Chemists discover structure of glucagon fibrils

June 24, 2019

Patients with type 1 diabetes have to regularly inject themselves with insulin, a hormone that helps their cells absorb glucose from the bloodstream. Another hormone called glucagon, which has the opposite effect, is given to diabetic patients to revive them if they become unconscious due to severe hypoglycemia.

The form of glucagon given to patients is powdered and has to be dissolved in liquid immediately before being injected, because if stored as a liquid, the protein tends to form clumps, also called amyloid fibrils. A new study from MIT reveals the structure of these glucagon fibrils and suggests possible strategies for altering the amino acid sequence so that the protein is less likely to become clumped.

"Insulin in solution is stable for many weeks, and the goal is to achieve the same solution stability with glucagon," says Mei Hong, an MIT professor of chemistry and one of the senior authors of the study. "Peptide fibrillization is a problem that the pharmaceutical industry has been working for many years to solve."

Using nuclear magnetic resonance (NMR) spectroscopy, the researchers found that the structure of glucagon fibrils is unlike any other amyloid fibrils whose structures are known.

Yongchao Su, an associate principal scientist at Merck and Co., is also a senior author of the study, which appears in Nature Structural and Molecular Biology. MIT graduate student Martin Gelenter is the lead author of the paper.

Fibril formation

Amyloid fibrils form when proteins fold into a shape that allows them to clump together. These proteins are often associated with disease. For example, the amyloid beta protein forms plaques associated with Alzheimer's disease, and alpha synuclein forms Lewy bodies in the neurons of Parkinson's disease patients.

Hong has previously studied the structures of other amyloid peptides, including one that binds to metals such as zinc. After giving a talk on her research at Merck, she teamed up with scientists there to figure out the structure of the fibrillized form of glucagon.

Inside the human body, glucagon exists as an "alpha helix" that binds tightly with a receptor found on liver cells, setting off a cascade of reactions that releases glucose into the bloodstream. However, when glucagon is dissolved in a solution at high concentrations, it begins transforming into a fibril within hours, which is why it has to be stored as a powder and mixed with liquid just before injecting it.

The MIT team used NMR, a technique that analyzes the magnetic properties of atomic nuclei to reveal the structures of the molecules containing those nuclei, to determine the structure of the glucagon fibrils. They found that the glucagon fibril consists of many layers of flat sheets known as beta sheets stacked on top of one another. Each sheet is made up of rows of identical peptides. However, the researchers discovered that, unlike any other amyloid fibril whose structure is known, the peptides run antiparallel to each other. That is, each strand runs in the opposite direction from the two on either side of it.

"All thermodynamically stable amyloid fibrils known so far are parallel packed beta sheets," Hong says. "A stable antiparallel beta strand amyloid structure has never been seen before."

In addition, the researchers found that the glucagon beta strand has no disordered segments. Each of the tens of thousands of peptide strands that make up the fibril is held tight in the antiparallel beta sheet conformation. This allows each peptide to form a 10-nanometer-long beta strand.

"This is an extremely stable strand, and is the longest beta strand known so far among any proteins," Hong says.

Stable structure

One major reason that glucagon fibrils are so stable is that side chains extending from the amino acids making up the glucagon peptides interact strongly with side chains of the peptides above and below them, creating very secure attachment points, also called steric zippers, that help to maintain the overall structure.

While all previously studied amyloid fibrils have a fixed set of residues that form the steric zippers, in glucagon fibrils, even-numbered residues from one strand and odd-numbered residues from the neighboring strand alternately form the steric zipper interface between two beta sheet layers. This conformational duality is another novel feature of the glucagon fibril structure.

"We can see from this structure why the fibril is so stable, and why it's so hard to prevent it from forming," Hong says. "To block it, you really have to change the identity of the amino acid residues. I'm now working with a colleague here to come up with ways to modify the sequence and break those stabilizing interactions, so that the peptide won't self-assemble to form this fibril."

Such alternative peptide sequences could remain shelf-stable for a longer period of time in solution, eliminating the need to mix glucagon with liquid before using it.
-end-
The research was funded by Merck Sharp and Dohme Corp., a subsidiary of Merck and Co., and the National Institutes of Health.

Massachusetts Institute of Technology

Related Peptides Articles from Brightsurf:

Peptides+antibiotic combination may result in a more effective treatment for leishmaniasis
A combination of peptides and antibiotics could be key to eliminating the parasite causing leishmaniasis and avoiding the toxicity to people and animals caused by current drugs.

Designer peptides show potential for blocking viruses, encourage future study
Chemically engineered peptides, designed and developed by a team of researchers at Rensselaer Polytechnic Institute, could prove valuable in the battle against some of the most persistent human health challenges.

Tracking down cryptic peptides
Using a newly developed method, researchers from the University of Würzburg, in cooperation with the University Hospital of Würzburg, were able to identify thousands of special peptides on the surface of cells for the first time.

Synthesis of prebiotic peptides gives clues to the origin of life on Earth
Coordination Compounds Lab of Kazan Federal University started researching prebiotic peptide synthesis in 2013 with the use of the ASIA-330 flow chemistry system.

Peptides that can be taken as a pill
Peptides represent a billion-dollar market in the pharmaceutical industry, but they can generally only be taken as injections to avoid degradation by stomach enzymes.

Harnessing psyllid peptides to fight citrus greening disease
BTI, USDA and UW scientists have identified peptides in the Asian citrus psyllid, an insect that spreads the bacterium that causes citrus greening disease (huanglongbing, HLB).

New technique has potential to protect oranges from citrus greening
Citrus greening, also called Huanglongbing (HLB), is devastating the citrus industry.

Researchers show what drives a novel, ordered assembly of alternating peptides
A team of researchers has verified that it is possible to engineer two-layered nanofibers consisting of an ordered row of alternating peptides, and has also determined what makes these peptides automatically assemble into this pattern.

Origin of life insight: peptides can form without amino acids
Peptides, one of the fundamental building blocks of life, can be formed from the primitive precursors of amino acids under conditions similar to those expected on the primordial Earth, finds a new UCL study published in Nature.

Ragon Institute study identifies viral peptides critical to natural HIV control
Investigators at the Ragon Institute of MGH, MIT and Harvard have used a novel approach to identify specific amino acids in the protein structure of HIV that appear critical to the ability of the virus to function and replicate.

Read More: Peptides News and Peptides Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.