CHAPTER 29: URONIC ACID PATHWAY

Chapter 29: Uronic Acid Pathway
The uronic acid pathway is another alternate cytoplasmic route for the metabolism of glucose 6-phosphate (Glc-6-P), which can convert it to uridine diphosphate glucuronate (UDP-glucuronate) for use in the biosynthesis of vitamin C and glycoproteins, or hepatic detoxification of endogenous and exogenous compounds. Like the hexose monophosphate shunt (HMS), the uronic acid pathway does not produce ATP via substrate-level phosphorylation. However, it does generate reducing equivalents in the form of NADH, which can subsequently give rise to ATP through mitochondrial oxidative phosphorylation.
Glucose 6-phosphate is converted to glucose 1-phosphate (Glc-1-P) in the first reaction of the uronic acid pathway, which then reacts with uridine triphosphate (UTP) to form the active nucleotide, UDP-glucose. These initial reactions, which are also involved in glycogen formation. Next, UDP-glucose is oxidized by an NAD+ -dependent dehydrogenase to yield UDP-glucuronate, using two moles of NAD+. (Note: At pH 7.4, most acids are dissociated; hence, the use of the name for the anionic (UDP-glucuronate) rather than the acid form (UDP-glucuronic acid) of this compound is more appropriate). The liver and, to a lesser extent, the kidneys use UDP-glucuronate for conjugation reactions with non-polar, lipophilic endogenous compounds (e.g., steroid hormones and bilirubin, a natural break-down product of heme), and with some lipophilic drugs and toxic substances entering from the diet. These two organs may then excrete these more polar, water-soluble conjugates into bile and/or urine. Since glucuronide conjugates are also more soluble in the watery medium of blood plasma, they are less bound to plasma proteins, and therefore are more likely to be filtered by the kidneys and excreted into urine following release from liver cells into blood.
The liver contains several isoforms of UDP glucuronosyltransferase (UGT), the enzyme used in these conjugation reactions. Some isoforms appear to be selective for endogenous compounds (e.g., bilirubin), while others target exogenous compounds. Hydrophobic compounds (like bilirubin, steroid hormones, xenobiotics, phenobarbital, zidovudine (AZT), and other drugs) typically have one or more hydroxyl groups that become linked to glucuronate through O-glycosidic bonds. Drug exposure is known to induce synthesis of the glucuronosyltransferase isoform specific for that drug, thereby, over time, enhancing excretion and promoting tolerance.
Cats appear to have a less diverse pattern of UGT isoform expression for exogenous compounds than do other species. This helps to explain why cats have difficulty conjugating, and therefore excreting various drugs from the body (e.g., aspirin, acetaminophen, diazepam, and morphine) that are typically metabolized through the hepatic glucuronide conjugation mechanism. Such differences may indeed reflect the evolutionary influence of the carnivorous diet of Feliform species, and resultant minimal exposure to plant-derived toxins which may otherwise keep these glucuronide conjugating isoforms functional.
Carbohydrate-protein complexes (i.e., glycosaminoglycans, mucopolysaccharides, and glycoproteins such as chondroitin sulfate, dermatan sulfate, and hyaluronic acid) can also be derived from UDP-glucuronate, and therefore become important in various body structures. Cartilage, bone, skin, umbilical cord, heart valves, arterial walls, cornea, and tendons are representative examples. Hyaluronic acid also occurs in synovial fluid and the vitreous humor of the eye; it serves to lubricate joints and to hold water in interstitial spaces.
In heparin, as in dermatan sulfate, the UDP glucuronate is epimerized to iduronate, and sulfate is added. This compound contains repeating disaccharide units of glucosamine and iduronate. It is produced and stored in mast cells, and it is also found in the liver, lung, and skin. Heparin is known for its anticoagulant and lipid-clearing properties. Heparan sulfate (not to be confused with heparin), is present on many cell surfaces, particularly endothelial cells in the walls of blood vessels (where it is produced). It contains repeating disaccharide units of glucosamine and glucuronate. Heparan sulfate is a negatively charged proteoglycan that participates in cell growth and cell-cell communication. It is also found in the basement membrane of the glomerulus, where it plays a major role in determining the charge selectiveness of the renal glomerular barrier. When this barrier is compromised, plasma proteins begin to appear in urine.
Since further reactions of UDP-glucuronate involve the sugar rather than its UDP form, the nucleotide must be removed. Next, glucuronate can be reduced by NADPH to yield L-gulonate. In most animals L-gulonate can be converted to L-ascorbate (vitamin C). However, in primates, fish, flying mammals (Chiroptera), songbirds (Passeriformes), and guinea pigs, this conversion does not occur because of the absence of the enzyme L-gulonolactone oxidase. Therefore, these organisms require vitamin C in their diets. Those species that synthesize ascorbate may also benefit from dietary supplementation at critical times (e.g., during rapid growth or stress).
A prolonged vitamin C deficiency can lead to scurvy, decreased resistance to some infections, and alterations in connective tissue structure. Signs of scurvy include small, subcutaneous hemorrhages, muscle weakness, soft swollen gums, and loose teeth. Vitamin C is a powerful reducing agent and antioxidant, thus influencing several oxidation-reduction reactions in the organism. It functions in the hydroxylation of proline, which is used for collagen formation, thus influencing connective tissue formation. Vitamin C helps to facilitate bile acid formation in the liver, and it also assists in the synthesis of catecholamines from tyrosine.
L-Gulonate also serves as a branch point between the uronic acid pathway and the HMS. Loss of CO2 from L-gulonate yields L-xylulose, which can then be converted to its D-isomer through a preliminary NADPH-dependent reduction to xylitol. This product is oxidized via an NAD+ -dependent reaction to D-xylulose, which can then enter the HMS following phosphorylation (as xylulose 5- phosphate).
The uronic acid pathway is known to be affected in several ways. For example, lack of the enzyme needed to convert L-xylulose to xylitol can produce essential or idiopathic pentosuria. In this disease considerable quantities of L-xylulose may appear in urine. Administration of aminopyrine or antipyrine have been reported to increase the excretion of L-xylulose in pentosuric subjects.
Additionally, xylitol is an artificial sweetner, and in small amounts can cause canine hypoglycemia, hypokalemia, and hypophosphatemia (through an increase in insulin release), and in some cases liver dysfunction.
Some drugs are known to increase the rate at which glucose enters the uronic acid pathway. For example, administration of barbiturates such as phenobarbital, or of chlorobutanol to rats has been reported to result in a significant increase in the conversion of glucose to glucuronate, L-gulonate, and L-ascorbate.
In summary, the uronic acid pathway has five major biosynthetic functions:

