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Title:   Author  item
                                            LI, LENONG - SAMUEL ROBERTS NOBLE FDN.item
                                            CHANG, ZHENZHAN - SAMUEL ROBERTS NOBLE FDN.item
                                            Pan, Zhiqiang

                                                         - *Peter*item
                                            FU, ZHENG-QING - ARGONNE NATIONAL LAB., ILitem
                                            WANG, XIAOQIANG - SAMUEL ROBERTS NOBLE FDN.
  Submitted to: Proceedings of the National Academy of Sciences (PNAS)
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 6/23/2008
                                    Publication  Date: 9/16/2008
                                    Citation: Li, L., Chang, Z., Pan, Z., Fu, Z., Wang, X. 2008. Modes of Heme-Binding and Substrate Access for Cytochrome P450 CYP74A Revealed by Crystal Structures of Allene Oxide Synthase. Proceedings of the National Academy of Sciences. 105(37):13883-13888.Interpretive Summary: Cytochrome P450s catalyze an enormously wide range of chemical reactions and have different enzymatic mechanisms and complex substrate specificities. Allene oxide synthase (AOS) is a P450 enzyme that plays a key role in the biosynthesis of oxylipin jasmonates which are involved in signal and defense reactions in higher plants. AOS and other CYP74 enzymes recognize specific fatty acid hydroperoxides in the synthesis of oxygenated lipids. The crystal structures of guayule (Parthenium argentatum) AOS (CYP74A2) and its complex with the substrate analog have been determined. This is the first plant P450 crystal structure being reported. The structures revealed a novel heme binding mode with an unusually long heme binding loop and a unique I-helix, as well as two channels through which substrate and product may access and leave the active site.  These crystal structures provide a basis for exploring the interactions between enzyme and substrates for deciphering the catalytic mechanism of AOS and the related P450s, and reveal the first insights into their structure-function relationship.
                                Technical Abstract: Cytochrome P450s exist ubiquitously in all organisms and are involved in many biological processes. Allene oxide synthase (AOS) is a P450 enzyme that plays a key role in the biosynthesis of oxylipin jasmonates which are involved in signal and defense reactions in higher plants. The crystal structures of guayule (Parthenium argentatum) AOS (CYP74A2) and its complex with the substrate analog 13(S)-hydroxyoctadeca-9Z,11E-dienoic acid have been determined. The structures exhibit a classic P450 fold, but possess a novel heme binding mode with an unusually long heme binding loop and a unique I-helix. The structures also reveal two channels through which substrate and product may access and leave the active site. The entrances are defined by a loop between b3-2 and b3-3. Asn276 in the substrate binding site may interact with the substrate’s hydroperoxy group and play an important role in catalysis, and Lys282 at the entrance may control substrate access and binding. These studies provide both the first structural insights into AOS and related P450s, and a structural basis to understand the distinct reaction mechanism.


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