Properties in Protein Structure
The distinctive cyclic structure of proline's side chain locks its φ backbone dihedral angle at approximately −60°, giving proline an exceptional conformational rigidity compared to other amino acids. Hence, proline loses less conformational entropy upon folding, which may account for its higher prevalence in the proteins of thermophilic organisms. Proline acts as a structural disruptor in the middle of regular secondary structure elements such as alpha helices and beta sheets; however, proline is commonly found as the first residue of an alpha helix and also in the edge strands of beta sheets. Proline is also commonly found in turns, which may account for the curious fact that proline is usually solvent-exposed, despite having a completely aliphatic side chain. When proline is bound as an amide in a peptide bond, its nitrogen is not bound to any hydrogen, meaning it cannot act as a hydrogen bond donor, but can be a hydrogen bond acceptor. The distinct side chain/amine group interactions allow proline to aid in the formation of beta turns.
Multiple prolines and/or hydroxyprolines in a row can create a polyproline helix, the predominant secondary structure in collagen. The hydroxylation of proline by prolyl hydroxylase (or other additions of electron-withdrawing substituents such as fluorine) increases the conformational stability of collagen significantly. Hence, the hydroxylation of proline is a critical biochemical process for maintaining the connective tissue of higher organisms. Severe diseases such as scurvy can result from defects in this hydroxylation, e.g., mutations in the enzyme prolyl hydroxylase or lack of the necessary ascorbate (vitamin C) cofactor.
Sequences of proline and 2-aminoisobutyric acid (Aib) also form a helical turn structure.
Peptide bond formation with incoming Pro-tRNAPro is considerably slower than with any other tRNAs, which is a general feature of N-alkylamino acids. Peptide bond formation is also slow between an incoming tRNA and a chain ending in Proline; with the creation of Proline-Proline bonds slowest of all.
Read more about this topic: Proline
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