Biological complexity: how plants transport glucose (Introduction)

by David Turell , Saturday, January 26, 2019, 19:05 (2128 days ago) @ David Turell

A very complex protein structure has been discovered:

https://www.sciencedaily.com/releases/2019/01/190125094239.htm

"Sugar transport through Sugar Transport Proteins (STP) is unique to plants, and is important for the proper development of plant organs such as pollen. STPs are also used to concentrate sugars in specific tissues like fruit, and they play an important role in the plant defence against fungal attacks from e.g. rust and mildew.

"Sugar is generated in plant leaves by photosynthesis, and is transported as the disaccharide sucrose to other parts of the plant through the sieve tissue. In sink tissues such as roots, pollen and fruits, the plant can absorb the sugar either as sucrose or, after cleavage, as the monosaccharides glucose and fructose.

"Uptake of glucose and other monosaccharides is driven by STPs that move sugar through the otherwise impermeable cell membrane using an acid gradient. These proteins have some specific properties compared to similar proteins from animals or bacteria. They have an extremely high affinity for sugar; in fact, they bind 1000 times more strongly to sugars than similar proteins in humans. At the same time they maintain a very high level of activity over a broad pH spectrum compared to other acid-driven sugar transporters.

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"With the new structure, the researchers show that the STPs overall form resemble other sugar transporters from e.g. humans. But the structure also holds surprises. Peter Aasted Paulsen highlights a new domain that has not been described before. "Over the binding pocket where sugar is located, the STPs have a novel small domain that resembles a lid that is held in place with an unusual bond, a so-called disulfide bridge. It was a completely unexpected observation that immediately sparked the imagination."

"To investigate the function of the domain, the researchers made a version of the protein in which this bond was removed. With this change, the protein loses its ability to transport sugar efficiently at certain pH values. If you compare these results with an analysis of the structure, it can be seen that the lid is held in place by means of the bond, thereby creating a favourable environment for acid binding to a specific acid binding pocket. This binding causes a portion of the protein to be pushed toward the sugar molecule, thereby creating the very high affinity for sugar."

Comment: It is obvious such an intricate mechanism is irreducibly complex and had to be designed in a complete form from the beginning, not by stepwise evolution