Porphyran, a polysaccharide present in the cell walls of a red algae that is used notably in the preparation of sushi, is broken down specifically by an enzyme called porphyranase. This new enzymatic activity has been identified in marine bacteria and, surprisingly, in the bacteria that populate the gut of the Japanese. Scientists from CNRS and UPMC have explained this discovery by a transfer of genes between the bacteria, that allows the gut microbiota of the Japanese to acquire all the "machinery" it needs to consume the algae that surround sushi.

Their results are published in Nature on April 8, 2010.

Without intestinal flora, humans cannot break down the polysaccharides in their diet, which are one of the principal sources of energy for the brain. Indeed, intestinal bacteria contain enzymes that are known to "break down" polysaccharides (1), which are polymers made up of carbohydrates. They are essential because the human genome is not endowed with such enzymes.

Two research teams working at the Station Biologique in Roscoff (CNRS / UPMC) have been working on porphyranase, an enzyme that breaks down polysaccharides but whose true activity was previously unsuspected. These teams have thus discovered that porphyranase breaks down a highly specific molecule: porphyran, and not another substrate, as had previously been thought (2). Porphyran is a polysaccharide, one of the components in the walls of a red-colored marine algae called Porphyra. These algae are used to prepare Japanese sushi. According to historical documents, this alga has been consumed for many generations by the Japanese (3). Of considerable cultural importance in Japan, it has sometimes served as a gift or to pay certain taxes.

The researchers then demonstrated the process of recognition between the enzyme (porphyranase) and its substrate (porphyran). They were thus able to identify the "signature" of the sequence involved in this recognition (the specific site on the enzyme to which the reagent binds). As expected, this novel enzymatic activity was detected in marine bacteria. Further investigations led the scientists to compare genomic data regarding the gut microbiota of 13 Japanese individuals and 18 North Americans. They thus discovered that porphyranase was also present in the gut microbiota of the Japanese (but not in that of the North Americans).

The scientists suppose that the presence of this enzyme in the gut microbiota of the Japanese is directly linked to their dietary habits. As major consumers of Porphyra for several centuries, the Japanese have thus been in contact with the marine bacteria that contain porphyranases via their diet. Mirjam Czjzek and her team presume that a transfer of genes from marine bacteria to intestinal bacteria must have allowed the microbiota of the Japanese to accept the "machinery" required to break down the polysaccharides in Porphyra algae. These findings suggest that food associated with marine bacteria may constitute a means for the human gut microbiota to acquire new enzymes, which may, among other factors, explain their diversity.

Notes

(1) For example, cellulose and starch.

(2) It was previously thought that agarose, a carbohydrate polymer that is also extracted from red algae, was the substrate for this enzyme.

(3) Writings testify that the algae served as a form of payment in the 8th century.