Oil and water don't mix--which is the physical basis for organization in living systems

What is it about?

What does it mean to be organized? Think of the root word "organ." A body would be in trouble if it were not organized into organs. At the small level of the cell, the membrane is organized as a result of the structure of lipids. Lipids have two ends: a greasy end and a water-loving end. As a result, they self organize into a bilayer, also known as a membrane, which is the dividing line between the inside and the outside of a cell. Inside cells are found the ingredients of life, which includes proteins. It turns out, proteins are self-organized by the same physical forces, except proteins are made of amino acids. This paper deals with quantifying just how greasy or water-loving each amino acid is.

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Photo by Kai Dahms on Unsplash

Photo by Kai Dahms on Unsplash

Why is it important?

The protein folding problem has been around a long time, ever since it was found that globular proteins are made of linear chains of polypeptides which curl and wrap around themselves (i.e. fold), transforming a linear polymer into a defined 3D shape. The shape of a protein, such as an enzyme, plays a pivotal role in explaining how it works, so understanding folding is a big part of understanding proteins. The inside of a protein is somewhat analogous to the inside of a lipid bilayer--it is water hating, like a layer or droplet of oil. The tendency for amino acids to find themselves on the inside of a protein is closely related to their water loving or water hating properties, also known as their hydropathy. So knowing something about hydropathy tells you something about the likely location of an amino acid in the 3D structure of a folded protein, with more greasy ones on the inside, and more water loving ones on the outside, exposed to the aqueous environment.

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