Studies on the structure, mechanism and inhibition of serine palmitoyltransferase
Wadsworth, John Michael
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Sphingolipids and ceramides are essential components of cellular membranes and important signalling molecules. Because of a growing appreciation for their diverse biological roles, understanding of the biosynthesis and regulation of sphingolipids has recently become a key goal in drug discovery. Serine palmitoyltransferase (SPT) is a pyridoxal 5′-phosphate (PLP)-dependent enzyme that catalyses the condensation between L-serine and a long-chain acyl thioester such as palmitoyl-CoA (C16-CoA). This first step in sphingolipid biosynthesis is conserved in all organisms studied to date, from microbes to man. The fungal natural product myriocin is a potent inhibitor of SPT; however, the molecular details of inhibition are not fully understood. Myriocin contains a long alkyl chain and a polar head group thus it displays features of both SPT substrates. Therefore, the prevailing hypothesis is that inhibition of SPT occurs because myriocin acts as a mimic of a key transition state of the catalytic mechanism. Through a combination of UV-vis spectroscopy, mass spectrometry, x-ray crystallography and enzyme inhibition assays it has been possible to study the interaction between S. paucimobilis SPT and myriocin. I have shown that myriocin initially forms an inhibitory PLP:myriocin aldimine complex in the active site that displays a Ki of 967 nM. Interestingly, this complex is susceptible to unexpected, slow enzymatic degradation. The mechanism for myriocin breakdown has been elucidated as a retro-aldol type reaction, which results in cleavage of the C2-C3 bond producing a C18 aldehyde. This aldehyde is then capable of covalently modifying the active site lysine265, forming a second (suicide) inhibitory complex and rendering the enzyme catalytically inactive. Substitution of the active site lysine produced SPT K265A, an inactive enzyme that did not catalyse the breakdown of the PLP:myriocin complex. However, the determination of the crystal structure of the SPT K265A:PLP-myriocin complex revealed that the myriocin had undergone decarboxylation. Nevertheless, this SPT:PLP:decarboxymyriocin structure revealed details about myriocin’s mechanism of inhibition for the first time. The novel mechanism of myriocin degradation has implications on the structure activity relationship (SAR) and design of drugs targeted towards SPT, the role of feedback regulation by long chain aldehydes and further expands the range of reactions catalysed by this important enzyme. As well as inhibition studies the structure of bacterial SPT was also examined by preparing an N-terminally truncated S. paucimobilis SPT. This version, shortened by 21 amino acids, was ~5-fold slower than the wild-type enzyme and suggests that the N-terminus may play a role in catalysis. Additional work has been undertaken to study an unusual membrane-bound viral SPT, composed of two naturally fused open reading frames (SPT2-SPT1) with the proposed SPT2 domain at the N-terminus and the SPT1 domain at the C-terminus. To study soluble mimics of this interesting fusion I prepared a bacterial S. paucimobilis SPT fused wild-type and mutant construct and isolated a fused SPT2-SPT1 with what appears to be single PLPbinding site.