TEAS Test Questions on the Action of Enzymes

In looking through sample TEAS test questions to address for my pre-nursing students I came across these two, both on the topic of enzyme action:

An enzyme processing a substrate A + B –> AB

 

1. Which of the following statements about enzymes is not true?

A. Enzymes are catalysts. 
B. Almost all enzymes are proteins. 
C. Enzymes operate most efficiently at optimum pH. 
D. Enzymes are destroyed during chemical reactions.

2. Which of the following is considered a model for enzyme action?

A. Lock and Key model 
B. Enzyme interaction model 
C. Transformation model 
D. Transcription model

Enzyme action is key to cell survival, indeed, it is the basis of all that a cell does.  Conventiently, Question #1 help define exactly what an enzyme is.

Which of the following statements about enzymes is not true?

Before answering the question, I have to stop and remind test-takers to read carefully. If I were to get this question, the first thing I would do is underline, box, or otherwise note the word ‘NOT’ so that it is clearly obvious and I will not later come back and ‘read through’ this word. Even if I can answer the question easily, I would still mark this. So, restated:

Which of the following statements about enzymes is NOT true?

Instead of a multiple-choice question, this should be approached as a series of T/F questions. I wouldn’t actually re-write these (the object is not to make more work for you, but, to make your questions clear)

1. A.     Enzymes are catalysts. 


True –  An enzyme is a catalyst. The clearest definition of what an enzyme is, is… ‘Enzymes act as biological catalysts.’ To further emphasize this, let’s define a catalyst by skipping down to ‘D’.

D. Enzymes are destroyed during chemical reactions

False – A catalyst is defined as a substance that takes part in a reaction, but is not consumed (altered / changed) by the reaction. Therefore, if enzymes are catalysts, then this must be false. You could stop here, but just to be sure, I always read through the other answers to make sure that there is no other answer that also appears true.

B.     Almost all enzymes are proteins.

1. 

Well…. I have a difficult time with this one. ‘Almost’ is not the kind of word you want to see in a question like this. Many enzymes are proteins, but many are also ribozymes, meaning enzymes composed of RNA. The ribosome is an excellent example of a ribozyme, consisting of mostly rRNA and a small protein component as well.

Luckily, we have already seen that ‘D’ is clearly false, while this is merely a questionable answer.


C. Enzymes operate most efficiently at optimum pH. 


Each enzyme may have its own optimal conditions

True – Enzymes, like all molecules, will have an optimal pH. This is because pH changes will result in changes in how a molecule folds. As I have mentioned many times before, ‘Form Dictates Function’ – if a molecule folds correctly, it will function correctly.; if it folds incorrectly, then it will (almost always) function incorrectly, or not at all.

Which of the following statements about enzymes is NOT true?

D. Enzymes are destroyed during chemical reactions

Next question…

2. Which of the following is considered a model for enzyme action?

A. Lock and Key model 
B. Enzyme interaction model 
C. Transformation model 
D. Transcription model

A.           Lock and Key model 


The lock and key model describes how an enzyme and its substrate fit together precisely as a key fits a lock. This analogy describes both the precision of the fit as well as the specificity a key has for its lock.

 B.              Enzyme interaction model 


These are just words. They sound right, but that’s as far as it goes.

C.           Transformation model

Again, these are just words. In this case, not even the right words. ‘Transformation’ refers to either the transfer of DNA into a cell (as with plasmid DNA into a bacteria) or the mutation of DNA in a cell such that it ‘transforms’ into a cancer cell.


D. Transcription model

            ‘Transcription’ refers to the copying of genetic information from DNA to RNA as in the central dogma.

 

 

How Do Enzymes Lower the Activation Energy?

In class today we discussed three mechanisms for how enzymes function. These three mechanisms are:

1. Bringing substrates together in a way that promotes the appropriate interaction.

2. Another way they function is to couple exergonic and endergonic reactions.

3. They also lower the Activation Energy of the reactions they catalyze

 

Ashley asked a good followup question: how, exactly, do enzymes lower the activation energy in reactions?

I’ve looked around for a good explanation of this without finding much satisfaction. One explanation found on Wiki.answers was that, according to the induced fit model, when substrates bind to enzymes, there is strain put on the molecule that promotes the reaction. But this answer felt unsatisfactory for a couple reasons: First, it was unsupported by any links to primary data and second, it’s really only a partial answer – i.e., does it take energy to get the enzyme into this strained conformation? If so, where does that energy come from (perhaps the particle’s movement?) If it doesn’t require energy, why do the substrates get themselves into this position?

Enzyme placing strain on substrate

A better answer was found from Science magazine back in 2004. The article, Mireia Garcia-Viloca, Jiali Cao, Martin Karplus, Donald G. Truhlar. “How Enzymes Work: Analysis by Modern Rate Theory and Computer Simulations.”Science. 303:186 – 195 (9 January 2004), refers back to a statement by Linus Pauling where he posited that the only actual power enzymes have is in stabilizing a transition state. What does this mean? It means essentially the same thing as was stated above – however it also refers to a kind of ‘in between’ state that substrates enter while bound to the enzymes that makes it easy to convert into the ‘desired’ product.

Apparently, people have observed this lowering of the activation energy, but it’s been difficult for even those who study these things to define. I’ve posted the article on blackboard if any of you are interested in taking a peak at it.