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4C Focus on Health & Medicine: Conversion to Ethanol

4C Focus on Health & Medicine: Conversion to Ethanol

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THE ATP YIELD FROM GLUCOSE



757



The ethanol in beer, wine, and other alcoholic beverages is obtained by the fermentation of sugar,

quite possibly the oldest example of chemical synthesis. The carbohydrate source determines the

type of alcoholic beverage formed. The sugars in grapes are fermented to form wine, barley malt

is used for beer, and the carbohydrates in corn or rye form whiskey. When the fermented ethanol

reaches a concentration of about 12%, the yeast cells that provide the enzymes for the process

die and fermentation stops. Any alcoholic beverage with an ethanol concentration of greater than

12% must be distilled to increase the ethanol content. While ethanol consumption is considered

socially acceptable in most societies, the irresponsible consumption of alcoholic beverages is a

major health and social problem in many parts of the world (Section 14.6).



Fermentation plays a key role in

the production of bread, beer,

and cheese.



Fermentation plays a role in forming other food products. Cheese is produced by fermenting

curdled milk, while yogurt is prepared by fermenting fresh milk. When yeast is mixed with flour,

water, and sugar, the enzymes in yeast carry out fermentation to produce the CO2 that causes

bread to rise. Some of the characteristic and “intoxicating” odor associated with freshly baked

bread is due to the ethanol that has evaporated during the baking process.



PROBLEM 24.12



What role does NADH play in the conversion of pyruvate to lactate? What role does NADH

play in the conversion of pyruvate to ethanol?



PROBLEM 24.13



(a) In what way(s) is the conversion of pyruvate to acetyl CoA similar to the conversion of

pyruvate to ethanol? (b) In what way(s) are the two processes different?



PROBLEM 24.14



(a) In what way(s) is the conversion of pyruvate to lactate similar to the conversion of pyruvate

to ethanol? (b) In what way(s) are the two processes different?



24.5 THE ATP YIELD FROM GLUCOSE

How much ATP is generated from the complete catabolism of glucose (C6H12O6) to carbon

dioxide (CO2)? To answer this question we must take into account the number of ATP molecules

formed in the following sequential pathways:











the glycolysis of glucose to two pyruvate molecules

the oxidation of two pyruvate molecules to two molecules of acetyl CoA

the citric acid cycle

the electron transport chain and oxidative phosphorylation



To calculate how much ATP is generated, we must consider both the ATP formed directly in reactions, as well as the ATP produced from reduced coenzymes after oxidative phosphorylation.

As we learned in Section 23.6, each NADH formed in the mitochondrial matrix provides the

energy to yield 2.5 ATPs, while each FADH2 yields 1.5 ATPs. In contrast, the NADH formed

during glycolysis is generated in the cytoplasm outside the mitochondria. Although it cannot

pass through the mitochondrial membrane, its electrons and energy can be transferred to other

molecules—NADH or FADH2—ultimately yielding 1.5 or 2.5 ATPs for each NADH. To simplify calculations, we will use the value 2.5 ATP/NADH consistently in calculations.

Finally, any calculation must also take into account that glucose is split into two three-carbon

molecules at step [4] of glycolysis, so the ATP yield in each reaction must be doubled after

this step. With this information and Figure 24.6 in hand, we can now determine the total yield of

ATP from the complete catabolism of glucose.



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CARBOHYDRATE, LIPID, AND PROTEIN METABOLISM







FIGURE 24.6 The ATP Yield from the Aerobic Metabolism of Glucose to CO2

C6H12O6

glucose

Glycolysis



2 ATP



2 NADH



5 ATP



2 CH3COCO2−

pyruvate

2 CO2



2 NADH



5 ATP



2 CH3COSCoA

acetyl CoA

Citric acid cycle

Electron transport chain

Oxidative phosphorylation



2 GTP



2 ATP



6 NADH



15 ATP



2 FADH2



3 ATP



4 CO2

Total ATP yield = 32 ATP



The complete catabolism of glucose forms six CO2 molecules and 32 ATP molecules.