1. Synthesis of the sugar moieties of various classes of glycoproteins.
2. Participation in heparin and heparan sulfate formation.
3. Production of UDP-glucuronate for various conjugation reactions.
4. L-ascorbate (vitamin C) formation.
5. To serve as a minor route for the formation of pentoses (e.g., D-xylulose), which can enter the HMS.

Like the HMS, the uronic acid pathway is an alternate cytoplasmic route for the metabolism of Glc-6-P. It does not produce ATP via substrate-level phosphorylation; however, it does generate reducing equivalents which can give rise to ATP through mitochondrial oxidative phosphorylation.

SUMMARY

The uronic acid pathway is an alternate cytoplasmic route for the metabolism of glucose 6-phosphate (Glc-6-P). It converts Glc-6-P to uridine diphosphate glucuronate (UDP-glucuronate) for the biosynthesis of vitamin C and glycoproteins, as well as hepatic detoxification. The pathway does not produce ATP but generates NADH, which can be used to produce ATP through oxidative phosphorylation. The pathway involves the conversion of Glc-6-P to glucose 1-phosphate (Glc-1-P), which then reacts with uridine triphosphate (UTP) to form UDP-glucose. UDP-glucose is oxidized to UDP-glucuronate, which is used for conjugation reactions in the liver and kidneys. The liver contains isoforms of UDP glucuronosyltransferase (UGT), the enzyme involved in these conjugation reactions. Cats have a less diverse pattern of UGT isoform expression, which explains their difficulty in conjugating and excreting certain drugs. UDP-glucuronate is also used to produce carbohydrate-protein complexes, such as glycosaminoglycans and glycoproteins. The pathway is also involved in the formation of heparin, heparan sulfate, and L-ascorbate (vitamin C). A deficiency in vitamin C can lead to scurvy and alterations in connective tissue structure. L-Gulonate, a product of the pathway, serves as a branch point between the uronic acid pathway and the hexose monophosphate shunt (HMS). The uronic acid pathway has several biosynthetic functions, including the synthesis of sugar moieties in glycoproteins, participation in heparin and heparan sulfate formation, production of UDP-glucuronate for conjugation reactions, L-ascorbate formation, and the formation of pentoses. Overall, the uronic acid pathway is an important metabolic pathway with various functions in the body.

