Among the twenty amino acids encoded by the genetic code, proline is unique in having a cyclic structure with its side chain connected to the amino group to create a secondary amine. As a consequence of its cyclic structure, proline constrains the structure of proteins where it occurs, disrupting alpha-helices. Isomerization between the cis and trans forms of proline in proteins is isomerized by peptidyl-prolyl isomerases like the cyclophilins that contribute to protein folding and are components of signal transduction pathways. Proline biosynthesis begins with the ATP-driven phosphorylation and reduction of the carboxyl side chain of glutamate. The resulting glutamate gamma-semialdehyde then spontaneously cyclizes in a non-enzymatic reaction to produce delta1-pyrroline-5-carboxylate. This reduction of this intermediate creates the final pathway product, proline. In mammals, proline biosynthesis can occur through another route, through the urea cycle. In this pathway, arginine serves are the first intermediate that is converted to ornithine.
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