D-脯氨酸是天然氨基酸 L-脯氨酸的异构体。已发现人血浆和唾液中的 D-氨基酸含量相对较高。这些氨基酸可能源自细菌，但也有证据表明它们是通过氨基酸消旋酶活性内源产生的。

Fibronectin Adhesion-promoting Peptide (Heparin Binding Peptide) is one of the heparin-binding amino acid sequences found in the carboxy-terminal heparin-binding domain of fibronectin.

This is amino acids 86 to 100 fragment of class II-associated invariant chain peptide called CLIP. The major histocompatibility complex class II molecule displays peptide fragments of foreign proteins to trigger a defensive reaction from the immune system. Before insertion of the foreign peptides into the binding groove, a place-holding peptide CLIP is removed. This is accomplished by the molecule DM, which is shown to increase the dissociation rate of a CLIP peptide from class II.