Sugar, Spice, and Antimicrobial Peptides
By Cameron Grant, majoring in Biochemistry
October 25, 2021
Hot peppers are used for their spicy heat in all kinds of dishes, from Mexican to Indian cuisine.
Source: Photo by Shankar S., CC-BY 2.0
Within every plant cell, a plant’s most basic unit of life, there are an estimated 25 billion proteins (essential molecules that control cellular transport, structure, enzymes, communication, and more). These proteins are made up of long, twisting chains of peptides called polypeptide chains. Proteomics is the study of the proteome, an entire set of proteins expressed by a genome or organism. A surprisingly underexplored subset of the field is the search for bioactive plant peptides within a proteome. These special peptides can demonstrate anti-inflammatory, antioxidant, and most importantly, antimicrobial activities. To be “antimicrobial” is to be active against microbes, tiny living organisms that can’t be seen with the human eye (for example, bacteria). Further findings show that AMPs (antimicrobial peptides) could help to address the ever-growing issue of antibiotic resistance towards current day medicine. Research done by Kevin Culver and the Hicks Laboratory team here at The University of North Carolina at Chapel Hill has led to the discovery of AMPswithin Capsicum spp., more commonly known as the hot pepper.
PhD Candidate Kevin Culver, a member of the Hicks Lab at the UNC Chapel Hill Department of Chemistry
Image #2 Source: Kevin Culver
The lab’s search for these AMPs (antimicrobial peptides) begins with mining through a large universal protein database. These special peptides tend to share some conserved characteristics, typically being expressed as a protein containing a signal peptide and being cysteine-rich, with cysteine being a specific kind of amino acid. The sequence mining allows scientists to determine a list of predicted antimicrobial peptides within the plant proteome. With their AMP cheat sheet in hand, the Hicks laboratory is ready to dive into plant experimentation.
The team extracts likely peptides from plants and digest them with the enzyme trypsin. Trypsin digests the extracted sequences into smaller, more easily identifiable chains. The digested peptides are separated, ionized and placed into the mass spectrometer, a device that uses an ion’s mass and charge to identify its molecular weight. The elements discrete identities can be determined by comparison with standardized masses on the periodic table. Researchers can then identify the AMPs in the original plant extract by comparing it to their “AMP cheat sheet”. After predicting the AMPs, the peptides are screened for microbial activity. If something appears to be actively fighting bacteria, the data can be used to figure out which peptide is responsible.
Dr. Culver has translated his research to tangible applications within the medical field. Using these steps in their analysis, they found that a trypsin-digested C. chinense x frutescens (a ghost pepper) proteome included AMPs that exhibited antimicrobial activity against gram negative bacteria, which are known to cause pneumonia, E.Coli, meningitis, bloodstream infections, and much more. “The hope is that more knowledge of antimicrobial peptides will contribute to helping address this larger issue of antibiotic resistance.” Kevin notes of their findings. After all, bacteria mutate rapidly, meaning it wouldn’t take long for a new antibiotic-resistant bacterium to comprise a large chunk of a bacterial population. “[These AMPs] are alternative molecules in which it tends to be harder for bacteria to develop resistance.” 1
The plant kingdom remains an untapped resource for modern drug discovery. The Hicks lab will only continue to identify more AMPs in different kinds of plants, so keep an eye out for their future findings on your pharmacy shelf -- the next miracle drug could come from any plant, whether it be a pepper, papaya, or potato.
Robotic preparation of mass spectrometry analyzing protein patterns of a sample.
Source: The Web site of the National Cancer Institute (https://www.cancer.gov)
References:
Interview with Kevin Culver, PhD Candidate 09/21/21
Too Hot to Handle: Antibacterial Peptides Identified in Ghost Pepper Kevin D. Culver, Jessie L. Allen, Lindsey N. Shaw, and Leslie M. Hicks Journal of Natural Products 2021 84 (8), 2200-2208 DOI: 10.1021/acs.jnatprod.1c00281