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Chapter 6 - Biochemistry

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chichi's version from 2017-04-25 14:54

Section 1

Question Answer
One of the enzymes involved in glycolysis, aldose, requires Zn2+ for catalysis. Under conditions of zinc deficiency, when the enzyme may lack zinc, it would be referred in as the:apoenzyme
Which of the following is not among the six internationally accepted classes of enzymes?Polymerases
Enzymes are potent catalysts because they:lower the activation energy for the reactions they catalyze
The role of an enzyme in an enzyme-catalyzed reaction is to:increase the rate at which substrate is converted into product
Which one of the following statements is true of enzyme catalysts?They can increase the reaction rate for a given reaction by a thousand fold or more.
Which one of the following statements is true of enzyme catalysts?The lower the activation energy for the conversion of substrate to product.
Which of the following statements is false?For S--->P, a catalyst shifts the reaction equilibrium to the right
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Section 2

Question Answer
Enzymes differ from other catalysts in that only enzymes:display specificity toward a single reactant
Which of the following is true of the binding energy derived from enzyme-substrate interations?It is sometimes used to hold substrates in the optimal orientation for reaction.
The concept of "induced fit" refers to the fact that"substrate binding may induce a conformational change in the enzyme, which then brings catalytic groups into proper orientation.
The benefit of measuring the initial rate of a reaction Vo is that at the beginning of a reaction:changes in [S] are negligible, so [S] can be treated as a constant
Which of the following statements about a plot Vo vs. [S] for an enzyme that follows Michaelis-Menten kinetics is false?At very high [S], the velocity curve becomes a horizontal line that intersects the y-axis at Km.
Michaelis and Menten assumed that the overall reaction for an enzyme-catalyzed reacion could be written as E+S (k1)--><--(k-1) ES -->(k2) E+P Using this reaction, the rate of breakdown of the enzyme-substrate complex can be described by the expression: k-1 [ES] + k2 [ES]
The steady state assumption, as applied to enzyme kinetics, implies:the ES complex is formed and broken down at equivalent rates
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Section 3

Question Answer
Which of these statements about enzyme-catalysed reactions is false?The activation energy for the catalyzed reaction is the same as for the uncatalyzed reaction, but the equilibrium constant is more favorable in the enzyme-catalyzed reaction.
The following data were obtained in a study of an enzyme known to follow Michaelis-Menten kinetics: The Km for the enzyme is approximately:turnover number
In a plot of 1/V against 1/[S] for an enzyme-catalyzed reaction, the presence of a competitive inhibitor will alter the:intercept on the 1/[S] axis
In competitive inhibition, an inhibitor:binds reversibly at the active site
Vmax for an enzyme-catalyzed reaction:is twice the rate observed when the concentration of substrate is equal to the Km.
Enzyme X exhibits maximum activity at pH=6.9. X shows a fairly sharp decrease in its activity when the pH goes much lower than 6.4. One likely interpretation of this pH activity is that:a His residue on the enzyme is involved in the reaction
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Section 4

Question Answer
Both water and glucose share an -----OH that can serve as a substrate for a reaction with the terminal phosphate of ATP catalyzed by hexokinase. Glucose, however, is about a million times more reactive as a substrate than water. The best explanation is that:the larger glucose binds better to the enzyme; its induces a conformational change is hexokinase that brings active-site amino acids into position for catalysis
A good transition-state analog:binds to the enzyme more tightly than the substrate.
A transition-state analog:resembles the transition-state structure of the normal enzyme-substrate complex
The role of the metal ion (Mg2+) in catalysis by enolase is tofacilitate general base catalysis
Which of the following statements about allosteric control of enzymatic activity is false?Heterotropic allosteric effects compete with substrate for binding sites
A small molecule that decreases the activity of an enzyme by binding to a site other than the catalytic site is termed a(n):allosteric inhibitor
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Section 5