• Glycolysis yields a net of two ATP molecules. The two molecules of NADH formed during

step [6] of glycolysis yield five additional ATPs.

C6H12O6

glucose



2 CH3COCO2−



+



2 ATP



+



2 NADH



pyruvate

2 × (2.5 ATP/NADH) = 5 ATP



• Oxidation of two molecules of pyruvate to acetyl CoA in the mitochondria forms two

NADH molecules that yield five ATP molecules.

2 CH3COCO2−

pyruvate



2 CH3COSCoA



+



2 CO2



+



2 NADH



acetyl CoA

2 × (2.5 ATP/NADH) = 5 ATP



• Beginning with two acetyl CoA molecules, the citric acid cycle (Figure 23.7) forms two GTP

molecules, the energy equivalent of two ATPs, in step [5]. The six NADH molecules and two

FADH2 molecules also formed yield an additional 18 ATPs from the electron transport chain

and oxidative phosphorylation. Thus, 20 ATPs are formed from two acetyl CoA molecules.

2 × (1.5 ATP/FADH2) = 3 ATP

2 CH3COSCoA

acetyl CoA



4 CO2



+



2 GTP

2 ATP



+



6 NADH



+



2 FADH2



6 × (2.5 ATP/NADH) = 15 ATP



Adding up the ATP formed in each pathway gives a total of 32 molecules of ATP for the complete catabolism of each glucose molecule. Most of the ATP generated from glucose metabolism results from the citric acid cycle, electron transport chain, and oxidative phosphorylation.



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GLUCONEOGENESIS



759



Glucose is the main source of energy for cells and the only source of energy used by the brain.

When energy demands are low, glucose is stored as the polymer glycogen in the liver and

muscles. When blood levels of glucose are low, glycogen is hydrolyzed to keep adequate blood

glucose levels to satisfy the body’s energy needs.

Blood glucose levels are carefully regulated by two hormones. When blood glucose concentration rises after a meal, insulin stimulates the passage of glucose into cells for metabolism. When

blood glucose levels are low, the hormone glucagon stimulates the conversion of stored glycogen

to glucose.



PROBLEM 24.15



How much ATP results from each transformation?

a. glucose → 2 pyruvate

b. pyruvate → acetyl CoA



c. glucose → 2 acetyl CoA

d. 2 acetyl CoA → 4 CO2



PROBLEM 24.16



What three reactions form CO2 when glucose is completely catabolized?



PROBLEM 24.17



What three reactions form a nucleoside triphosphate (GTP or ATP) directly when glucose is

completely catabolized?



PROBLEM 24.18



What is the difference in ATP generation between the aerobic oxidation of glucose to CO2 and

the anaerobic conversion of glucose to lactate?



24.6 GLUCONEOGENESIS

Now that we have learned how glucose is converted to CO2 and energy, we can also consider one

other metabolic pathway that involves glucose.

• Gluconeogenesis is the synthesis of glucose from noncarbohydrate sources—lactate,

amino acids, or glycerol.



As such, gluconeogenesis is an anabolic pathway rather than a catabolic pathway since it

results in the synthesis of glucose from smaller molecules. Gluconeogenesis occurs in the liver

when the body has depleted both available glucose and stored glycogen reserves. Such a condition results during sustained vigorous exercise or fasting.

For example, the lactate that builds up in muscles as a result of vigorous exercise can be transported to the liver where it is oxidized to pyruvate, which is then converted to glucose by means

of gluconeogenesis. The newly synthesized glucose is then utilized for energy or stored in the

muscle as glycogen.

OH

2 CH3



C

H

lactate



O

CO2







oxidation



2 CH3



C



CO2−



pyruvate



gluconeogenesis



C6H12O6

glucose



Conceptually, gluconeogenesis is the reverse of glycolysis; that is, two molecules of pyruvate are

converted to glucose by a stepwise pathway that passes through all of the same intermediates

encountered in glycolysis. In fact, seven of the 10 steps of gluconeogenesis use the same enzymes

as glycolysis. Three steps in glycolysis (steps [1], [3], and [10]), however, must use different

enzymes to be energetically feasible for gluconeogenesis to occur. As a result, gluconeogenesis

provides a mechanism for synthesizing new glucose molecules from other substrates.

