In the well-fed state, galactose can enter glycolysis or contribute to glycogen storage
Administration of galactose during hypoglycemia induces an increase in blood glucose
Glycolysis Glucose
Figure I-12-5.Galactose Metabolism
An important source of galactose in the diet is the disaccharide lactose present in milk. Lactose is hydrolyzed to galactose and glucose by lactase associated with the brush border membrane of the small intestine. Along with other monosaccharides, galactose reaches the liver through the portal blood.
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Once transported into tissues, galactose is phosphorylated (galactokinase), trapping it in the cell. Galactose 1-phosphate is converted to glucose 1-phos- phate by galactose 1-P uridyltransferase and an epimerase.
Important enzymes to remember are galactokinase and galactose 1-phosphate uridyltransferase.
•Genetic deficiencies of these enzymes produce galactosemia. Cataracts, a characteristic finding in patients with galactosemia, result from conversion of the excess galactose in peripheral blood to galactitol in the lens of the eye, which has aldose reductase.
•Accumulation of galactitol in the lens causes osmotic damage and cataracts.
•The same mechanism accounts for the cataracts in diabetics because aldose reductase also converts glucose to sorbitol, which causes osmotic damage.
•Deficiency of galactose 1-phosphate uridyltransferase produces a more severe disease because, in addition to galactosemia, galactose 1-P accumulates in the liver, brain, and other tissues.
Galactosemia
Galactosemia is an autosomal recessive trait resulting from a defective gene encoding galactokinase or galactose 1-P uridyltransferase. There are over 100 heritable mutations that can cause it (incidence 1/60,000 births). Galactose will be present in elevated amounts in the blood and urine and can result in decreased glucose synthesis and hypoglycemia.
The parents of a 2-week-old infant being breastfed returned to the hospital because the infant frequently vomited, had a persistent fever, and looked yellow since birth. The physician quickly observed that the infant had early hepatomegaly and cataracts. Blood and urine tests were identified elevated sugar (galactose and, to a smaller extent, galactitol) in the blood and urine. The doctor told the parents to bottle-feed the infant with lactose-free formula supplemented with sucrose. Subsequently, the infant improved.
Galactosemia symptoms often begin around day 3 in a newborn and include the hallmark cataracts. Jaundice and hyperbilirubinemia do not resolve if the infant is treated with phototherapy. In the galactosemic infant, the liver, which is the site of bilirubin conjugation, develops cirrhosis. Vomiting and diarrhea occur after milk ingestion because although lactose in milk is hydrolyzed to glucose and galactose by lactase in the intestine, the galactose is not properly metabolized. Severe bacterial infections (E. coli sepsis) are common in untreated galactosemic infants. Failure to thrive, lethargy, hypotonia, and mental retardation are other common and apparent features. Many U.S. states have mandatory screening of newborns for galactosemia. If an infant is correctly diagnosed within the first several weeks of life through a newborn screening heel prick test, formulas containing galactose-free carbohydrates are given. The life expectancy will then be normal with an appropriate diet.
Clinical Correlate
Lactose Intolerance
Primary lactose intolerance is caused by a hereditary deficiency of lactase, most commonly found in persons of Asian and African descent. Secondary lactose intolerance can be precipitated at any age by gastrointestinal disturbances such as celiac sprue, colitis, or viral-induced damage to intestinal mucosa, which is why kids with diarrhea should drink clear liquids, and not milk.
Common symptoms of lactose intolerance include vomiting, bloating, explosive and watery diarrhea, cramps, and dehydration. The symptoms can be attributed to bacterial fermentation of lactose to a mixture of CH4, H2, and small organic acids. The acids are osmotically active and result in the movement of water into the intestinal lumen.
Diagnosis is based on a positive hydrogen breath test after an oral lactose load. Treatment is by dietary restriction of milk and milk products (except unpasteurized yogurt, which contains active Lactobacillus) or by lactase pills.
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Part I ● Biochemistry
FRUCTOSE METABOLISM
Intestine
Sucrose
Fruits, honey
Sucrase
Glucose
Fructose
Blood
Other tissues phosphorylate
Fructose
fructose slowly through hexokinase
Liver
Fructokinase
Fructokinase deficiency
Kidney
is benign
Fructose
1-P
Aldolase B (fructose 1-P
aldolase activity) deficiency:
Aldolase B
• Lethargy, vomiting
• Liver damage,
DHAP
Glyceraldehyde
hyperbilirubinemia
• Hypoglycemia
• Hyperuricemia,
lactic acidosis
• Renal proximal tubule
Glycolysis
defect (Fanconi)
Glycogenesis
Glyceraldehyde 3-P
Gluconeogenesis
Note
Because dihydroxyacetone phosphate and glyceraldehyde (the products of fructose metabolism) are downstream from the key regulatory and ratelimiting enzyme of glycolysis (PFK-1), a high-fructose drink supplies a quick source of energy in both aerobic and anaerobic cells.
