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Uridine diphosphate (UDP)-glucose and UDP-galactose are substrates for many glycosyltransferase

reactions.

Lactose is formed from UDP-galactose and glucose.

UDP-glucose is oxidized to UDP-glucuronate, which forms glucuronide derivatives of various

hydrophobic compounds, making them more readily excreted in urine or bile than the parent

compound.

Glycoproteins and glycolipids contain various types of carbohydrate residues.The carbohydrates in glycoproteins can be either O-linked or N-linked and are synthesized in the

endoplasmic reticulum and the Golgi apparatus.

For O-linked carbohydrates, the carbohydrates are added sequentially (via nucleotide sugar

precursors), beginning with a sugar linked to the hydroxyl group of the amino acid side chains of

serine or threonine.

For N-linked carbohydrates, the branched carbohydrate chain is first synthesized on dolichol

phosphate and then transferred to the amide nitrogen of an asparagine residue of the protein.

Glycolipids belong to the class of sphingolipids that add carbohydrate groups to the base ceramide

one at a time from nucleotide sugars.

Defects in the degradation of glycosphingolipids leads to a class of lysosomal diseases known as

the sphingolipidoses.

Table 27.7 summarizes the diseases discussed in this chapter. REVIEW QUESTIONS—CHAPTER 27

1.Which one of the following best describes a mother with galactosemia caused by a deficiency of

galactose 1-P uridylyltransferase?

A. She can convert galactose to UDP-galactose for lactose synthesis during lactation.

B. She can form galactose 1-P from galactose.

C. She can use galactose as a precursor to glucose production. D. She can use galactose to produce glycogen.

E. She will have lower-than-normal levels of serum galactose after drinking milk.

2.The immediate carbohydrate precursors for glycolipid and glycoprotein synthesis are which of the

following?

A. Sugar phosphates B. Sugar acids

C. Sugar alcohols

D. Nucleotide sugarsE. Acyl-sugars

3.A newborn is diagnosed with neonatal jaundice. In this patient, the bilirubin produced lacks which

one of the following carbohydrates? A. Glucose

B. Gluconate C. Glucuronate D. Galactose E. Galactitol

4.The nitrogen donor for the formation of amino sugars is which one of the following?

A. Ammonia B. Asparagine C. Glutamine D. Adenine E. Dolichol

5.Which one of the following glycolipids would accumulate in a patient with Sandhoff disease?

A. GM1

B. Lactosyl-ceramide

C.Globoside

D.Glucocerebroside

E.GM3

6.A defect in which one of the following enzymes would severely affect an individual’s ability to

specifically metabolize galactose? A. UDP-glucose pyrophosphorylase B. Hexokinase

C. Glucose 6-P dehydrogenase D. Triose kinase

E. Pyruvate carboxylase

7.A woman, shortly after giving birth to her first child, was discovered to be unable to synthesize

lactose. Analysis of various glycoproteins in her serum indicated that there was no defect in the

carbohydrate chains, nor was the carbohydrate content of her cell-surface glycolipids altered. This

woman may have a mutation in which one of the following enzymes or class of enzymes? A. A glucosyltransferase

B. A galactosyltransferase C. Lactase

D. α-Lactalbumin

E. A lactosyltransferase

8.A 27-year-old man of Mediterranean descent developed hemolytic anemia after being prescribed a

drug that is a potent oxidizing agent. The anemia results from which one of the following?

A. A lowered concentration of oxidized glutathione B. A lowered concentration of reduced glutathione

C. Increased production of NADPHD. A reduction of hydrogen peroxide levels E. An increase in the production of glucose 6-P

9.A patient’s blood is being typed by exposing it to either type A or type B antibodies. Which result

would be expected if the patient had type AB blood? A. No reaction to A antibodies

B. No reaction to B antibodies

C. No reaction to either A or B antibodies D. Reaction to both A and B antibodies

E. A reaction to A antibodies but not to B antibodies

10.Inherited defects in the pentose phosphate shunt pathway could lead to which one of the following?