• Glycolysis is a catabolic process that converts glucose to pyruvate.

• Gluconeogenesis is an anabolic process that synthesizes glucose from pyruvate.



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CARBOHYDRATE, LIPID, AND PROTEIN METABOLISM







FIGURE 24.7 The Cori Cycle—Glycolysis and Gluconeogenesis



[1] Glycolysis in muscle



2 CH3COCO2−

pyruvate



C6H12O6

glucose



[4] Transport to the muscle



2 CH3CH(OH)CO2−

lactate

[2] Transport to the liver



2 CH3COCO2−

pyruvate



C6H12O6

glucose



2 CH3CH(OH)CO2−

lactate



[3] Gluconeogenesis in the liver



Steps in the Cori cycle:

[1] The catabolism of glucose in muscle forms pyruvate, which is reduced to lactate when the

oxygen supply is limited.

[2] Lactate is transported to the liver.

[3] Oxidation of lactate forms pyruvate, which is then converted to glucose by the 10-step

process of gluconeogenesis.

[4] Glucose is transported back to the muscle.



Cycling compounds from the muscle to the liver and back to the muscle is called the Cori cycle.

The Cori cycle involves the four operations shown in Figure 24.7.

Because the brain relies solely on glucose as an energy source, gluconeogenesis is a mechanism

that ensures that the brain has a supply of glucose even when a diet is low in carbohydrates and

glycogen reserves are low. Gluconeogenesis is not a commonly used metabolic pathway when

carbohydrate intake is high. On the other hand, a low carbohydrate diet makes gluconeogenesis an important pathway that converts noncarbohydrate substrates—fats and amino acids—to

needed glucose.



PROBLEM 24.19



From what you learned about glycolysis in Section 24.3, what is the starting material and product

of each of the following steps in gluconeogenesis: (a) step [1]; (b) step [3]; (c) step [10]?



24.7 THE CATABOLISM OF TRIACYLGLYCEROLS

The first step in the catabolism of triacylglycerols, the most common lipids, is the hydrolysis of

the three ester bonds to form glycerol and fatty acids, which are then metabolized in separate

pathways.

O

CH2

CH



O

O



C

O



R



C

O



R



CH2



CH2 O C R

triacylglycerol



smi26573_ch24.indd 760



+



3 H2O



lipase



H



C



O



OH

OH



CH2



OH



+



3 HO



C



R



fatty acid



glycerol



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THE CATABOLISM OF TRIACYLGLYCEROLS



761



24.7A GLYCEROL CATABOLISM

The glycerol formed from triacylglycerol hydrolysis is converted in two steps to dihydroxyacetone phosphate. Phosphorylation of glycerol with ATP forms glycerol 3-phosphate, which is

then oxidized with NAD+.

CH2

H



C



ATP



OH



ADP



CH2



OH



H



C



glycerol

kinase



CH2 OH

glycerol



NAD+ NADH + H+



OH

OH



CH2

C



CH2 O P

glycerol

3-phosphate



glycerol 3-phosphate

dehydrogenase



OH

O



CH2 O P

dihydroxyacetone

phosphate



O





=



P



O



P



intermediate in glycolysis

and gluconeogenesis



O−



Since dihydroxyacetone phosphate is an intermediate in both glycolysis and gluconeogenesis,

two pathways are available, depending on the energy needs of the organism.



PROBLEM 24.20



Analyze the phosphorylation of glycerol in step [1] of glycerol metabolism by looking at the

functional groups, the reagent, and the name of the enzyme, as discussed in Section 24.2.



24.7B FATTY ACID CATABOLISM BY β-OXIDATION

Fatty acids are catabolized by 𝛃-oxidation, a process in which two-carbon acetyl CoA units are

sequentially cleaved from the fatty acid. Key to this process is the oxidation of the β carbon to

the carbonyl group of a thioester (RCOSR'), which then undergoes cleavage between the α and

β carbons.

This carbon is oxidized.