Figure I-12-6.Fructose Metabolism
Fructose is found in honey and fruit and as part of the disaccharide sucrose (common table sugar). Sucrose is hydrolyzed by intestinal brush border sucrase, and the resulting monosaccharides, glucose and fructose, are absorbed into the portal blood. The liver phosphorylates fructose and cleaves it into glyceraldehyde and DHAP. Smaller amounts are metabolized in renal proximal tubules.
Important enzymes to remember are fructokinase and fructose 1-P aldolase (aldolase B).
•Genetic deficiency of fructokinase is benign and often detected incidentally when urine is checked for glucose with a dipstick.
•Fructose 1-phosphate aldolase deficiency is severe because of accumulation of fructose 1-phosphate in the liver and renal proximal tubules. Symptoms are reversed after removing fructose and sucrose from the diet. Cataracts are not a feature of this disease because fructose is not an aldose sugar and therefore not a substrate for aldose reductase in the lens.
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Chapter 12 ● Glycolysis and Pyruvate Dehydrogenase
Hereditary Fructose Intolerance
Hereditary fructose intolerance is an autosomal recessive disease (incidence 1/20,000) due to a defect in the gene that encodes aldolase B in fructose metabolism. In the absence of the enzyme, fructose challenge results in an accumulation of fructose 1-phosphate in hepatocytes and thereby sequestering of inorganic phosphate in this substance. The drop in phosphate levels prevents its use in other pathways, such as glycogen breakdown and gluconeogenesis. Eventually, the liver becomes damaged due to the accumulation of trapped fructose 1-phosphate.
A 4-month-old infant was breastfed and developing normally. The mother decided to begin the weaning process and started to feed the baby with fruit juices. Within a few weeks, the child became lethargic and yellow-skinned, vomited frequently, and had frequent diarrhea. The mother thought that the child might have had a food allergy and took the child to a clinic for testing. It found that the child had sugar in the urine but did not react with the glucose dipsticks.
If diagnosed early to alleviate complications, a person with fructose intolerance on a diet that excludes fructose and sucrose will develop normally and have a normal lifespan. However, complete exclusion of these sugars is difficult, especially with their widespread use as nutrients and sweeteners. Failure to correct the diet and prolonged fructose ingestion could eventually lead to proximal renal disorder resembling Fanconi syndrome.
PYRUVATE DEHYDROGENASE
Pyruvate from aerobic glycolysis enters mitochondria, where it may be converted to acetyl-CoA for entry into the citric acid cycle if ATP is needed, or for fatty acid synthesis if sufficient ATP is present. The pyruvate dehydrogenase (PDH) reaction is irreversible and cannot be used to convert acetyl-CoA to pyruvate or to glucose. PDH in the liver is activated by insulin, whereas in the brain and nerves the enzyme (actually, a complex of 5 enzymatic activities) is not responsive to hormones.
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Part I ● Biochemistry
Glucose
GLYCOLYSIS
Pyruvate
NAD
CoA
–
NADH
Pyruvate dehydrogenase
–
ATP
NADH
–
+
Calcium
CO2
Acetyl CoA
CITRIC ACID
FATTY ACID
CYCLE
SYNTHESIS
CO2 + H2O
Fatty acids
Figure I-12-7.Pyruvate Dehydrogenase
Cofactors and coenzymes used by pyruvate dehydrogenase include:
•Thiamine pyrophosphate (TPP) from the vitamin thiamine
•Lipoic acid
•Coenzyme A (CoA) from pantothenate
•FAD(H2) from riboflavin
•NAD(H) from niacin (some may be synthesized from tryptophan)
Pyruvate dehydrogenase is inhibited by its product acetyl-CoA. This control is important in several contexts and should be considered along with pyruvate carboxylase, the other mitochondrial enzyme that uses pyruvate (introduced in gluconeogenesis, Chapter 14).
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Thiamine Deficiency
Chapter 12 ● Glycolysis and Pyruvate Dehydrogenase
High-Yield
Thiamine deficiency is commonly seen in alcoholics, since alcohol interferes with thiamine absorption from the intestine. Patients may develop a complex of symptoms associated with Wernicke peripheral neuropathy and Korsakoff psychosis. Symptoms include:
•Ataxia
•Ophthalmoplegia, nystagmus
•Memory loss and confabulation
•Cerebral hemorrhage
Congestive heart failure may be a complication (wet beri-beri) owing to inadequate ATP and accumulation of ketoacids in the cardiac muscle.
Similar to pyruvate dehydrogenase, 2 enzyme complexes use thiamine:
•Branched-chain ketoacid dehydrogenase (metabolism of branchedchain amino acids)
Thiamine deficiency significantly impairs glucose oxidation, causing highly aerobic tissues (e.g., brain and cardiac muscle) to fail first. In addition, branchedchain amino acids are sources of energy in brain and muscle.
Clinical Correlate
If thiamine deficiency is suspected, give IV thiamine prior to glucose (dextrose) administration (to prevent lactic acidosis).
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Part I ● Biochemistry
Review Questions
Select the ONE best answer.
