Vitamin C biosynthesis (2024)

Also known as ascorbic acid, vitamin C is a smallcarbohydrate molecule first identified in the 1920s by Albert von SzentGyörgyi, who discovered that it was able to prevent and cure scurvy. Scurvy isa pathological life-threatening condition suffered by people who do not haveaccess to fruits or vegetables for long periods of time. A decade earlier,Kazimierz Funk had prepared a list of nutritional factors, called vitamins,whose deficiencies cause severe diseases in humans. In his list, Funk used theletter "C" to designate a factor still unidentified, but known to preventscurvy. Later on, Szent Györgyi and Haworth chemically identified "C" asascorbic acid, and named it so because ascorbic means "anti-scurvy."Over the next century, what we now know as vitamin C became one of the mostpopular drugs in human history.

Why is this molecule so well-known? Apart from itsdeficiency causing scurvy in humans, vitamin C is also vitally important toother species. Neither animals nor plants can live without vitamin C, and it istherefore surprising that some animals (some fishes and birds, and a few mammals,including guinea pigs and humans) have lost the capability to produce it overthe course of evolution. How did this happen?

In each step of a biosynthetic pathway, a substrate is converted into a product in a reaction catalysed by an enzyme. The product of the first reaction becomes the substrate of the second one, and the steps of the pathway run sequentially until the final product is produced (Figure 1). In 1957, the famous biochemist Albert Lehninger studied vitamin C biosynthesis in animals, and realized that, unlike many species, such as cats and dogs, which can biosynthesize their own vitamin C supply, humans are unable to do so. Human cells cannot perform the crucial last step of vitamin C biosynthesis, the conversion of l-gulono-g-lactone into ascorbic acid, which is catalysed by the enzyme gulonolactone oxidase. As a follow-up to Lehninger's work several years later, Nishikimi and co-workers observed that the gene that codes for gulonolactone oxidase is actually present in humans, but is not active due to the accumulation of several mutations that turned it into a non-functional pseudogene (Nishikimi & Yagi 1991). Notably, not only all humans, but also gorillas, chimps, orangutans, and some monkeys have this inborn genetic flaw, meaning that the loss of vitamin C biosynthesis must have occurred first in one of our primate ancestors. But how can something so crucial for survival be eliminated through the course of evolution? Typically, we expect that positive traits should be retained during evolution, and as vitamin C is beneficial, how would natural selection remove such a crucial biosynthetic capability? Indeed, individuals carrying the mutation(s) in the gene encoding gulonolactone oxidase should have had less chance of surviving and reproducing. However, the opposite occurred, and those who had lost vitamin C biosynthesis survived. How can we explain this apparent paradox?

To better understand how and why the loss of vitamin C occurred,we need to understand the benefits of it. Scurvy is a deadly disease thatoccurs in vertebrates that are unable to synthesize vitamin C when their dietdoes not include fresh fruit and vegetables, rich sources of the vitamin.Historically, this disease killed many sailors, who did not have suchperishable foods available during their long voyages at sea. Scurvy takes sometime to develop in a human with a vitamin-C-free diet, and when it does it canshow a range of symptoms. These include lassitude, neurological dysfunction,and, more commonly, dramatic defects in blood vessel and bone integrity. Theselatter symptoms are the most easily recognized because they cause skin spots,bleeding of gums, and loose teeth, as well as bone and cartilage fragility.

In the late 1950s, the new tools of biological chemistryallowed the identification of another essential role for vitamin C that helpedexplain these fundamentally disabling symptoms. In 1962, through analysis ofthe radioactivity incorporated into collagen using a tritiated version of theamino acid proline, Stone and Meisterdiscovered that vitamin C is used as a co-substrate by peptidyl-prolylhydroxylase, an enzyme that catalyzes the selective modification of proline tohydroxyproline. This modification is essential for proper collagen folding.Consequently, the lack of vitamin C results in the formation of non-functionalcollagen in blood vessels and bones, which accounts for most of the severe boneand blood vessel related symptoms. The variety of scurvy symptoms beyond thosestemming from collagen defects occur because vitamin C is also a co-substratefor multiple enzymes involved in biosynthesis, including the synthesis ofdopamine, an important neurotransmitter, and carnitine which helps mitochondriakeep in pace with the demand for energy production. Lack of these two compoundshelps explain the neurological dysfunction and lassitude symptoms of scurvy.

Nowadays, scurvy is not as widespread as it used to be,although cases still occur among people with unhealthy eating habits. However,vitamin C became quite popular in the 20^th century, not for its rolein the prevention of scurvy, but for its potent "antioxidant" function. Theidentification of reactive oxygen species (ROS), such as hydrogen peroxide andsuperoxide ions, as molecules that are potentially harmful for biologicalmembranes and other cell components, has intensified interest in things thatare anti-ROS, known as antioxidants. These compounds are able to react with dangerousoxidants and keep cells and tissues healthy. Indeed, vitamin C is one of thebest physiological non-toxic antioxidants because it is so efficient: it reactswith many different kinds of ROS. There is a common misconception thatantioxidants are always beneficial, when rather they are complex molecules thatare part of intricate systems ensuring proper cell function. Regardless,because of the integral role that vitamin C plays inside a cell, be itantioxidant or co-substrate, preserving its biosynthesis should have been aselective advantage. Why did humans lose this ability?

Like humans, other animals unable to synthesize vitamin Ccan always find a supply of it in other organisms that synthesize it on theirown, namely plants. Humans who consume regular portions of fresh fruit (ortablets, which are less effective) will avoid the consequences of not makingtheir own vitamin C. Inclusion of vitamin C in the human diet explains why ournon-synthesizing ancestors did not become extinct, as they found this aneffective compensation for the mutations in the gulonolactone oxidase gene.However, biochemists speculate that there may have been some concurrentadvantage of this mutation that caused it to persist and spread in the humanlineage. For instance, since one of the products of the reaction catalysed bygulonolactone oxidase is hydrogen peroxide, Halliwell suggested in 2001 thatthe loss of biosynthesis balanced the "cost" of production, since the advantageof producing one vitamin C molecule would be lost by the production of thisreactive oxygen species (Halliwell 2001).

More recently, Grano and De Tullio proposed anotherhypothesis, based on the studies by Knowleset al. In 2003, Knowles et al.demonstrated that vitamin C regulates a key stress-induced transcription factorcalled Hypoxia Inducible Factor 1α (HIF1α), a protein that, when activated,regulates the expression of hundreds of stress-related genes. Notably, theactivation of HIF1α occurs in the absence of adequate oxygen or vitamin Csupply. Grano and De Tullio therefore proposed that organisms that have lostvitamin C biosynthesis have an advantage: they can finely regulate HIF1αactivation on the basis of the dietary intake of vitamin C (Grano & DeTullio 2007). When vitamin C supply is sufficient, the HIF transcriptionfactor is less active than in conditions of vitamin C deficiency. In otherwords, the lack of vitamin C biosynthesis allows our bodies to know more aboutour nutritional status and consequently set the proper baseline of HIF1αexpression. It is like a sensitive titration system.

There is a third yet still unexplored possibility. We knowfrom other studies that pseudogenes are not inert, but can have a significantrole in epigenetic control of gene expression (Poliseno et al. 2010). Could this also apply to the human gulonolactoneoxidase pseudogene? Time (and much research) will tell.