Purdue University Graduate School
20221.7.26 Nunez_Thesis_Final.pdf (1.76 MB)


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posted on 2021-07-26, 19:18 authored by Brianne S NunezBrianne S Nunez
Cyclic peptides are important sources of medicines.
They are advantageous compared to linear peptides because they possess lower flexibility, which allows for high-affinity target binding and enhanced proteolytic stability. Unfortunately, achieving head-to-tail cyclization of peptides is quite challenging, as it is hard to control efficiency and regiospecificity of peptide macrocyclization. Many have attempted to improve peptide cyclization, including the use of different synthetic reagents as well as synthetic techniques to allow amide-bond formation and promote cyclization. While these strategies have offered great potential solutions, the aim of this study is to explore an alternative strategy that utilizes biocatalysis as a method of achieving successful peptide cyclization. Biocatalysis is the use of enzymes as natural process catalysts under artificial in vitro conditions. Biocatalysis is often more environmentally friendly and safer compared to traditional organic synthesis methods. Non-ribosomal peptide synthetases (NRPSs) are one of the major sources of cyclic peptides in nature. These are systems of large multifunctional proteins are organized into functional domains that act as an assembly line to generate peptide natural products. Normally, the thioesterase domain is responsible for hydrolysis and cyclization of the peptide. Recently, a novel cyclase (SurE) that is physically discrete from the NRPS was discovered. Based on this unique quality, we hypothesized that SurE would be easier to express compared to thioesterase domains and, for this reason, SurE could be a fantastic biocatalyst for the cyclization of peptides. To test this, we designed and generated an expression vector for SurE. We then expressed and purified the SurE protein. We also synthesized three linear peptides of varying lengths. To test for SurE activity, we attempted to add N-acetylcysteamine (SNAC) to mimic its native substrate. Unfortunately, we were unable to successfully attach the SNAC to our linear peptide. To combat this issue, a new synthesis strategy is currently being developed. This work is currently ongoing in the Parkinson lab, with the aim being to test the SurE protein, as well as other PBP-like cyclases, on other modified linear peptides and demonstrate whether the protein has the ability to cyclase a wide scope of peptides.


Degree Type

  • Master of Science


  • Chemistry

Campus location

  • West Lafayette

Advisor/Supervisor/Committee Chair

Elizabeth Parkinson

Additional Committee Member 2

Jean Chmielewski

Additional Committee Member 3

Hilkka Kenttamaa