August 11, 2018 | David F. Coppedge

One Wrong-Handed Amino Acid Can Cause Vertigo

The rocks in your head that give you a sense of balance are finely tuned to spiral in a certain way.

We’ve mentioned chirality before in articles on the origin of life. One of the difficulties for secular scientists is explaining how life ended up with one-handed (chiral) building blocks, like amino acids and sugars, when both are equally likely to form by the laws of physics. That remains a major unsolved problem in origin-of-life studies, because proteins cannot work unless 100% one-handed.

Another case of chirality just turned up in the human body: inside the inner ear, little crystals of calcium carbonate will fail to work if one amino acid goes right-handed instead of left-handed.

Tiny crystals of calcium carbonate help us keep our balance.

In a press release from McGill University on “Chirality Switching in Biomineral Structures,” researchers found one reason why some people develop vertigo. We all have little rocks in vestibules (sacs) within our inner ears, called otoconia. Those crystals are not just chips off the old block; they are carefully constructed by “proteins that guide the biomineralization process.” When otoconia grow correctly, they spiral in a certain direction. The scientists found that a single wrong-handed amino acid can make them spiral the opposite way, leading to failure in the balance organ.

Remarkably, they could also make the helicoids turn clockwise or counterclockwise by simply using only either the left-handed or right-handed version of the amino acids aspartic acid and glutamic acid – these being abundant in proteins that guide biomineralization processes in many organisms.

“This work provides key information towards understanding how organisms can have biomineralized structures rotating in opposite directions,” says McKee, senior author of the study. “Our findings also give insight into pathologic chiral malformations that might arise in human otoconia, structures in the inner ear whose normal functioning maintains balance by gravity sensing and by detecting linear acceleration. Our work predicts how pathologic chiral malformations might arise in human otoconia and could one day be used to develop therapies for vertigo (loss of balance) based on this understanding”.

Snail (DFC)

The paper in Science Advances, the open-access journal of the AAAS, gives the details of how the crystals grow. They spiral one way, then at a certain point, the spirals switch and grow the other way, overlapping as they grow. Adsorption of the one-handed enzyme on the crystal explains how the crystal can start growing with a left-handed spiral, and then switch to a right-handed spiral as layers build up. The outcome is finely dependent on the enzyme’s sequence. The authors say that “chiral switching” operates not only in the inner ear otoconia, but in snail shells that on rare occasions spiral the opposite way from normal.

This layer-by-layer rotational circular growth model for vaterite structures, which also provides a chiral switching mechanism, may be part of organic contributions to chiral switches observed in mineralized structures in biology, where biomineralized shells and other structures such as coccolith skeletons and pathologic human vaterite otoconia have chiral structures. Not only do the data support the postulate that simple acidic amino acids are potentially involved in producing chiral mineralized architectures both large and small, but they also additionally suggest that more complex chiral biomolecules may provide additional hierarchical instructions for extended structure in biomineralization.

The paper calls science’s lack of understanding of the origin of chirality an “Achilles’ heel” of biology, but ends on a biomimetic note:

In summary, this work demonstrates that two imperfect oriented attachment growth mechanisms operate in establishing chiral hierarchical vaterite helicoids induced by the adsorption of a single chiral enantiomer of an acidic amino acid onto a vaterite crystal, and whose helicoid chirality can be switched. These findings advance our understanding of why, despite a lack of d-enantiomer [right-handed] amino acids in biology—the “Achilles heel” of understanding the origin of enantiomeric pairs of biominerals from chiral amino acids—various enantiomorphs of biomineral may indeed be formed by homochiral biomolecules (more specifically l-acidic amino acids, as found in nature). The findings also describe a new organizational function involving mineral crystal-biomolecule interactions over different length scales that may prove useful in the development of novel composite/hybrid functional materials. The findings also allowed us to build a growth model for pathologic human inner ear otoconia that predicts other pathologic forms—as yet to be discovered—whose formation may one day be modified by therapies based on this understanding.

A sidebar in the press release explains chirality in molecules.

How about that; another fine-tuning example in biology, and a clever trick that scientists want to imitate! Your body is more wonderful than you can possibly imagine. It makes you dizzy just thinking about it. Be thankful today if you can bend over without losing your balance. Be thankful anyway, even if you do lose your balance. You can read this, can’t you?

 

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