A. Ineffective oxidative phosphorylation caused by dysfunctional mitochondria B. An inability to carry out reductive detoxification

C. An inability to produce fructose 6-P and glyceraldehyde 3-P for five-carbon sugar biosynthesis

D. An inability to generate NADH for biosynthetic reactions E. An inability to generate NADH to protect cells from ROS ANSWERS TO REVIEW QUESTIONS

1.The answer is B. Galactose metabolism requires the phosphorylation of galactose to galactose 1-

P,

which is then converted to UDP-galactose (which is the step that is defective in the patient), and

then epimerized to UDP-glucose. Although the mother cannot convert galactose to lactose because

of the enzyme deficiency, she can make UDP-glucose from glucose 6-P, and once she has made

UDP-glucose, she can epimerize it to form UDP-galactose and can synthesize lactose (thus, A is

incorrect). However, because of her enzyme deficiency, the mother cannot convert galactose 1-P

to UDP-galactose or UDP-glucose, so the dietary galactose cannot be used for glycogen synthesis

or glucose production (thus, C and D are incorrect). After ingesting milk, the galactose levels will

be elevated in the serum because of the metabolic block in the cells (thus, E is incorrect).

2.The answer is D. Nucleotide sugars, such as UDP-glucose, UDP-galactose, and CMP-sialic acid,

donate sugars to the growing carbohydrate chain. The other activated sugars listed do not

contribute to glycolipid or glycoprotein synthesis.

3.The answer is C. Bilirubin is conjugated with glucuronic acid residues to enhance its solubility.

Glucuronic acid is glucose oxidized at position 6; gluconic acid is glucose oxidized at position 1

and is generated by the HMP shunt pathway. The activated form of glucuronic acid is UDPglucuronate.

4.The answer is C. Glutamine donates the amide nitrogen to fructose 6-P to form glucosamine 6-P.

None of the other nitrogen-containing compounds (A, B, and D) donate their nitrogen to

carbohydrates. Dolichol contains no nitrogens and is the carrier for carbohydrate chain synthesis

of N-linked glycoproteins.

5.The answer is C. Sandhoff disease is a deficiency of both hexosaminidase A and B activity,

resulting from loss of activity of the β-subunit in both of these enzymes. The degradative step at

which amino sugars need to be removed from the glycolipids would be defective, such thatgloboside and GM2 accumulate in this disease. The other answers are incorrect; GM1 does

contain an amino sugar, but it is converted to GM2 before the block in Sandhoff disease is

apparent.

6.The answer is A. Galactose is phosphorylated by galactokinase and the galactose

1-P formed

reacts with UDP-glucose to form glucose 1-P and UDP-galactose. The enzyme that catalyzes this

reaction is galactose 1-P uridylyltransferase. UDP-glucose is absolutely required for further

galactose metabolism. UDP-glucose is formed from UTP and glucose 1-P by the enzyme UDPglucose pyrophosphorylase, so a deficiency in this enzyme would lead to reduced galactose

metabolism, caused by a lack of UDP-glucose. Galactose is phosphorylated by galactokinase, not

by hexokinase. Glucose 6-P dehydrogenase is not required for galactose metabolism; it converts

glucose 6-P to 6-phosphogluconate in the first step of the HMP shunt pathway. Triose kinase is

also not required for galactose metabolism because this enzyme converts glyceraldehyde to

glyceraldehyde 3-P. Pyruvate carboxylase is also not required for galactose metabolism, because

it converts pyruvate to oxaloacetate.

7. The answer is D. Lactose synthase is composed of two subunits, a galactosyltransferase and α-

lactalbumin. In the absence of α-lactalbumin, the galactosyltransferase is active in glycoprotein

and glycolipid synthesis but is relatively inactive for lactose synthesis. At birth, α-lactalbumin is

induced and alters the specificity of the galactosyltransferase such that lactose can now be

synthesized. Because glycoprotein and glycolipid synthesis was normal, the mutation could not be

in the galactosyltransferase subunit. Lactase is the enzyme that digests lactose in the intestine, and

a lactosyltransferase enzyme, if such an enzyme did exist, would not answer the question, because

it would require lactose as a substrate. A glucosyltransferase is not required for

lactose synthesis (lactose synthase is).

