Researchers from Wageningen University, along with colleagues from the University of Groningen and the University of Pavia (Italy), have unravelled the mechanism that plays a role in the natural production of vitamin C. In this process, a molecular gatekeeper blocks the entrance to the reaction centre of a crucial enzyme.
The biological production of vitamin C in plants, fungi and many animals is a complicated process that involves enzymes. A large group of these catalysts need oxygen to function well. In plants, a chemical, cytochrome C, replaces the function of oxygen. Cytochrome C or oxygen ensures that the co-factor flavin in the enzyme's action centre is brought back to its original state after reaction. Because of this restoration, the enzyme is ready for a new reaction.
The research team wondered why the one group of enzymes reacted with oxygen and the other, closely related group did not. How does the oxygen reach the centre of the enzyme, which consists of about 500 hundred linked building blocks (amino acids) of different sizes and forms. This string of building blocks is, as it were, bunched up into a little lump with 'holes, caverns and tunnels' in between. Oxygen has to seep through this little lump or clear a path through the tangle of amino acids in order to penetrate the hidden flavin in the centre.
Imagine, the researchers said, that in some enzymes oxygen can reach the enzyme's centre through tunnels and holes. You should then be able to discover the route using the structure. Unfortunately, there was no crystalline structure of the enzyme in question on hand. There was, however, one other possibility. By laying side by side all of the individual building blocks of the enzymes that react with oxygen and those that do not, the differences should become clear.
Comparing both analyses brought a subtle difference to light. Only one building block, number 113, at the end of a possible route turned out to be a bit different. This difference relates to the amino acid alanine. When alanine was replaced by the smaller building block glycine at that position, it turned out that the enzyme was suddenly oxygen permeable. And not just a little bit. The difference is so large it's as if a dam has burst: a factor of 400.
How is it possible that one building block in a construction of 500 blocks can have so much effect? The researchers support the tunnel theory: the building block alanine has four different protrusions, while glycine has only three. Alanine's extra protrusion, a methyl group, blocks the tunnel and prevents oxygen from penetrating the centre. At this site, alanine works as a gatekeeper and it keeps the door tightly shut.
But, why isn't the gate just simply open? Evidently, having a strict gatekeeper has its advantages. It turns out that the aggressive substance hydrogen peroxide ('domestic bleach') forms in the reaction with oxygen. Hydrogen peroxide accelerates the ageing of cells and a plant, which makes a lot of vitamin C, does not like this.
The way is now open to prepare vitamin C in a natural way. However, the chemical route already exists, is cheap and yields an identical product. The deciphered mechanism is, however, also applicable to similar biochemical reactions, for example, the preparation of vanilla. Additionally, the deciphered process can mean a step forward in synthetic biology in which products that do not occur or hardly occur in nature can be produced in a natural way.
The article in which the team reports its finding has been declared Paper of the Week by the Journal of Biological Chemistry, an honour given to only one in every hundred articles.
- Leferink et al. Identification of a Gatekeeper Residue That Prevents Dehydrogenases from Acting as Oxidases. Journal of Biological Chemistry, 2008; 284 (7): 4392 DOI: 10.1074/jbc.M808202200
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