OUTLINE

• The uronic acid pathway is an alternate cytoplasmic route for the metabolism of glucose 6-phosphate (Glc-6-P)
• It converts Glc-6-P to uridine diphosphate glucuronate (UDP-glucuronate)
• UDP-glucuronate is used in the biosynthesis of vitamin C and glycoproteins, as well as hepatic detoxification of endogenous and exogenous compounds
• The pathway does not produce ATP via substrate-level phosphorylation, but generates reducing equivalents in the form of NADH
• Glucose 6-phosphate is converted to glucose 1-phosphate (Glc-1-P) in the first reaction of the pathway
• Glc-1-P reacts with uridine triphosphate (UTP) to form UDP-glucose
• UDP-glucose is oxidized by an NAD+-dependent dehydrogenase to yield UDP-glucuronate
• The liver and kidneys use UDP-glucuronate for conjugation reactions with non-polar, lipophilic compounds
• Glucuronide conjugates are excreted into bile and/or urine
• The liver contains several isoforms of UDP glucuronosyltransferase (UGT), the enzyme used in conjugation reactions
• Cats have difficulty conjugating and excreting certain drugs due to a less diverse pattern of UGT isoform expression
• UDP-glucuronate can also be used to produce carbohydrate-protein complexes such as glycosaminoglycans, mucopolysaccharides, and glycoproteins
• Heparin, dermatan sulfate, and heparan sulfate are examples of compounds derived from UDP-glucuronate
• UDP-glucuronate can be reduced to L-gulonate, which can be converted to L-ascorbate (vitamin C) in most animals
• Some animals, including primates, fish, flying mammals, songbirds, and guinea pigs, cannot convert L-gulonate to L-ascorbate and require dietary vitamin C
• Prolonged vitamin C deficiency can lead to scurvy and alterations in connective tissue structure
• L-gulonate also serves as a branch point between the uronic acid pathway and the hexose monophosphate shunt (HMS)
• Lack of the enzyme needed to convert L-xylulose to xylitol can produce essential

QUESTIONS

Qcard 1:

Question: What is the uronic acid pathway?

Answer: The uronic acid pathway is an alternate cytoplasmic route for the metabolism of glucose 6-phosphate (Glc-6-P), which converts it to uridine diphosphate glucuronate (UDP-glucuronate) for various biosynthetic functions.

Qcard 2:

Question: What is the role of UDP-glucuronate in the uronic acid pathway?

Answer: UDP-glucuronate is produced in the uronic acid pathway and is used for conjugation reactions with endogenous and exogenous compounds, as well as for the biosynthesis of vitamin C and glycoproteins.

Qcard 3:

Question: How are glucuronide conjugates excreted from the body?

Answer: Glucuronide conjugates are excreted into bile and/or urine by the liver and kidneys, respectively, due to their increased solubility in water.

Qcard 4:

Question: What is the role of UDP glucuronosyltransferase (UGT) in the uronic acid pathway?

Answer: UDP glucuronosyltransferase (UGT) is the enzyme used in conjugation reactions in the uronic acid pathway, allowing for the attachment of glucuronate to hydrophobic compounds.

Qcard 5:

Question: What are some examples of carbohydrate-protein complexes derived from UDP-glucuronate?

Answer: Examples include glycosaminoglycans, mucopolysaccharides, and glycoproteins such as chondroitin sulfate, dermatan sulfate, and hyaluronic acid.

Qcard 6:

Question: What is the significance of heparin and heparan sulfate in the body?

Answer: Heparin is known for its anticoagulant and lipid-clearing properties, while heparan sulfate participates in cell growth, cell-cell communication, and plays a major role in the renal glomerular barrier.

Qcard 7:

Question: What happens in species that cannot convert L-gulonate to L-ascorbate?

Answer: Species that lack the enzyme L-gulonolactone oxidase, such as primates, fish, flying mammals, songbirds, and guinea pigs, require vitamin C in their diets.

