Tag Archives: enzyme

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. Image

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.


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Posted by on January 1, 2014 in Uncategorized


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Allosteric Enzymes

I have found that many readers find my blog with the search terms ‘allosteric enzymes.’ But in looking back to the post that I wrote describing these enzymes I find it a bit wanting. So I’ve decided to write a new post on that topic here.


Consider first what an enzyme is: biological catalysts.

And, what is a catalyst? It’s something that is involved in a chemical reaction, but is not changed by the reaction.

-If a catalyst was a person it might be a matchmaker who, through their personal network, sets people up together to see if they are compatible. But like matchmakers, catalysts just bring molecules together, they don’t actually become part of the couple themselves. Catalysts remain unchanged at the end of the reaction ready to do the same job again.



Enzymes do their work by binding their substrates in a location called the active site. In the active site, the reaction takes place usually breaking one molecule into two or joining two molecules into one or some other such reaction.


Figure 1

Figure 1: Enzyme with active site for substrates


So, enzymes catalyze reactions in biological systems. They are typically proteins, but can also be RNA molecules that fold up to have the same properties.

In my class, I always emphasize that

Form Dictates Function.’

That is, in cells (or even outside of them) biomolecules work or don’t work because of their form. In this case, if an enzyme is folded into the proper conformation, it may bind molecules and facilitate a reaction to take place. If they are not in a conformation to bind the molecules, they won’t do it and the reaction does not take place.


Figure 2: An unregulated enzyme. This enzyme is always in the active conformation capable of processing substrates (A) into product (B)

Some enzymes might appear to be always in the proper conformation and always catalyze reactions amongst the molecules around them (like the one pictured above in Figure 2). However, some others are shapeshifters, that are sometimes in a conformation favorable to catalyzing the reaction (an active conformation)

Other times, these enzymes are in conformations unfavorable to catalyzing the reaction (an inactive conformation).


But what dictates what conformation an enzyme is in?

One thing might be whether there are other molecules that bind to the enzymes in the nearby (micro-)environment. These molecules are called effectors and reasonably enough, they bind a site on the enzyme called an effector site. An effector site is a binding site on the enzyme that exists anywhere outside of the active site.

Figure 3

Figure 3: Allosteric enzyme with active site and effector site. A) Effector unbound, Active site in active conformation – capable of processing substrate B) Effector bound, Active site in inactive conformation – incapable of processing substrate


When an effector molecule binds to the effector site, the shape of the whole enzyme changes to a new conformation. In the illustration above, the active form is shown when the effector is unbound (A), the inactive form is shown when the effector is bound (B). In this case, binding of the effector caused a change to the inactive form, so the effector is called an inhibitor. It is possible that another enzyme is in the inactive form when the effector is unbound, but changes into an active conformation when bound. In that case, the effector would be an activator.


Enzymes that change shape like this are called allosteric enzymes. ‘Allo-‘ translates as different and ‘–steric’ translates as shape, so these are enzymes that change their shape (from active to inactive forms).


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Posted by on December 8, 2012 in Uncategorized


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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.”Science303:186 – 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.


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Posted by on September 20, 2012 in Uncategorized


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Enzyme regulation


Effector Actions may be Inhibitory (top) or Activating (bottom)

I’ve been looking for an animation (or something similar) that would illustrate how enzyme activity can be regulated by effectors. However, this seems to be lacking… I’m thinking about making one myself and will post it here if I do.

Recall that effectors are the molecules that bind to enzymes outside of the active site and have either a positive (activator) or negative (inhibitor) effect on the enzyme’s activity. Regardless of the positive or negative effect, both of these molecules are considered effectors and bind to an effector site. The effector site can be any location on the enzyme other than the active site. Inhibitory effectors are called non-competitive inhibitors because they do not compete with the binding of substrate directly at the active site. Activating effectors are called activators. Because these molecules bind away from the active site, they do not directly interact with the substrate, but instead influence the enzyme’s ability to bind (and/or modify) substrate by changing the shape of the enzyme. For this reason, enzymes that bind effectors are called allosteric enzymes, from the greek allo– ‘other’ and steric ‘shape/ conformation.’

Allosteric enzymes can be in one of two conformations: Active Conformation or Inactive Conformation. The conformation state that these enzymes exist in is dependent upon the presence or absence of their effector proteins.

Competitive Inhibition

The other major class of regulator is that of competitive inhibitors, these molecules bind directly to an enzyme’s active site and prevent substrate processing. This is a much more straightforward kind of inhibition and does not alter a enzyme’s shape into an inactive conformation.

I hope this helps to organize your thoughts about enzyme regulation. I plan on reviewing this in class this week using a couple example problems.

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Posted by on September 19, 2012 in Uncategorized


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Enzyme Regulation Animation

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Posted by on September 18, 2012 in Uncategorized


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