8.The answer is B. The patient has glucose 6-P dehydrogenase deficiency and cannot generate

NADPH from glucose 6-P. In the red blood cells, which lack mitochondria, this is the only

pathway through which NADPH can be generated. In the absence of NADPH, the molecule which

protects against oxidative damage (glutathione) is oxidized preferentially to protect membrane

lipids and proteins. There is only limited glutathione in the membrane, so once it is oxidized, it

needs to be converted back to reduced glutathione (the protective form). The enzyme that does

this, glutathione reductase, requires NADPH to supply the electrons to reduce the oxidized

glutathione. In the absence of NADPH, the glutathione cannot be reduced, and the protection

offered by reduced glutathione is eliminated, leading to membrane damage and cell lysis. Because

the life of a red cell is so short (120 days in circulation), there is usually sufficient glutathione to

protect the cell in the absence of an exogenous oxidizing agent. However, once such an agent is

present (such as a drug), the red cells are easily lysed, leading to the anemia. The hemolytic

anemia is not caused by a lowering of hydrogen peroxide levels nor by an increase in the

production of glucose 6-P.

9.The answer is D. A person with type AB blood would carry both the A and B antigens, so the

blood cells would bind to antibodies directed against either the type A antigen or the type B

antigen. Type O blood, which does not express either the type A or type B antigens, would not

react with either antibody. Type A blood would react to antibodies to antigen A but not to antigenB. Type B blood would react to antibodies to antigen B but not to antibodies directed against

antigen A.

10.The answer is B. The pentose phosphate pathway is operative in the cytosol of all cells because

all cells require NADPH for reductive detoxification. NADPH is also used for fatty-acid

synthesis and detoxification of medications. NADPH does not donate electrons to complex I of the

electron-transfer chain, so oxidative phosphorylation would not be impaired. Fructose 6-P and

glyceraldehyde 3-P can be generated via glycolysis; the pentose phosphate pathway is not

required for their synthesis. NADPH is used for both biosynthetic reactions and to protect cells

from ROS, not NADH (which is generated via glycolysis).28 Gluconeogenesis and Maintenance

of Blood Glucose Levels

For additional ancillary materials related to this chapter, please visit thePoint. During fasting, many of the reactions of glycolysis are reversed as the liver produces glucose to maintain

blood glucose levels. This process of glucose production is called gluconeogenesis. Gluconeogenesis, which occurs primarily in the liver, is the pathway for the synthesis of glucose from

compounds other than carbohydrates. In humans, the major precursors of glucose are lactate, glycerol,

and amino acids, particularly alanine. Except for three key sequences, the reactions of gluconeogenesis

are reversals of the steps of glycolysis (Fig. 28.1). The sequences of

gluconeogenesis that do not use

enzymes of glycolysis involve the irreversible, regulated steps of glycolysis. These three sequences are

the conversion of (1) pyruvate to phosphoenolpyruvate (PEP), (2) fructose 1,6-bisphosphate to fructose 6-

phosphate, and (3) glucose 6-phosphate to glucose.Some tissues of the body, such as the brain and red blood cells, cannot synthesize glucose on their

own but depend on glucose for energy. On a long-term basis, most tissues also require glucose for other

functions, such as synthesis of the ribose moiety of nucleotides or the carbohydrate portion of

glycoproteins and glycolipids. Therefore, to survive, humans must have mechanisms for maintaining

blood glucose levels.

After a meal containing carbohydrates, blood glucose levels rise (Fig. 28.2). Some of the glucose

from the diet is stored in the liver as glycogen. After 2 or 3 hours of fasting, this glycogen begins to be

degraded by the process of glycogenolysis, and glucose is released into the blood. As glycogen stores

decrease, adipose triacylglycerols are also degraded, providing fatty acids as an alternative fuel andglycerol for the synthesis of glucose by gluconeogenesis. Amino acids are also released from the muscle

to serve as gluconeogenic precursors.

During an overnight fast, blood glucose levels are maintained by both glycogenolysis and

gluconeogenesis. However, after approximately 30 hours of fasting, liver glycogen stores are mostly

depleted. Subsequently, gluconeogenesis is the only source of blood glucose. Changes in the metabolism of glucose that occur during the switch from the fed to the fasting state are

regulated by the hormones insulin and glucagon. Insulin is elevated in the fed state, and glucagon is

elevated during fasting. Insulin stimulates the transport of glucose into certain cells such as those in

muscle and adipose tissue. Insulin also alters the activity of key enzymes that regulate metabolism,

stimulating the storage of fuels. Glucagon counters the effects of insulin, stimulating the release of stored

fuels and the conversion of lactate, amino acids, and glycerol to glucose. Blood glucose levels are maintained not only during fasting but also during exercise, when muscle

cells take up glucose from the blood and oxidize it for energy. During exercise, the liver supplies glucose

to the blood by the processes of glycogenolysis and gluconeogenesis. THE WAITING ROOM

