Question

All of the following are true about glucuronic acid except A. it is a charged molecule at physiological pH. B. as a UDP derivative, it can be decarboxylated to a component used in proteoglycan synthesis. C. it is a precursor of ascorbic acid in humans. D. its formation from glucose is under feedback control by a UDP-linked intermediate. E. it can ultimately be converted to xylulose 5 -phosphate and thus enter the pentose phosphate pathway.

Short answer

Answer: Glucuronic acid is a precursor of ascorbic acid in humans.

Step-by-step solution

Evaluate Statement A

Glucuronic acid is a charged molecule at physiological pH. This statement is true, as glucuronic acid has a carboxyl group that ionizes at physiological pH, making it a negatively charged molecule.

Evaluate Statement B

As a UDP derivative, glucuronic acid can be decarboxylated to a component used in proteoglycan synthesis. This statement is true. UDP-glucuronic acid is a precursor for the synthesis of glycosaminoglycans which are components of proteoglycans.

Evaluate Statement C

Glucuronic acid is a precursor of ascorbic acid in humans. This statement is false. Ascorbic acid (Vitamin C) is an essential vitamin for humans, meaning it cannot be synthesized by our body and must be obtained through diet. Glucuronic acid is not a precursor for ascorbic acid in humans.

Evaluate Statement D

The formation of glucuronic acid from glucose is under feedback control by a UDP-linked intermediate. This statement is true. The UDP-glucose dehydrogenase enzyme converts UDP-glucose to UDP-glucuronic acid, and this reaction is under feedback control by the levels of UDP-linked intermediates.

Evaluate Statement E

Glucuronic acid can ultimately be converted to xylulose 5-phosphate and thus enter the pentose phosphate pathway. This statement is also true. Glucuronic acid can be converted to xylulose 5-phosphate through a series of enzymatic reactions, which can then enter the pentose phosphate pathway. In conclusion, the false statement among the given options is Statement C: Glucuronic acid is not a precursor of ascorbic acid in humans.

Key concepts

Proteoglycan Synthesis

Proteoglycans are vital components of the extracellular matrix in tissues, providing structural support and playing key roles in cell signaling. The synthesis of proteoglycans involves a complex series of enzymatic processes, one of which includes the transformation of UDP-glucuronic acid.

This substance serves as a vital precursor for the glycosaminoglycan chains that are characteristic of proteoglycans. Specifically, UDP-glucuronic acid supplies the glucuronic acid residues that are alternating in these long sugar chains, which are later modified and attached to a core protein to form a complete proteoglycan. Thus, the production of UDP-glucuronic acid is a crucial initial step in the biosynthesis of proteoglycans.

UDP-Glucuronic Acid

UDP-glucuronic acid is a nucleotide sugar formed from UDP-glucose through the action of the enzyme UDP-glucose dehydrogenase. This compound has a key role as an intermediate in various biochemical pathways, particularly in conjugation reactions, where it facilitates the detoxification of drugs and toxins in the liver.

Additionally, as previously mentioned, it is essential in the synthesis of proteoglycans. At a physiological pH, UDP-glucuronic acid carries a negative charge due to the ionization of its carboxyl group, making it soluble and active in cellular environments.

Pentose Phosphate Pathway

The pentose phosphate pathway (PPP) is an essential metabolic pathway parallel to glycolysis. It is critical for generating reducing power in the form of NADPH and for producing ribose 5-phosphate, which is needed for nucleotide synthesis. Moreover, it provides a route for the transformation of glucose derivatives into various other sugars.

Glucuronic acid, through a series of reactions, can be converted into xylulose 5-phosphate, a pentose that is a component of the PPP. This highlights the integrative nature of metabolic pathways and the versatility of substrates like glucuronic acid that can contribute to multiple processes within the cell.

Ascorbic Acid in Humans

Ascorbic acid, commonly known as vitamin C, is an essential nutrient for humans; it is required for the biosynthesis of collagen, absorption of iron and proper immune function. Unlike most animals, humans lack the ability to synthesize ascorbic acid due to the absence of the enzyme L-gulonolactone oxidase, which is involved in its biosynthesis from glucose derivatives in other organisms.

Therefore, ascorbic acid must be obtained through the diet. This distinguishes humans from other species that can produce ascorbic acid endogenously through metabolic pathways including those that involve glucuronic acid.

UDP-Glucose Dehydrogenase

UDP-glucose dehydrogenase is a crucial enzyme that catalyzes the oxidation of UDP-glucose to UDP-glucuronic acid. This reaction is not only a step in glucuronidation reactions but also constitutes a regulation point in the synthesis of UDP-glucuronic acid. The activity of this enzyme is under feedback control by UDP-linked intermediates, ensuring that a balance is maintained between the demand and supply of UDP-glucuronic acid in various biochemical pathways.

The enzyme's function exemplifies the regulated nature of metabolic processes and emphasizes the coordinated control necessary for maintaining metabolic homeostasis in the cell.