CH2



CH2



CH2



O

CH2

β



CH2 C

α

This C



These C’s come off as acetyl CoA.



C bond is cleaved.



Fatty acid oxidation begins with conversion of the fatty acid to a thioester with coenzyme A.

This process requires energy, which comes from the hydrolysis of two P O bonds in ATP to

form AMP, adenosine monophosphate. Much like the beginning of glycolysis requires an energy

investment, so, too, the initial step of fatty acid oxidation requires energy input.

ATP



O

CH3(CH2)14



CH2 CH2

stearic acid



C



OH



+



HS



AMP + 2 HPO42−



CoA

acyl CoA synthetase



O

CH3(CH2)14



CH2 CH2

an acyl CoA



C



SCoA



Once the product, an acyl CoA, is inside the mitochondrion, the process of β-oxidation is set to

begin. β-Oxidation requires four steps to cleave a two-carbon acetyl CoA unit from the acyl CoA,

as shown with the 18-carbon fatty acid stearic acid in Figure 24.8.

In step [1], FAD removes two hydrogen atoms to form FADH2 and a double bond between the

α and β carbons of the thioester. Water is added to the double bond in step [2] to place an OH

group on the β carbon to the carbonyl group, which is then oxidized in step [3] to form a carbonyl

group. The NAD+ oxidizing agent is reduced to NADH in step [3] as well. Finally, cleavage of the

bond between the α and β carbons forms acetyl CoA and a 16-carbon acyl CoA in step [4].

• As a result, a new acyl CoA having two carbons fewer than the original acyl CoA is formed.



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CARBOHYDRATE, LIPID, AND PROTEIN METABOLISM







FIGURE 24.8 𝛃-Oxidation of a Fatty Acid

This bond is cleaved in β-oxidation.

O

18 C’s



CH3(CH2)14



CH2 CH2

α

β

C18 acyl CoA



acyl CoA

dehydrogenase



C



SCoA



FAD



1 Oxidation



FADH2

O



CH3(CH2)14



CH

β



CH

α



C

H2O



enoyl CoA hydratase



CH3(CH2)14



OH



H



O



CH

β



CH



C



This bond is cleaved.



SCoA



O

C



CH2



SCoA



HSCoA



acyl CoA

acyltransferase

O



16 C’s



CH3(CH2)14



C



3 Oxidation



NADH + H+



O

C



2 Hydration



NAD+



β-hydroxyacyl CoA

dehydrogenase



CH3(CH2)14



SCoA



4 Cleavage



O

+



SCoA



C16 acyl CoA



CH3



C



SCoA



2 C’s



The following equation summarizes the important components of β-oxidation for a general acyl

CoA, RCH2CH2COSCoA. Each four-step sequence forms one molecule each of acetyl CoA,

NADH, and FADH2.

O

R



CH2



CH2



O

R



C



C



SCoA



+



NAD+



+



FAD



HSCoA



+



H2O



NADH



+



H+



O

SCoA



+



CH3



C



SCoA



+



+



FADH2



Once the 16-carbon acyl CoA is formed in step [4], it becomes the substrate for a new four-step

β-oxidation sequence. This type of metabolic pathway is called a spiral pathway, because the



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THE CATABOLISM OF TRIACYLGLYCEROLS



763



same set of reactions is repeated on increasingly smaller substrates. The process continues until a

four-carbon acyl CoA is cleaved to form two acetyl CoA molecules. As a result:

• An 18-carbon acyl CoA is cleaved to nine two-carbon acetyl CoA molecules.

• A total of eight cycles of 𝛃-oxidation are needed to cleave the eight carbon–carbon bonds.

O

CH3CH2



CH2CH2



CH2CH2



CH2CH2



CH2CH2



CH2CH2



CH2CH2



CH2CH2



CH2



C



SCoA



C18 acyl CoA

acetyl CoA

C16 acyl CoA



All bonds in red are cleaved, one at

a time, beginning near the carbonyl

end of the acyl CoA.



acetyl CoA

C14 acyl CoA

5 cycles



5 acetyl CoA



C4 acyl CoA



2 acetyl CoA



Thus, complete β-oxidation of the acyl CoA derived from stearic acid forms:

• 9 CH3COSCoA molecules (from the 18-carbon fatty acid)

• 8 NADH (from eight cycles of 𝛃-oxidation)

• 8 FADH2 (from eight cycles of 𝛃-oxidation)



β-Oxidation of unsaturated fatty acids proceeds in a similar fashion, although an additional step

is required. Ultimately every carbon in the original fatty acid ends up as a carbon atom of

acetyl CoA.



