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In the laboratory, the activity of enzymes is studied in large excess of water in a homogeneous medium. But in nature, enzymes often act in dense environments and on insoluble molecules that can modify their activity. This is particularly the case in the plant cell wall which is reshaped by many enzymes during the cell's life.

To unravel the functioning of enzymes in the cell wall, the diffusion of an active or heat-inactivated enzyme was studied in model gels mixing pectins and cellulose, the two main components of the cell wall. This study was conducted in collaboration with the DISCO beamline of the SOLEIL synchrotron.

The auto-fluorescence properties of the enzyme under deep UV radiation and the resolution of the SOLEIL synchrotron instruments made it possible to track the movement of the enzymes in these model gels.

The results show that the migration of the enzyme is:

- related to its activity: the inactive enzyme does not move. The heat released during the catalysis would be at the origin of the displacement of the active enzyme;

- favoured by the presence of cellulose in the gel, suggesting that the mixture of the two polysaccharides allows the formation of pores where the diffusion of the enzyme is facilitated.

Thus, in the plant, the enzymes that develop the polymer network in the wall during the life of the cell (plant growth, fruit maturation, etc.) are able to move from a few hundred micrometers to a few millimeters as soon as they are active.