Qcard 8:

Question: What are the consequences of vitamin C deficiency?

Answer: Prolonged vitamin C deficiency

Mind Map: Uronic Acid Pathway

Central Idea: Uronic Acid Pathway is an alternate cytoplasmic route for the metabolism of Glucose 6-phosphate (Glc-6-P) that has various biosynthetic functions.

Main Branches:

1. Glucose 6-phosphate metabolism
2. UDP-glucuronate synthesis
3. Conjugation reactions
4. Biosynthesis of glycoproteins and vitamin C
5. Pentose formation

Glucose 6-phosphate metabolism

• Glucose 6-phosphate converted to Glucose 1-phosphate (Glc-1-P)
• Glc-1-P reacts with Uridine triphosphate (UTP) to form UDP-glucose

UDP-glucuronate synthesis

• UDP-glucose oxidized to UDP-glucuronate using NAD+
• UDP-glucuronate used for conjugation reactions and biosynthesis

Conjugation reactions

• Liver and kidneys use UDP-glucuronate for conjugation with endogenous and exogenous compounds
• Conjugates excreted into bile and/or urine

Biosynthesis of glycoproteins and vitamin C

• UDP-glucuronate used for the synthesis of glycosaminoglycans, mucopolysaccharides, and glycoproteins
• Important for body structures like cartilage, bone, skin, etc.
• UDP glucuronate epimerized to iduronate in heparin and dermatan sulfate
• Heparin has anticoagulant and lipid-clearing properties
• Heparan sulfate participates in cell growth and cell-cell communication

Pentose formation

• UDP-glucuronate converted to L-gulonate
• L-gulonate can be converted to L-ascorbate (vitamin C) in most animals
• Primates, fish, flying mammals, songbirds, and guinea pigs require dietary vitamin C
• Vitamin C deficiency can lead to scurvy and alterations in connective tissue structure

Sub-branches:

• Glucose 6-phosphate metabolism
  • Glucose 1-phosphate formation
  • UDP-glucose formation
• UDP-glucuronate synthesis
  • Oxidation of UDP-glucose
  • NAD+ as a cofactor
• Conjugation reactions
  • Liver and kidneys as major organs

Study Plan: Chapter 29: Uronic Acid Pathway

Day 1: Introduction and Glucose 6-phosphate Conversion

• Read and understand the introduction to the uronic acid pathway.
• Focus on the conversion of glucose 6-phosphate to glucose 1-phosphate (Glc-1-P).
• Understand the reaction between Glc-1-P and uridine triphosphate (UTP) to form UDP-glucose.
• Take notes on the key concepts and reactions.

Day 2: UDP-Glucose Oxidation and Conjugation Reactions

• Review the conversion of UDP-glucose to UDP-glucuronate using NAD+-dependent dehydrogenase.
• Understand the role of UDP-glucuronate in hepatic detoxification and conjugation reactions.
• Learn about the liver and kidney's use of UDP-glucuronate for conjugation with endogenous and exogenous compounds.
• Take notes on the significance of glucuronide conjugates and their excretion.

Day 3: UDP Glucuronosyltransferase (UGT) and Isoforms

• Study the different isoforms of UDP glucuronosyltransferase (UGT) present in the liver.
• Understand the selective nature of UGT isoforms for endogenous and exogenous compounds.
• Learn about the role of hydrophobic compounds and their conjugation with glucuronate.
• Take note of the impact of drug exposure on UGT isoform synthesis and enhanced excretion.

Day 4: UGT Isoform Expression in Cats and Carbohydrate-Protein Complexes

• Focus on the less diverse pattern of UGT isoform expression in cats for exogenous compounds.
• Understand the difficulties cats face in conjugating and excreting certain drugs.
• Learn about the role of UDP-glucuronate in the synthesis of carbohydrate-protein complexes.
• Study the importance of glycosaminoglycans, mucopolysaccharides, and glycoproteins in various body structures.

Day 5: Vitamin C Metabolism and Other Functions of Uronic Acid Pathway

• Review the conversion of UDP-glucuronate to L-gulonate and its further conversion to L-ascorbate (vitamin C).
• Understand the implications of L-gulonolactone oxidase absence in certain organisms.
• Study the consequences of vitamin C deficiency, including scurvy and