SAMPLE PROBLEM 24.3



ANALYSIS



Consider lauric acid, CH3(CH2)10CO2H. (a) How many molecules of acetyl CoA are formed

from complete β-oxidation? (b) How many cycles of β-oxidation are needed for complete

catabolism?

The number of carbons in the fatty acid determines the number of molecules of acetyl CoA

formed and the number of times β-oxidation occurs.

• The number of molecules of acetyl CoA equals one-half the number of carbons in the

original fatty acid.

• Because the final turn of the cycle forms two molecules of acetyl CoA, the number of

cycles is one fewer than the number of acetyl CoA molecules formed.



SOLUTION



PROBLEM 24.21



Since lauric acid has 12 carbons, it forms six molecules of acetyl CoA from five cycles of

β-oxidation.

For each fatty acid: [1] How many molecules of acetyl CoA are formed from complete

catabolism? [2] How many cycles of β-oxidation are needed for complete oxidation?

a. arachidic acid (C20H40O2)



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b. palmitoleic acid (C16H30O2)



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CARBOHYDRATE, LIPID, AND PROTEIN METABOLISM



24.7C THE ENERGY YIELD FROM FATTY ACID OXIDATION

How much energy—in terms of the number of molecules of ATP formed—results from the

complete catabolism of a fatty acid? To determine this quantity, we must take into account the

ATP cost for the conversion of the fatty acid to the acyl CoA, as well as the ATP production from

the coenzymes (NADH and FADH2) and acetyl CoA formed during β-oxidation. The steps are

shown in the accompanying How To procedure.



HOW TO

EXAMPLE

Step [1]



Determine the Number of Molecules of ATP Formed from a Fatty Acid

How much ATP is formed by the complete catabolism of stearic acid, C18H36O2?

Determine the amount of ATP required to synthesize the acyl CoA from the fatty acid.

• Since the conversion of stearic acid (C17H35COOH) to an acyl CoA (C17H35COSCoA) requires the hydrolysis of

two P O bonds, this is equivalent to the energy released when 2 ATPs are converted to 2 ADPs.

• Thus, the first step in catabolism costs the equivalent of 2 ATPs—that is, –2 ATPs.



Step [2]



Add up the ATP generated from the coenzymes produced during 𝛃-oxidation.

• As we learned in Section 24.7B, each cycle of β-oxidation produces one molecule each of NADH and FADH2. To

cleave eight carbon–carbon bonds in stearic acid requires eight cycles of β-oxidation, so 8 NADH and 8 FADH2

are produced.

8 NADH × 2.5 ATP/NADH = 20 ATP

8 FADH2 × 1.5 ATP/FADH2 = 12 ATP

From reduced coenzymes:



32 ATP



• Thus, 32 ATPs would be produced from oxidative phosphorylation after the reduced coenzymes enter the

electron transport chain.



Step [3]



Determine the amount of ATP that results from each acetyl CoA, and add the results for steps [1]–[3].

• From Section 24.7B, stearic acid generates nine molecules of acetyl CoA, which then enter the citric acid cycle

and go on to produce ATP by the electron transport chain and oxidative phosphorylation. As we learned in

Section 23.6, each acetyl CoA results in 10 ATPs.

9 acetyl CoA ì 10 ATP/acetyl CoA = 90 ATP



Totaling the values obtained in steps [1]–[3]:

(–2) + 32 + 90 = 120 ATP molecules from stearic acid

Answer



PROBLEM 24.22



Calculate the number of molecules of ATP formed by the complete catabolism of palmitic acid,

C16H32O2.



PROBLEM 24.23



Calculate the number of molecules of ATP formed by the complete catabolism of arachidic

acid, C20H40O2.



