One may well think that both forms of chiral molecules ought to be equally common in nature. But when we study the molecules of the cells in close-up, it is evident that nature mainly uses one of the two enantiomers. That is why we have – and this applies to all living material, both vegetable and animal – amino acids, and therefore peptides, enzymes and other proteins, only of one of the mirror image forms. Carbohydrates and nucleic acids like DNA and RNA are other examples.
For more info, see also an interesting video "What is chirality and how did it get in my molecules?" at http://ed.ted.com/lessons/michael-evans-what-is-chirality-and-how-did-it-get-in-my-molecules