1.A 10-month-old child is being evaluated for the underlying cause of a hemolytic anemia. In the diagram below, the oxygen dissociation curve for hemoglobin in his erythrocytes is compared with the curve obtained with normal red cells.
% Saturation
Normal
100
50 Patient
40 80 120 pO2 (mm Hg)
A deficiency of which enzyme is most likely to account for the hemolytic anemia in this patient?
A.Glucokinase
B.Glucose 6-P dehydrogenase
C.Pyruvate carboxylase
D.Glutathione reductase
E.Pyruvate kinase
2.A breast-fed infant begins to vomit frequently and lose weight. Several days later she is jaundiced, her liver is enlarged, and cataracts are noticed in her lenses. These symptoms are most likely caused by a deficiency of
A.galactose 1-P uridyltransferase
B.lactase
C.glucose-6-phosphatase
D.galactokinase
E.aldolase B
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Chapter 12 ● Glycolysis and Pyruvate Dehydrogenase
3.Following an early-morning run, a 29-year-old man consumes an allAmerican breakfast consisting of cereal, eggs, bacon, sausage, pancakes with maple syrup, doughnuts, and coffee with cream and sugar. Which of the following proteins will most likely be activated in his liver after breakfast?
A.Cytoplasmic PEP carboxykinase
B.Plasma membrane GLUT-4 transporter
C.Cytoplasmic phosphofructokinase-2
D.Mitochondrial carnitine transporter
E.Cytoplasmic glycogen phosphorylase
Items 4 and 5
A 55-year-old alcoholic was brought to the emergency department by his friends. During their usual nightly gathering at the local bar, he had passed out and they had been unable to revive him. The physician ordered an injection of thiamine followed by overnight parenteral glucose. The next morning the patient was alert and coherent, serum thiamine was normal, and blood glucose was 73 mg/dL (4 mM). The IV line was removed and he was taken home.
4.Which of the following enzymes is thiamine-dependent and essential for glucose oxidation in the brain?
A.Transketolase
B.Transaldolase
C.Succinyl-CoA thiokinase
D.Acetyl-CoA carboxylase
E.Pyruvate dehydrogenase
5.At the time of discharge from the hospital, which of the following proteins would have no significant physiologic activity in this patient?
A.Malate dehydrogenase
B.Glucokinase
C.α-Ketoglutarate dehydrogenase
D.GLUT 1 transporter
E.Phosphofructokinase-1
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Part I ● Biochemistry
Answers
1.Answer: E. A right-shift in the O2 binding curve is indicative of abnormally elevated 2,3-BPG secondary to a defect in red cell anaerobic glycolysis. Only pyruvate kinase participates in this pathway.
2.Answer: A. Cataracts + liver disease in a milk-fed infant = classic galactosemia.
3.Answer: C. Only PFK-2 will be insulin-activated in the postprandial period.
4.Answer: E. Most important TPP-dependent enzymes include pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, and transketolase. Transketolase is in the HMP shunt and is not strictly essential for glucose oxidation.
5.Answer: B. After an overnight fast (plasma glucose 73 mg/dL), the liver is producing glucose and glucokinase activity would be insignificant
(high Km, low insulin). The other proteins would be needed for aerobic glucose oxidation in the brain or for hepatic gluconeogenesis.
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Citric Acid Cycle and
13
Oxidative Phosphorylation
Learning Objectives
Solve problems concerning citric acid cycle
Explain information related to electron transport chain
Understand concepts of oxidative phosphorylation
CITRIC ACID CYCLE
The citric acid cycle (also called Krebs cycle or tricarboxylic acid cycle), is in the mitochondria. Although oxygen is not directly required in the cycle, the pathway will not occur anaerobically because NADH and FADH2 will accumulate if oxygen is not available for the electron transport chain.
The primary function of the cycle is oxidation of acetyl-CoA to carbon dioxide. The energy released from this oxidation is saved as NADH, FADH2, and guanosine triphosphate (GTP). The overall result of the cycle is represented by the following reaction:
Acetyl-CoA
2 CO2
3 NAD + FAD + GDP + Pi
3 NADH + FADH2 + GTP
Notice that none of the intermediates of the citric acid cycle appear in this reaction, not as reactants or as products. This emphasizes an important (and frequently misunderstood) point about the cycle.
•It does not represent a pathway for the net conversion of acetyl-CoA to citrate, to malate, or to any other intermediate of the cycle.
•The only fate of acetyl-CoA in this pathway is its oxidation to CO2.
•Thus, the citric acid cycle does not represent a pathway by which there can be net synthesis of glucose from acetyl-CoA.
The cycle is central to the oxidation of any fuel that yields acetyl-CoA, including glucose, fatty acids, ketone bodies, ketogenic amino acids, and alcohol. There is no hormonal control of the cycle, as activity is necessary irrespective of the fed or fasting state. Control is exerted by the energy status of the cell through allosteric activation or deactivation. Many enzymes are subject to negative feedback.