Question Answer
Allosteric enzymes:usually have more than one polypeptide chain
Which of the following has not been shown to play a role in determining the specificity of protein kinases?Disulfide bonds near the phosphorylation site
How is trypsinogen converted to trypsin?Proteolysis of trypsinogn forms trypsin
Define the terms "cofactor" and "coenzyme."A cofactor is any chemical component required for enzye activity; it includes both organic molecules, called "coenzymes," and inorganic ions.
Michaelis-Menten kinetics is sometimes referred to as "saturation" kinetics. Why?According to the Michaelis-Menten model of enzyme-substrate interaction, when [S] becomes very high, an enzyme molecule's active site will become occupied wit a new substrate molecule as soon as it releases a product. Therefore, at very high [S], Vo does not increase with additional substrate, and the enzyme is said to be "saturated" with substrate
Why is the Lineweaver-Burk (double reciprocal) plot (see Box 6, p. 206) more useful than the standard V vs. [S] plot in determining kinetic constants for an enzyme? The plot of V vs. [S] is hyperbolic; maximum velocity is never acheived experimentally, because it is impossible to do experiments at infinitely high [S]. The Lineweaver-Burk transformation of the Michaelis-Mentenn equation produces a linear plot that can be extraploated to infinitie [S] (where 1/[S] becomes zero), allowing a determination of Vmax.
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Section 6

Question Answer
How does the total enzyme concentration affect turnover number and Vmax?The turnover number, kcat, is the number of substrate molecules converted to products in a given time by a single enzyme molecule, so turnover number is not affected by the total enzyme conventration, [Et]. For any given reaction, however, Vmax can change because Vmax is the product of turnover number x the total enzyme concentration, or Vmax = kcat [Et].
Methanol (wood alcohol) is highly toxic because it is converted to formaldehyde in a reaction catalyzed by the enzyme alcohol dehydrogenase: NAD+ + methanol --> NADH+ + H+ + formaldehyde Part of the medical treatment for methanol poisoning is to administer ethanol (ethyl alcohol) in amounts large enough to cause intoxication under normal circumstances. Explain this in terms of what you know about examples of enzymatic reactions.Ethanol is a structural analog of methanol, and competes with methanol for the binding site of alcohol dehydrogenase, slowing the conversion of methanol to formaldehyde, allowing its clearance by the kidneys. The effect of ethanol is that of a competitive inhibitor.
Why does pH affect the activity of an enzyme?The state of ionization of several amino acid side chains is affected by pH, and the activity of many enzymes requires that certain of the amino acid residue side chains be in a specific ionization state.
Chymotrypsin belongs to a group of porteolytic enzymes called the "serine proteases" may of which have an Asp, His, and Ser residue that are crucial to the catalytic mechanism. The serine hydroxyl functions as a nucleophile. What do other two amino acids do to support this nucleophilic reaction?In chymotrypsin, histidine functions as a general base, accepting a proton from the serine hydroxyl, thereby increasing serine's reactivity as a nucleophile. The negatively charged Asp stabilizes the positive charge the develops on the His.
On the enzyme hexokinase, ATP reacts with glucose to produce glucose 6-phosphate and ADP five orders of magnitude faster than ATP reacts wit H2O to form phosphate and ADP. The instrinsic chemical reactivity of the -----OH group in water is about the same as that of the glucose molecule, an water can certainly fit into the active site. Explain this rate differential in two sentences or less.The binding of glucose to hexokinase induces a conformation change tat brings the amino acid residues that facilitate the phosphoryl transfer into position in the active site. Binding of water alone does not induce this conformation change.
Explain how a biochemist might discover that a certain enzyme is allosterically regulated.The enzyme would show kinetics that do not fit the Michaelis-Menten equation; the plot of V vs. [S] would be sigmoidal, not hyperbolic. The enzymes kinetics would be affected by molecules other than substrate(s).
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Section 7

Question Answer
What is a zymogen (proenzyme)? Explain briefly with an example.A zymogen is an inactive form of an enzyme that is activated by on or more proteolytics cleavages in its sequence. Chymotrypsinogen, trypsinogen, and proelastase are all zymogens, becoming chymotrypsin, trypsin, and elastase, respectively, after proper cleavage.
The form in which the ping pong mechanism binds substrates is identified as which type of mechanism?The ping-pong mechanism is a non-sequential mechanism. A product is released after the first substrate is bound.
What are two characteristics of an enzyme that catalyzes a reaction through the ping-pong mechanism?One, a product is seen before the second substrate is bound. Two, binding of the first substrate causes the enzyme to change into an intermediate form that will bind the second substrate. Three, the plot of 1/v vs. 1/[A] as [B] changes will be parallel lines.
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