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Questions about Enzymes

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Questions about Enzymes

Postby samoyan » Sat Nov 17, 2007 5:29 am

I had a test yesterday and I`m not sure whether I answered some questions correct or not. Here are some about enzymes.

1). A change in environmental pH can inhibit the proper functioning of an enzyme by:

a) changing the charge of its active site.
b) altering the enzyme`s primary structure.
c) altering the specificity of the enzyme.
d) a, b, c
e) a and c


First, I chose D, but know I`m not sure about B. Primary structure of enzyme is its linear sequence of amino acids. Low or high pH can definitely affect the charge of enzyme`s active site and alter the specificity of enzyme. Sometimes pH can denature enzymes and if an enzyme (protein) is denatured, its amino acids can disassemble.


2). You are testing the conditions of a particular chemical reaction and set up one reaction tube with enzyme and another without enzyme. At the end of two hours you analyze the tubes. You find ample product in the tube with enzyme but none in the tube without enzyme. Based on that information which of the following must be true?

a) the reaction requires an enzyme to proceed.
b) the reaction is non-spontaneous.
c) the reaction requires energy to proceed.
d) this reaction is exergonic (exothermic).
e) none of the above statements must be true.

Answer A as a correct is obvious, but the answer C is also correct. Enzymes lower activation energy so that reactions can proceed at normal temperatures. I`m a little bit confused which choice will be 100% correct.

What do you think about these questions?

Thank you.
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Postby mith » Sat Nov 17, 2007 7:31 am

1.To change the primary sequence you would need to break some polypeptide bonds--changing amino acids or cutting the chain somewhere.

2. All reactions require activation energy whether less or more because you require a transition state.
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Postby victor » Sat Nov 17, 2007 11:45 am

what the option C means is that the reaction is endergonic (requires free energy to proceed). Actually, the things happen in the tube with enzyme is that the substrate is in the metastable state, that is the substrate is thermodynamically favored to react but can't react unless there's an addition of catalyst or a little bit of energy to surpass the activation energy (Ea).
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Postby samoyan » Sat Nov 17, 2007 5:06 pm

Thank you for explanation. Which answers do you think are correct ones?

1). E
2). D

I have one more question. Is it possible that very low or high pH will denature a protein (enzyme) completely, so that it lost its original linear sequence of amino acids?
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Postby MrMistery » Sat Nov 17, 2007 9:26 pm

theoretically, no
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Postby h2so4hurts » Sun Nov 18, 2007 12:29 am

Changing the pH to either extreme will not break the peptide bonds.
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Postby victor » Sun Nov 18, 2007 2:19 am

samoyan wrote:Thank you for explanation. Which answers do you think are correct ones?

1). E
2). D


Umm...for number 2, I think that the answer is B. Because, even though A is likely the correct answer, I think that enzyme is not the only catalyst in all chemical reaction (you can add some metal which acts like catalyst). The answer is not C also, because if you said that the reaction needs energy to proceed, then the reaction is endergonic/endothermic.
My reason for choosing B is that non-spontaneous implies that the reactants are in the metastable state which are able to do a spontaneous reaction if a catalyst is added.

For your next question, you have to differ between denaturation and hydrolysis. When you refer until the protein lost its primary structure, it means that the protein undergo hydrolysis, not denaturation. I still don't know how to break peptide bonds, but I'll search for it. My prediction is with strong bases because peptide (amide) bonds tends to be acidic.
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Postby mith » Sun Nov 18, 2007 2:31 am

A spontaneous reaction is thermodynamically favorable but not necessarily fast. A non-spontaneous reaction will not happen without energy input.
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