Mcdb 110 Problem Set 5

Front 1

Define phosphate group transfer potentials. What information is provided by comparing the phosphate group transfer potential values of various phosphorylated compounds?

Back 1

Phosphate group transfer potentials refer to the change in free energy that results in a reaction in which a phosphate bond is broken. Comparing the values of various phosphorylated compounds will yield information as to which ones may be useful to couple to non-spontaneous reactions.

Front 2

Why are many metabolites phosphorylated?

Back 2

1. The addition of this charged group retains the compound in the cell.

2. The higher energy bond lowers the activation energy.

3. Can be used to make ATP via substrate level phosphorylation

Front 3

Define substrate level phosphorylation.

Back 3

Substrate level phosphorylation is the transfer of a phosphate group from a metabolic intermediate to ADP directly through an enzymatic pathway.

Front 4

What is the advantage of using ATP over pyrophosphate as a major carrier of energy for a cell?

Back 4

ATP is used over pyrophosphate because its more complex structure allows it to bind enzymes in multiple ways.

Front 5

What is the non-catabolic function of glycolysis?

Back 5

The non-catabolic function of glycolysis is the production of substrates needed for anabolic reactions

Front 6

If one function of glycolysis is to obtain energy in catabolism, why is ATP consumed in the first half of the pathway?

Back 6

ATP is consumed in the first part of the pathway to prime the sugars so they can be broken down later into high energy compounds capable of producing more ATP than is initially used.

Front 7

Given that many steps of glycolysis have an unfavorable (positive) ΔGo ', why does glycolysis proceed in the cell when all enzymes are active?

Back 7

Glycolysis proceeds because the products of the reactions are reactants in the next step in the pathway (and sometimes other pathways) and are always kept at low levels. By the Law of Mass Action or Le Chatlier’s principle, this drives reactions forward.

Front 8

What step in glycolysis commits metabolized glucose to the glycolytic pathway?

Back 8

The conversion of D-fructose-6-phosphate to D-fructose-1,6-bisphosphate commits glucose to the glycolytic pathway

Front 9

What is the key enzyme used in the committed step of glycolysis and how it is positively and negatively regulated?

Back 9

The key enzyme is phosphofructokinase (PFK). It is positively regulated by AMP and ADP and negatively regulated by ATP and citrate

Front 10

Normally a regulated enzyme is inhibited by a downstream product; however, one of the allosteric inhibitors of this enzyme is actually a substrate for the enzyme. Explain this apparent paradox.

Back 10

When ATP is at low levels, it also serves as a substrate for PFK. This paradox is explained by the fact that ATP levels measure the energy available in a cell. Glycolysis makes ATP, increasing the energy level of a cell. If the energy level is too high, then ATP can inhibit PFK. In addition, ATP bound to PFK increases the Km for fructose-6- phosphate

Front 11

What is channeling and how does it increase the efficiency of glycolysis within a cell?

Back 11

Metabolite channeling—some glycolytic enzymes associate with each other in a cell. It increases the efficiency of glycolysis because the products of one enzymatic reaction are very close to the next glycolytic enzyme. Chemically labile reactants are protected and do not need to diffuse through the cell until they find the next enzyme.

Front 12

What is the important functional difference between NADH and NADPH?

Back 12

NADPH is primarily used in anabolism. NADH is primarily used in catabolism.

Front 13

Why do these two different forms exist in cells? (NADH vs NADPH)

Back 13

They both exist in cells because even though they both act as electron acceptors and donors,they serve different pathways, and because they are differentially recognized, this allows cells to control the flow of reductive power towards catabolism or anabolism

Front 14

Under what circumstances and why do some cells carry out fermentation?

Back 14

Cells carry out fermentation under anaerobic conditions to regenerate NAD+. This results in the rapid output of energy, because NAD+ quickly becomes available for reduction to NADH.

Front 15

What needs of a cell can be provided by the pentose phosphate pathway?

Back 15

The pentose phosphate pathway is primarily anabolic- it produces metabolic precursors

Front 16

What can be produced by the pentose phosphate pathway but not by glycolysis?

Back 16

NADPH, ribose-5-phosphate, and erythrose-4-phosphate.

Front 17

What do you suppose is the rationale that some athletes use in eating a high carbohydrate meal the evening before an event?

Back 17

They hope to increase readily available energy stores by the breakdown of carbohydrates through glycolysis and the TCA cycle

Front 18

Depending on the organism, sucrose can be cleaved to monosaccharides by either hydrolysis (using the enzyme sucrase) or phosphorolysis (using sucrose phosphorylase). (The latter is analogous to the reaction catalyzed by glycogen phosphorylase.) Calculate the ATP yield per mole of sucrose metabolized by fermentation of sucrose to ethanol starting with either a) hydrolytic or b) phosphorolytic cleavage of sucrose.

Back 18

First of all, since the conversion of pyruvate to ethanol does not yield any ATP, we only need to be concerned with getting to pyruvate.

Look at table.

