Enzymes, Energy, and Catalysis

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I have provided you with a Biology Quiz in multiple choice format with an answers provided (although it is presented below, please see attached response for better formatiing and diagrams). In the second attachment, you will find a tutorial explaining the correct answers for each question.

I hope this is helpful.

(Source: Adapted from Farabee, M. J. ©1992, 1994, 1997, 1999, 2000, 2001. "Online Biology Book.")

Biology Quiz Tutorial: Energy, Enzymes, and Catalysis

Question 1 Tutorial: Features of enzyme catalyzed reactions.
1. Which statement about enzyme catalyzed reactions is NOT true?
A. Enzymes form complexes with their substrates.
B. Enzymes lower the activation energy for chemical reactions.
C. Enzymes change the K eq for chemical reactions. CORRECT ANSWER
D. Many enzymes change shapes slightly when substrate binds.
E. Reactions occur at the "active site" of enzymes, where a precise 3D orientation of amino acids is an important feature of catalysis.

Features of Enzyme Catalyzed Reactions

Enzymes are biological catalysts. Catalysts lower the activation energy for reactions. The lower the activation energy for a reaction, the faster the rate. Thus enzymes speed up reactions by lowering activation energy. Many enzymes change shape when substrates bind. This is termed "induced fit", meaning that the precise orientation of the enzyme required for catalytic activity can be induced by the binding of the substrate.
Enzymes have active sites. The enzyme active site is the location on the enzyme surface where substrates bind, and where the chemical reaction catalyzed by the enzyme occurs. There is a precise -substrate interaction that occurs at the active site stabilized by numerous weak interactions (hydrogen bonds, electrostatic interactions, hydrophobic contacts, and van der Waals forces).
Enzymes form complexes with their substrates. The binding of a substrate to an enzyme active site is termed the "enzyme-substrate complex". A generic equation for complex formation is as follows:

Enzymes do not:
Change the equilibruim constant for a reaction. Keq depends only on the difference in energy level between reactants and products.
Change DG for a reaction. As shown in the graphs above, enzymes only lower activation energy, but do not change the difference in energy levels between reactants and products.
Convert a non-spontaneous reaction into a spontaneous reaction.
Question 2 Tutorial: Equilibrium constant for sucrose hydrolysis.

2. The equilibrium constant for the conversion of the disaccharide sucrose to the simple sugars glucose and fructose is 140,000. What can you conclude about the reaction: sucrose + H2O glucose + fructose?
A. It is a closed system.
B. It never reaches equilibrium.
C. It is spontaneous, starting with sucrose. CORRECT ANSWER
D. The equilibrium constant increases when the starting concentration of sucrose is increased.
E. At equilibrium, the concentration of sucrose is much higher than the concentrations of glucose and fructose.

Understanding activation energy:
The equilibrium constant for the conversion of the disaccharide sucrose to the simple sugars glucose and fructose is 140,000. What can you conclude about the reaction: sucrose + H2O glucose + fructose?
Reaction equations
The equation for the chemical reaction described in this problem is as follows:

This is a hydrolysis reaction. The disaccharide glucose is hydrolyzed to the two monosaccharides glucose and fructose. Biological hydrolysis reactions are almost always spontaneous (exergonic).
The balance between the forward and reverse reactions is known as the chemical equilibruim, which is defined as the ratio of the concentration of products and reactants at equilibruim when there is no further change in concentrations.
For this reaction,

Free energy and chemical equilibrium
This chemical reaction can run in both the forward and reverse directions, but the products glucose and fructose are at a lower energy level than the reactant sucrose.
The difference between a spontaneous and nonspontaneous reaction can be distinguished by the following relationships:
Spontaneous Reaction Non-Spontaneous Reaction
Keq > 1 Keq < 1
G < 0 G > 0
Exergonic Endergonic
Forward reaction favored Reverse reaction favored
The hydrolysis of sucrose
This is a spontaneous reaction beginning with sucrose because Keq > 1.

Question 3: Tutorial for the Kinetics of an allosteric enzyme.
3. In regards to the kinetics of an allosteric enzyme, which of the following graphs shows the results of reaction rate vs substrate concentration for an allosteric enzyme in the absence and presence of an allosteric inhibitor?

A. 1 CORRECT ANSWER
B. 2
C. 3
D. 4
E. None of the graphs.

The Kinetics of an allosteric enzymes:

Allosteric enzymes
Enzymes with multiple subunits have quarternary structure. One consequence of multiple subunits is that individual catalytic subunits each have their own active site. This means that an enzyme with quarternary structure can bind more than one substrate molecule. Allostery means "different shape". Allosteric enzymes change shape between active and inactive shapes as a result of the binding of substrates at the active site, and of regulatory molecules at other sites. In the simple case of an allosteric enzyme with an active and inactive form, the change in reaction rate with increasing substrate concentration is typically an "S-shaped" curve. For more information on allosteric enzyme, see the tutorial for question 14.

Binding of effectors to regulatory subunits
Allosteric enzymes may also have regulatory subunits that bind either activators or inhibitors. Activators and Inhibitors are termed "effectors". Inhibitors cause the allosteric enzyme to adopt the inactive shape. Activators promote the active shape.
An equilibrium exists between the active and inactive shapes. The amount of active and inactive enzyme is dependent on the relative concentrations of substrate and inhibitor, as suggested by the diagram:

The binding of an allosteric inhibitor causes the enzyme to adopt the inactive conformation, and promotes the cooperative binding of a second inhibitor.

An excess of substrate can overcome the inhibitor effect. Substrate binding causes the enzyme to assume the active conformation, and promotes the cooperative binding of additional substrate, leading to product formation.
The meaning of the S-shaped Curves with and without inhibitor

The shape of the curve minus inhibitor is described in more detail in the tutorial to question 14. As the substrate ...