Al M., a known alcoholic, was brought to the emergency department by his landlady, who stated

that he had been drinking heavily for the past week. During this time, his appetite had gradually

diminished, and he had not eaten any food for the past 3 days. He was confused, combative, tremulous,

and sweating profusely. His speech was slurred. His heart rate was rapid (110 beats/minute). As his

blood pressure was being determined, he had a grand mal seizure. His blood glucose, drawn just before

the onset of the seizure, was 28 mg/dL or 1.6 mM (reference range for overnight fasting blood glucose, 80

to 100 mg/dL or 4.4 to 5.6 mM). His blood ethanol level drawn at the same time was 295 mg/dL

(intoxication level, i.e., “confused” stage, 150 to 300 mg/dL).

Emma W. presented to the emergency department 3 days after being discharged from the hospitalfollowing a 7-day admission for a severe asthma exacerbation. She was intubated and required high-dose

intravenous methylprednisolone (a synthetic anti-inflammatory glucocorticoid) for

the first 4 days of her

stay. After 3 additional days on oral prednisone, she was discharged on substantial pharmacologic doses

of this steroid and instructed to return to her physician’s office in 5 days. She presented now with marked

polyuria (increased urination), polydipsia (increased thirst), and muscle weakness. Her blood glucose

was 275 mg/dLor 15 mM (reference range, 80 to 100 mg/dLor 4.4 to 5.6 mM).

Dianne A. took her morning insulin but then didn’t feel well, so she did not eat and took a nap.

When her friend came over later, she was unresponsive. The friend called an ambulance, and

Dianne was rushed to the hospital emergency department in a coma. Her pulse and blood pressure at

admission were normal. Her skin was flushed and slightly moist. Her respirations were slightly slow.

Ann R. continues to resist efforts on the part of her psychiatrist and family physician to convince

her to increase her caloric intake. Her body weight varies between 97 and 99 lb, far below the

desirable weight for a woman who is 5 ft 7 in tall. In spite of her severe diet, her fasting blood glucose

levels range from 55 to 70 mg/dL. She denies having any hypoglycemic symptoms. Otto S. has complied with his calorie-restricted diet and aerobic exercise program. He has lost

another 7 lb and is closing in on his goal of weighing 154 lb. He notes increasing energy during the

day, and he remains alert during lectures and assimilates the lecture material noticeably better than he did

before starting his weight-loss and exercise program. He jogs for 45 minutes each morning before

breakfast.

I. Glucose Metabolism in the Liver

Glucose serves as a fuel for most tissues of the body. It is the major fuel for certain tissues such as the

brain and red blood cells. After a meal, food is the source of blood glucose. The liver oxidizes glucose

and stores the excess as glycogen. The liver also uses the pathway of glycolysis to convert glucose to

pyruvate, which provides carbon for the synthesis of fatty acids. Glycerol 3-phosphate, produced from

glycolytic intermediates, combines with fatty acids to form triacylglycerols, which are secreted into the

blood in very-low-density lipoprotein (VLDL; explained further in Chapter 31). During fasting, the liver

releases glucose into the blood, so that glucose-dependent tissues do not suffer from a lack of energy. Two

mechanisms are involved in this process: glycogenolysis and gluconeogenesis. Hormones, particularly

insulin and glucagon, dictate whether glucose flows through glycolysis or whether the reactions are

reversed and glucose is produced via gluconeogenesis.

The measurement of ketones (acetoacetate and β-hydroxybutyrate; see Chapter 3) in the

blood and urine can indicate the level of starvation or the presence of diabetic ketoacidosis

(DKA). There are several methods to detect ketones. One is the use of reagent strips for urine

(based on the reaction of sodium nitroprusside with acetoacetate), but this method does not detect

the major blood ketone (β-hydroxybutyrate). A cyclic enzymatic method has been developed to

overcome this, in which blood or plasma samples are incubated with acetoacetate decarboxylase,which removes all acetoacetate from the sample (converting it to acetone and carbon dioxide).