Front 19

If it were possible to label glucose with the radioisotope 14C at any position or combination of positions, which form or forms of labeled glucose would give the most radioactivity in CO2 and the least in ethanol when anaerobically catabolized?

Back 19

The carbon in the CO2 that is liberated in fermentation comes from the carboxylic acid on pyruvate, which is either C3 or C4 from glucose. Labeling either of these carbons would give the most radioactivity in CO2, and labeling any of the other carbons would give the least

Front 20

Which labeled carbon or carbons of glucose, if it were metabolized aerobically, would result in the most rapid appearance of label in CO2?

Back 20

In aerobic metabolism, the first CO2 to be liberated is during the TCA cycle. It is the byproduct of the conversion of pyruvate to acetyl-CoA. It is the same carbon lost in fermentation to ethanol, so either C3 or C4 from glucose

Front 21

Bonetti, a California winemaker, was plagued one year with underripe grapes resulting from high rainfalls and few sunny days. At the end of the fermentation season, he was disappointed to find that his chardonnay had a final alcohol concentration of only10% (10 g / 100 ml). A good chardonnay usually has about 12-13% ethanol. What was the molar concentration of glucose or its equivalent in the initial grape juice before yeast was added?

Back 21

The molecular weight of ethanol is 46. So a solution that is 10g/100ml of ethanol has a concentration of (10/46)/0.1 L = 2.17 M. Because 1 glucose yields 2 ethanol, the starting concentration of glucose is half the final concentration of ethanol, or 1.085 M

Front 22

How many moles of sucrose per liter would he need to add to the juice that he started with to produce a wine containing 13% ethanol (i.e, to increase the final content of ethanol by 3%)?

Back 22

To get 13% ethanol, the ethanol concentration will be (13/46)/0.1L = 2.83 M, so you need 2.83 – 2.17 = 0.66 M more ethanol. Remember sucrose is one fructose and one glucose so one sucrose can yield four ethanol. So the sucrose you need is 0.66 M / 4 = 0.165M.

Front 23

Given:

glucose → 2 ethanol + 2 CO2 ΔGo ' = -229 kJ / mol glucose + 6 O2 →6 CO2 + 2 H2O ΔGo ' = -2870 kJ / mol

ATP → ADP + Pi ΔGo ' = -30.5 kJ / mol

Calculate the number of moles of ATP that could in principle be synthesized by the conversion of one mole of ethanol into CO2 + H2O assuming an efficiency of 44%

Back 23

To calculate free energy from converting one mole of ethanol to CO2 + H2O, reverse the first equation (and the sign of the free energy) and add it to the second equation: The energy from one mole of ethanol is -2641/2 = -1320.5 kJ/mol. At 44% efficiency, that energy value is reduced to 0.44 x (-1320.5) = -581 kJ/mol

Front 24

Given that aerobic catabolism of pyruvate produces 15 ATPs with ~44% efficiency, what does this tell you about the relative energy content of ethanol and pyruvate?

Back 24

The number of ATP that would make is 581/30.5 = 19 ATP per ethanol, which is more than the 15 ATP one gets from pyruvate through TCA, so ethanol has a higher energy content than pyruvate.

Front 25

If the C-1 carbon of glucose were labeled with a 14C radioisotope and the labeled glucose allowed to undergo glycolysis, which carbon atom on pyruvate would be labeled?

Back 25

C-3, the methyl carbon, on pyruvate would be labeled.

Front 26

If the C-4 carbon of glucose were labeled with a 14C radioisotope and the labeled glucose allowed to undergo glycolysis, which carbon atom on pyruvate would be labeled?

Back 26

C-1, the carboxylic carbon, on pyruvate would be labeled.

Front 27

Is it possible to achieve net synthesis of oxaloacetate by adding acetyl CoA to a mixture containing the enzymes and cofactors of the TCA cycle?

Back 27

. No, because oxaloacetate is recycled during TCA and any oxaloacetate produced would be used to consume more acetyl-CoA.

Front 28

What is the function of the glyoxylate cycle and how does it differ from the TCA cycle?

Back 28

The glyoxylate cycle nets oxaloacetate, and ultimately glucose, from precursors like acetyl-CoA. This cycle stops the TCA cycle at isocitrate and goes through an alternate pathway that doesn’t lose the two carbons introduced by acetyl-CoA.

Front 29

Succinate dehydrogenase activity is affected by oxaloacetate. Would you expect the enzyme activity to be enhanced or inhibited by oxaloacetate? Explain.

Back 29

Succinate dehydrogenase would be enhanced by oxaloacetate because a large amount of oxaloacetate would indicate that the cell can go through the TCA cycle and succinate dehydrogenase is an enzyme in the pathway

Front 30

Oxygen must be present in cells for the TCA cycle to progress; however, it is not an intermediate or reactant in the cycle. Why is oxygen necessary for continued operation of the TCA cycle?

Back 30

O2 is needed because it is required for oxidative phosphorylation, the pathway to recycle NADH to NAD+ and FADH2 to FAD in aerobic environments. These recycling events are where 3 ATPs are netted per 1 NADH and 2 per FADH2.