How does the energy generated in fatty acid catabolism compare to the energy generated from

glucose catabolism? In Section 24.5, we determined that one molecule (or one mole) of glucose

generates 32 molecules (or moles) of ATP, while the How To calculation demonstrates that one

molecule (or one mole) of stearic acid generates 120 molecules (or moles) of ATP.

To compare these values, we need to compare the amount of ATP generated per gram of each

material. To carry out a calculation of this sort we need to use the molar masses of glucose

(180 g/mol) and stearic acid (284 g/mol).



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KETONE BODIES



765



molar mass

conversion factor



For glucose:



A grizzly bear uses stored body fat

as its sole energy source during

its many months of hibernation.

β-Oxidation of fatty acids releases

sufficient energy to maintain a

constant body temperature between

32 and 35 oC and operate all necessary cellular processes.



For stearic acid:



32 ATP



1 mol



×



mol glucose



120 ATP

mol stearic acid



180 g



1 mol



×



284 g



=



=



0.18 mol ATP

g glucose



0.42 mol ATP

g stearic acid



over twice as much

energy per gram



This calculation demonstrates that a fatty acid produces over twice as much energy per gram

(in terms of moles of ATP generated) than glucose. This is why lipids are so much more effective as energy-storing molecules than carbohydrates.



PROBLEM 24.24



Use the number of molecules of ATP formed from the complete catabolism of palmitic acid,

C16H32O2, in Problem 24.22 to calculate the molecules (or moles) of ATP formed per gram of

palmitic acid (molar mass 256 g/mol).



PROBLEM 24.25



Another way to compare the energy content of molecules is to compare the number of ATPs

they generate per carbon atom they contain. (a) How many ATPs are formed per carbon when

glucose is completely catabolized? (b) How many ATPs are formed per carbon when stearic

acid is completely catabolized? (c) Do these data support or refute the fact that lipids are more

effective energy-storing molecules than carbohydrates?



24.8 KETONE BODIES

When carbohydrates do not meet energy needs, the body turns to catabolizing stored triacylglycerols, which generate acetyl CoA by β-oxidation of fatty acids. Normally the acetyl CoA is

metabolized in the citric acid cycle. When acetyl CoA levels exceed the capacity of the citric acid

cycle, however, acetyl CoA is converted to three compounds that are collectively called ketone

bodies—acetoacetate, β-hydroxybutyrate, and acetone.

O



O

2 CH3



C



SCoA



several steps



CH3



NADH + H+



O



C CH2 C

acetoacetate



NAD+



O−



OH

CH3



C



O

CH2



C



O−



H

β-hydroxybutyrate



CO2

O

CH3 C CH3

acetone



• Ketogenesis is the synthesis of ketone bodies from acetyl CoA.



Acetoacetate is first formed from two molecules of acetyl CoA by a multistep process. Acetoacetate is either reduced to β-hydroxybutyrate with NADH or undergoes decarboxylation to

form acetone. Ketone bodies are produced in the liver, and since they are small molecules that

can hydrogen bond with water, they are readily soluble in blood and urine. Once they reach

tissues, β-hydroxybutyrate and acetoacetate can be re-converted to acetyl CoA and metabolized for energy.



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CARBOHYDRATE, LIPID, AND PROTEIN METABOLISM



PROBLEM 24.26



Are any structural features common to the three ketone bodies?



PROBLEM 24.27



Why is the term “ketone body” a misleading name for β-hydroxybutyrate?



Under some circumstances—notably starvation, vigorous dieting, and uncontrolled diabetes—

when glucose is unavailable or cannot pass into a cell for use as fuel, ketone bodies accumulate, a

condition called ketosis. As a result, ketone bodies are eliminated in urine and the sweet odor of

acetone can be detected in exhaled breath. Sometimes the first indication of diabetes in a patient

is the detection of excess ketone bodies in a urine test.



