Why are d-amino acids not found in proteins?
Answers
Great question, and it is the tip of an iceberg.
For proteins to fold into their functional shapes, they must be polymers of the L-amino acids or their mirror images, the D-amino acids. The reason is, amino acids have a chiral center, and as you link them together form chains that want to twist one way but not the other. A mixture of D- and L- amino acids won’t form functional structures because one wants to twist the chain one way while the other wants to twist the chain in another. You would wind up with unfolded chains that would probably aggregate and precipitate.
A science fiction writer might imagine a “D-world” of all D-amino acids and proteins formed from them. The trouble is, the active sites of all the enzymes would be the mirror images of enzymes formed from L-amino acids and would not work on, for example, the D-sugars that dominate our world. The “D-world” would require that the stereochemistry of all biomolecules be flipped.
While we are on the subject, you could ask why DNA and RNA contain D- ribose and D-deoxyribose rather than their mirror images. Same problem. You need all of one kind of stereoisomer for the polymer to form a functional shape.
This presents a huge problem for people who think about abiogenesis. While they may conjure up ways to make amino acids and other building blocks from a chemical soup, they have no idea how to make chemically pure stereo-isomers. Organic chemists cannot do it in a lab without going through purification steps such as chromatography. A mixture of stereo-isomers of amino acids will polymerize because they react the same in chemical reactions, but the product is a mess.
Asymmetric synthesis is an inconvenient stumbling block for those who have a rosy view about origins of life.