Once this has been accomplished, β-hydroxybutyrate dehydrogenase is then incubated with the

sample, along with thionicotinamide adenine dinucleotide (thio-NAD+). The thio-NAD+ is

converted to thio-NADH, generating a colored product and acetoacetate. Thio-NADH absorbs

light at 405 nm, in the visible range, as compared to NADH, which absorbs at 340 nm, in the

ultraviolet range. The use of thio-NAD+ allows clinical laboratory instrumentation to be used. The

acetoacetate is then recycled back to β-hydroxybutyrate, in which NADH is converted to NAD+.

The β-hydroxybutyrate produced is then cycled back to acetoacetate, generating more thio-NADH.

The cycling enhances the sensitivity of the assay. Once equilibrium is reached, one can calculate,

from the change in absorbance per minute, the concentration of the β-hydroxybutyrate in the

sample.

II. Gluconeogenesis

Gluconeogenesis, the process by which glucose is synthesized from noncarbohydrate precursors, occurs

mainly in the liver under fasting conditions. Under the more extreme conditions of starvation, the kidney

cortex also may produce glucose. For the most part, the glucose produced by the kidney cortex is used by

the kidney medulla, but some may enter the bloodstream.

Starting with pyruvate, most of the steps of gluconeogenesis are simply reversals of those of

glycolysis (Fig. 28.3). In fact, these pathways differ at only three points. Enzymes involved in catalyzing

these steps are regulated so that either glycolysis or gluconeogenesis predominates, depending on

physiologic conditions.Most of the steps of gluconeogenesis use the same enzymes that catalyze the process of glycolysis. The

flow of carbon, of course, is in the reverse direction. Three reaction sequences of gluconeogenesis differ

from the corresponding steps of glycolysis. They involve the conversion of pyruvate to PEP and the

reactions that remove phosphate from fructose 1,6-bisphosphate (fructose 1,6-bisP) to form fructose 6-

phosphate (fructose 6-P) and from glucose 6-phosphate (glucose 6-P) to form glucose (see Fig. 28.3). The

conversion of pyruvate to PEP is catalyzed during gluconeogenesis by a series of enzymes instead of the

single enzyme used for glycolysis. The reactions that remove phosphate from fructose 1,6-bisP and from

glucose 6-P each use single enzymes that differ from the corresponding enzymes of glycolysis. Although

phosphate is added during glycolysis by kinases, which use adenosine triphosphate (ATP), it is removed

during gluconeogenesis by phosphatases that release inorganic phosphate (Pi) via hydrolysis reactions.

Comatose patients in DKA have the smell of acetone (a derivative of the ketone body acetoacetate) on their breath. In addition, DKA patients have deep, relatively rapid respirations typical of acidotic patients (Kussmaul respirations). These respirations result from anacidosis-induced stimulation of the respiratory center in the brain. More CO2 is exhaled in an

attempt to reduce the amount of acid in the body: H+ + HCO3 → H2CO3 → H2O + CO2 (exhaled).

These signs are not observed in a hypoglycemic coma.

The severe hyperglycemia of DKA also causes osmotic diuresis (i.e., glucose entering the

urine carries water with it), which, in turn, causes a contraction of blood volume. Volume

depletion may be aggravated by vomiting, which is common in patients with DKA. DKA may

cause dehydration (dry skin), low blood pressure, and a rapid heartbeat. These respiratory and

hemodynamic alterations are not seen in patients with hypoglycemic coma. The flushed, wet skin

of hypoglycemic coma is in contrast to the dry skin observed in DKA. A. Precursors for Gluconeogenesis

The three major carbon sources for gluconeogenesis in humans are lactate, glycerol, and amino acids,

particularly alanine. Lactate is produced by anaerobic glycolysis in tissues such as exercising muscle or

red blood cells as well as by adipocytes during the fed state. Glycerol is released from adipose stores of

triacylglycerol, and amino acids come mainly from amino acid pools in muscle, where they may be

obtained by degradation of muscle protein. Alanine, the major gluconeogenic amino acid, is produced in

the muscle from other amino acids and from glucose (see Chapter 37). Because ethanol metabolism only

gives rise to acetyl coenzyme A (acetyl-CoA), the carbons of ethanol cannot be used for gluconeogenesis.