HEALTH NOTE



An abnormally high concentration of ketone bodies can lead to ketoacidosis—that is, a lowering

of the blood pH caused by the increased level of β-hydroxybutyrate and acetoacetate. Although

the carbonic acid/bicarbonate buffer in the blood prevents drastic pH changes (Section 9.11),

even a small drop in pH can alter many critical biochemical processes.



Ketostix are a brand of test strips

that detect ketone bodies in urine.



Low carbohydrate diets, popularized by Dr. Robert Atkins in a series of diet books published

in the 1990s, restrict carbohydrate intake to induce the utilization of the body’s stored fat as its

main energy source. This increased level of fatty acid metabolism leads to an increased concentration of ketone bodies in the blood. Increased levels of ketone bodies in the urine—which can

be detected using ketone test strips—can be used as an indicator that the body has switched to

metabolic machinery that relies on fat rather than carbohydrates as its principal energy source.

Some physicians recommend that the level of ketone bodies be monitored to avoid the risk of

ketoacidosis.



PROBLEM 24.28



What type of enzymes catalyzes the conversion of acetoacetate to acetone?



PROBLEM 24.29



How is the concentration of acetyl CoA related to the production of ketone bodies?



24.9 AMINO ACID METABOLISM

HEALTH NOTE



After proteins are hydrolyzed in the stomach and intestines, the individual amino acids are reassembled to new proteins or converted to intermediates in other metabolic pathways. The catabolism of amino acids provides energy when the supply of carbohydrates and lipids is exhausted.

The catabolism of amino acids can be conceptually divided into two parts: the fate of the

amino group and the fate of the carbon atoms. As shown in Figure 24.9, amino acid carbon

skeletons are converted to pyruvate, acetyl CoA, or various carbon compounds that are part of

the citric acid cycle.



24.9A

Low carbohydrate diets such as the

Atkins plan induce the use of stored

fat for energy production to assist

weight loss.



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DEGRADATION OF AMINO ACIDS—THE FATE OF THE

AMINO GROUP



The catabolism of carbohydrates and triacylglycerols deals with the oxidation of carbon atoms

only. With amino acids, an amino group (–NH2) must be metabolized, as well. The catabolism

of amino acids begins with the removal of the amino group from the carbon skeleton and the formation of NH4+ by a two-step pathway: transamination followed by oxidative deamination.



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AMINO ACID METABOLISM



767







FIGURE 24.9 An Overview of the Catabolism of Amino Acids

Amino acids

Amino acid

catabolism

NH4+

Carbon skeleton



Urea

cycle



Urea



Pyruvate

Acetyl CoA



Citric acid

cycle



The breakdown of amino acids forms NH4+, which enters the urea cycle to form urea, and a

carbon skeleton that is metabolized to either pyruvate, acetyl CoA, or an intermediate in the

citric acid cycle.



• Transamination is the transfer of an amino group from an amino acid to an 𝛂-keto acid,

usually 𝛂-ketoglutarate.

The C–H and C–NH3+ groups are replaced by a C –

– O.

+



NH3

R



C



O

CO2−



+



H

amino acid



+



NH3



O



R' C CO2−

α-keto acid



R C CO2−

α-keto acid



transaminase



+



R'



C



CO2−



H

amino acid



The C –

– O is replaced by C–H and C–NH3+ groups.



In transamination, the amino group of the amino acid and the ketone carbonyl oxygen of

the 𝛂-keto acid are interchanged to form a new amino acid and a new 𝛂-keto acid. As an

example, transfer of an amino group from alanine to α-ketoglutarate forms pyruvate and glutamate, the completely ionized form of the amino acid glutamic acid.

+



NH3

CH3



C



O

CO2−



H

alanine



+



C CO2−

α-ketoglutarate



−O CCH CH

2

2

2



+



NH3



O

transaminase



CH3 C CO2−

pyruvate



+



The amino group is removed

from the amino acid.



−O



2CCH2CH2



C



CO2−



H

glutamate

(ionic form of glutamic acid)



Transamination removes the amino group to form a carbon skeleton that contains only

carbon, hydrogen, and oxygen atoms. These products are then degraded along other catabolic

pathways as described in Section 24.9B.



smi26573_ch24.indd 767



12/19/08 2:35:48 PM



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