the answer Choice here is going to be 20 millimolar per minute all right fantastic 10 says the graph below shows kinetic data obtained for flu virus enzyme activity as a function of substrate concentration in the presence and absence of two antiviral drugs all right so this is here let's just read the problem based on the graph which of the following statements is correct all right here's our graph let me zoom out so we can look at the graph read read the answer choices all right answer Choice a says both drugs are non-competitive Inhibitors B says Tamiflu increases the km value for this substrate compared to rolensa C says relenza increases the V Max value for the substrate compared to the Tamiflu and then D says both drugs are competitive Inhibitors of the viral enzyme Okay cool so if we remember this looks like a this plot looks like a line um a line Weaver Burke plot all right it's analogous in terms of information to your Michael mentin plot except that the X and Y axis um what would be the X and Y axis of your micromentis equation here in your line Weaver Burke it's the reciprocal all right so in your Michael mentis you have velocity versus substrate concentration in your line Weaver plot you have one over velocity for your y-axis and one over substrate concentration for your x-axis now when we talked about different feedback inhibitions specifically reversible Inhibitors we talked about competitive non-competitive and uncompetitive and we talked about the different line Weaver Burke plots for each of those all right what we talked about is that here at the x-intercept you have your 1 over km value and at the y-intercept you have a 1 over V Max one over not constant sorry one over your V Max value all right and the trend for competitive Inhibitors is that when you add an inhibitor all right the only thing that changes is your km for non-competitive Inhibitors the thing that changes it when you add an inhibitor is your V Max value your y-intercept and for um uncompetitive both your km and your V Max values change when you add an inhibitor cool now let's go back to this plot all right let's go back to this plot let's talk about it based on the graph when the substrate is present Tamiflu all right Tamiflu results in the same V Max all right so this is no inhibitor all right when we add Tamiflu they have the same V Max value what does change are there km values all right what is that a description of all right all right this is a description of competitive Inhibitors and specifically their km values differ right okay but Tamiflu has a higher km value all right those are all descriptors of competitive inhibition all right so what's happening when you add Tamiflu all right you have competitive inhibition cool beans non-competitive inhibition results in a decreased Vmax and the same km which is what we see when we add rolenza relenza and no inhibitor have different V Max values but they have the same k m all right and so when you add relanza you have non-competitive inhibition all right now because the question is only comparing the values between the two Inhibitors and not the enzyme without inhibitor the mechanism of inhibition is less important to determine than the value of KM and B Max all right and so for this question what we realize is that no both drugs are not competitive Inhibitors one is and the other is a competitive inhibitor so a is not correct B says Tamiflu increases the km value this is true because we determined that Tamiflu is competitive inhibition competitive inhibition increases you have an increase of KM when you add the inhibitor all right C says relenza increases the V Max all right rolenza decreases km because it's non-competitive all right so that's also not true and then D says both are competitive also not true one is competitive one is non-competitive so the only correct answer Choice here is B all right so 10 is B antastic 11 says that the conversion of ATP to cyclic amp and inorganic phosphate is most likely catalyzed by which class of enzyme all right this is great license I'm sorry and I will spell it just in case even my pronunciation you cannot tell what I'm saying these kinds of enzyme class this kind of enzyme class is responsible for the breakdown of a single molecule into two molecules without the addition of water or the transfer of electrons we covered this in the beginning of our review different classes of enzymes lies is often form cyclic compounds or double Pawns in the products that they break down to accommodate this so water was not a reactant and no cofactor was mentioned here they're just telling us that we're converting ATP to cyclic amp and inorganic phosphate and so we don't want to say that water was used because that's not the information we were given all right there's no transferring of functional groups we're simply breaking down this molecule into two molecules without the worry of water according to the problem and so the correct answer here is going to be C fantastic now 12 says which of the following is not a method by which enzymes decrease the activation energy for biological reactions now like we said what do enzymes do they lower the activation energy and they do so in a way that does not alter them enzymes are not altered by the process of catalysis right a molecule that breaks intramolecular bonds to provide activation energy would not be able to be reused all right so keeping that in mind which of the following is not a method by which enzymes decrease the activation energy all right they might modify the local charge environment they might form transient covalent bonds and they might act as electron donors or receptors what they won't do is break Bonds in the enzymes irreversibly all right because enzymes in Big Picture are not altered changed or or changed all right and so which of the following is not a method for enzymes to lower the activation energy that would be D breaking Bonds in the enzyme irreversibly nope not going to happen all right 13 says a certain Cooperative enzyme has four subunits two of which are bound to a substrate I'll just call all right which of the following statements can be made awesome so we didn't Focus too much on cooperativity but let me let's talk about it right Cooperative enzymes demonstrate a change in affinity for the substrate depending on how many substrate molecules are bound and whether the last change was accomplished because a substrate molecule was bound or or left the active side of the enzyme now because we cannot determine whether the most recent reaction was binding or dissociation we cannot say that the Affinity of the enzyme for the substrate has just increased or decreased all right and so what that eliminates is answer Choice A and B all right now we can make absolute comparisons though right the Unbound enzyme has the lowest affinity for the substrate and the enzyme with all but one step unit bound has the highest all right the increase in Infinity is not linear all right which also cancels out answer Choice C because it's not necessarily true that the Affinity of the enzyme for the substrate is at the average for this enzyme class what we can say is that the affinity for the enzyme for the substrate is greater than with one substrate bound and that would be answer Choice d all right looks like that is all I have for you I hope this was helpful let me know if you have any questions comments or concerns other than that good luck happy studying and have a beautiful beautiful day future doctors