Diabetes mellitus (DM) should be suspected if a venous plasma glucose level drawn regardless of when food was last eaten (a “random” sample of blood glucose) is “unequivocally elevated” (i.e., >200 mg/dL), particularly in a patient who manifests the classic

signs and symptoms of chronic hyperglycemia (polydipsia, polyuria, blurred vision, headaches,

rapid weight loss, sometimes accompanied by nausea and vomiting). To confirm the diagnosis, the

patient should fast overnight (10 to 16 hours), and the blood glucose measurement should be

repeated. Values of <100 mg/dL are considered normal. Values ≥126 mg/dL are indicative of DM.

The level of glycosylated hemoglobin (HbA1c) also can be measured to make the diagnosis, and if

>6.5%, it is diagnostic for DM. The levels of HbA1c can also gauge the extent of hyperglycemia

over the past 4 to 8 weeks and is used to guide treatment. Values of fasting blood glucose between

100 and 125 mg/dL are designated as impaired fasting glucose (IFG) (or prediabetes), and further

testing should be performed to determine whether these individuals will eventually develop overt

DM. Individuals with blood glucose levels in this range have been defined as having “prediabetes.” The determination that fasting blood glucose levels of 126 mg/dL, or a percentage

of glycosylated hemoglobin of >6.5%, is diagnostic for DM is based on data indicating that at

these levels of glucose or glycosylated hemoglobin, patients begin to develop complications of

DM, specifically retinopathy.

The renal tubular transport maximum in the average healthy subject is such that glucose will

not appear in the urine until the blood glucose level is >180 mg/dL. As a result, reagent tapes

(Tes-Tape or Dextrostix) designed to detect the presence of glucose in the urine are not sensitive

enough to establish a diagnosis of early DM.B. Formation of Gluconeogenic Intermediates from Carbon Sources

The carbon sources for gluconeogenesis form pyruvate, intermediates of the tricarboxylic acid (TCA)

cycle, or intermediates that are common to both glycolysis and gluconeogenesis. 1. Lactate, Amino Acids, and Glycerol

Pyruvate is produced in the liver from the gluconeogenic precursors lactate and

alanine. Lactate

dehydrogenase oxidizes lactate to pyruvate, generating reduced nicotinamide adenine dinucleotide

(NADH) (Fig. 28.4A), and alanine aminotransferase converts alanine to pyruvate (see Fig. 28.4B).

Glucocorticoids are naturally occurring steroid hormones. In humans, the major glucocorticoid is cortisol. Glucocorticoids are produced in the adrenal cortex in response

to various types of stress (see Chapter 41). One of their actions is to stimulate the degradation of

muscle protein. Thus, increased amounts of amino acids become available as substrates for

gluconeogenesis. Emma W. noted muscle weakness, a result of the muscle-degrading action of the

synthetic glucocorticoid prednisone, which she was taking for its anti-inflammatory effects.Although alanine is the major gluconeogenic amino acid, other amino acids, such as serine, serve as

carbon sources for the synthesis of glucose because they also form pyruvate, the substrate for the initial

step in the process. Some amino acids form intermediates of the TCA cycle (see Chapter 23), which can

enter the gluconeogenic pathway.

The carbons of glycerol are gluconeogenic because they form dihydroxyacetone phosphate (DHAP),

a glycolytic intermediate (see Fig. 28.4C). 2. Propionate

Fatty acids with an odd number of carbon atoms, which are obtained mainly from vegetables in the diet,

produce propionyl coenzyme A (propionyl-CoA) from the three carbons at the ω-end of the chain (see

Chapter 30). These carbons are relatively minor precursors of glucose in humans. Propionyl-CoA is

converted to methylmalonyl coenzyme A (methylmalonyl-CoA), which is rearranged to form succinyl

coenzyme A (succinyl-CoA), a four-carbon intermediate of the TCA cycle that can be used for

gluconeogenesis. The remaining carbons of an odd-chain fatty acid form acetyl-CoA, from which no net

synthesis of glucose occurs. In some species, propionate is a major source of carbon for gluconeogenesis.

Ruminants can produce massive amounts of glucose from propionate. In cows, the cellulose in grass is

converted to propionate by bacteria in the rumen. This substrate is then used to generate more than 5 lb of

glucose each day by the process of gluconeogenesis.

