Many biomolecules come in two versions which are a mirror image, both left and right hand. Cells generally use the left-hand version of the amino acids to produce proteins, and the uptake mechanisms are considered to share this choice. Scientists from the University of Groningen have now shown that a prokaryotic transport protein can transmit the two versions of aspartate aspartate with the same efficacy. A detailed analysis of the structure of the transporter shows why. The results were published in the journal eLife on April 24.
The role of life is known for more than a century. Many organic molecules are produced in two versions that have the same chemical formula and bonding between atoms, but are structurally one of the other mirror images. During the evolution, for some molecules the left version (L) is selected, while for others the mirror image (D) is used. This is a problem in the production of drugs, where sometimes only one version is effective, and the other version can cause serious side effects.
"Living organisms use L-amino acids in the production of proteins, but they will sometimes use D-amino acids, for example in the bacterial cell walls," explains Professor of Biochemistry Dirk Slotbum of the University of Groningen. The central nervous system of mammals has a transport protein for the neurotransmitter L-glutamate that can also transport the amino acid aspartate. "And this has proven to recognize both L-aspartate and D-aspartate."
This is against expectations. Since L-amino acids are functionally active compounds, it would be logical for transport proteins to choose only one hand. Slotboom: "This stems from the difference in structure. The recognition of the transporter requires that the molecule structure fit into the binding site." And as it is not possible to fit the left hand into the right glove, the binding of D-amino acids to the transport protein which evolved to accept L-amino acids is impossible.
So far, no real mechanical or structural studies have been performed to explain why a central nervous system conveyor seems to defy this logic. Therefore, Slotboy, together with colleague Albert Guskov, assistant professor and head of the laboratory for crystallography of biomolecular X-rays, decided to deal with this issue. Their research stream, Valentina Arkhipova, carried out a structural analysis of transport proteins, while Dr. student Gianluca Trunch has performed functional studies. For their experiments, they used the homologous transport protein found in microorganisms, which has a binding site that is almost identical to that of a mammalian conveyor.
Trinco showed that L-aspartate and D-aspartate were transported in the same way, driven by the transfer of three sodium ions. "In addition, the affinity for both substrates is similar," he says. Archipova studied the structure of the binding site with attached L- or D-aspartate. She noted that D-aspartame was housed with only minor rearrangements of the structure: "The key is that there is enough space for geometrically different D-aspartate to bind. The binding site is not like a glove, but rather as a patent.
In microorganisms, the protein transports only aspartate, which cells can use to build proteins and also to use it as a fuel or as a source of nitrogen. In mammals, the homologous protein transports glutamate into the central nervous system, where the amino acid is used as a neurotransmitter. The transport protein removes L-glutamate from the synaptic crevice, the part where the nerve impulse is transferred to another neuron.
There are indications that aspartate may also act as a neurotransmitter. "If that were the case, L- and D-aspartate could perform this function," says Slotboom. "The affinity for the two types of aspartate is very high, which may indicate a function and suggests that D-aspartate is also used for something." Interestingly, the transporter does not accept D-glutamate. is a matter of space: glutamate has an additional methylene group compared to aspartate. "And in D-glutamate, that methylene probably causes a collision with the binding site." It does not fit, not even in the customs.