β-Oxidation of fatty acids produces acetyl-CoA (see Chapter 30). Because the pyruvate

dehydrogenase reaction is thermodynamically and kinetically irreversible, acetyl-CoA does not form

pyruvate for gluconeogenesis. Therefore, if acetyl-CoA is to produce glucose, it must enter the TCA cycle

and be converted to malate. For every two carbons of acetyl-CoA that are converted to malate, two

carbons are released as CO2: one in the reaction catalyzed by isocitrate dehydrogenase and the other in

the reaction catalyzed by α-ketoglutarate dehydrogenase. Therefore, there is no net synthesis of glucose

from acetyl-CoA.

In a fatty acid with 19 carbons, how many carbons (and which ones) have the capability to

form glucose?

Only the three carbons at the ω-end of an odd-chain fatty acid that form propionyl-CoA are

converted to glucose. The remaining 16 carbons of a fatty acid with 19 carbons form acetylCoA, which does not form any net glucose.

C. Pathway of Gluconeogenesis

Gluconeogenesis occurs by a pathway that reverses many, but not all, of the steps of glycolysis.

1. Conversion of Pyruvate to Phosphoenolpyruvate

In glycolysis, PEP is converted to pyruvate by pyruvate kinase. In gluconeogenesis, a series of steps is

required to accomplish the reversal of this reaction (Fig. 28.5). Pyruvate is carboxylated by pyruvatecarboxylase to form oxaloacetate (OAA). This enzyme, which requires biotin, is the catalyst of an

anaplerotic (refilling) reaction of the TCA cycle (see Chapter 23). In gluconeogenesis, this reaction

replenishes the OAA that is used for the synthesis of glucose (Fig. 28.6). Excessive ethanol metabolism blocks the production of gluconeogenic precursors. Cells

have limited amounts of NAD, which exists either as NAD+ or as NADH. As the levels of

NADH rise, those of NAD+ fall, and the ratio of the concentrations of NADH and NAD+ ([NADH]/[NAD+]) increases. In the presence of ethanol, which is very rapidly oxidized in theliver, the [NADH]/[NAD+] ratio is much higher than it is in the normal fasting liver (see Chapter

33). High levels of NADH drive the lactate dehydrogenase reaction toward lactate. Therefore,

lactate cannot enter the gluconeogenic pathway, and pyruvate that is generated from alanine is

converted to lactate. Because glycerol is oxidized by NAD+ during its conversion to DHAP, the

conversion of glycerol to glucose is also inhibited when NADH levels are elevated. Consequently, the major precursors lactate, alanine, and glycerol are not used for gluconeogenesis

under conditions in which alcohol metabolism is high.

The CO2 that was added to pyruvate to form OAA is released in the reaction catalyzed by

phosphoenolpyruvate carboxykinase (PEPCK), which generates PEP (Fig. 28.7A). For this reaction,

guanosine triphosphate (GTP) provides a source of energy as well as the phosphate group of PEP.

Pyruvate carboxylase is found in mitochondria. In various species, PEPCK is located either in the cytosol

or in mitochondria, or it is distributed between these two compartments. In humans, the enzyme is

distributed about equally in each compartment.OAA, generated from pyruvate by pyruvate carboxylase or from amino acids that form intermediates

of the TCA cycle, does not readily cross the mitochondrial membrane. It is either decarboxylated to form

PEP by the mitochondrial PEPCK or it is converted to malate or aspartate (see Fig. 28.7B and C). The

conversion of OAA to malate requires NADH. PEP, malate, and aspartate can be transported into the

cytosol.

After malate or aspartate traverses the mitochondrial membrane (acting as a carrier of OAA) and

enters the cytosol, it is reconverted to OAA by reversal of the reactions given previously (see Fig. 28.7B

and C). The conversion of malate to OAA generates NADH. Whether OAA is transported across the

mitochondrial membrane as malate or aspartate depends on the need for reducing equivalents in the

cytosol. NADH is required to reduce 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate

(glyceraldehyde 3-P) during gluconeogenesis.

OAA, produced from malate or aspartate in the cytosol, is converted to PEP by the cytosolic PEPCK

(see Fig. 28.7A).

2. Conversion of Phosphoenolpyruvate to Fructose 1,6-Bisphosphate

The remaining steps of gluconeogenesis occur in the cytosol (Fig. 28.8). Starting