Ozempic has made headlines for its remarkable success in treating obesity and diabetes. However, it is just one in a rapidly growing class of drugs called peptide therapeutics that sits between small molecules (like aspirin) and biologics (like antibodies).
A UC Santa Barbara research team has developed a technique to efficiently synthesize unnatural amino acids and apply them to peptide construction. They hope that the methodology, published in Journal of the American Chemical Societywill greatly advance peptide research by giving scientists greater access to amino acids beyond the 22 found in nature.
The main advantage is that these amino acids come out of the process already in a form that can be used directly for the production of peptides, without additional modification steps. Compared to existing approaches, this is one of the simplest and most widely useful methods reported so far.”
Phil Kohnke, first author, PhD student in senior author Liming Zhang’s lab, Department of Chemistry & Biochemistry
The mechanism of life
Amino acids are the building blocks of proteins, making them among the most fundamental biological molecules. Joining between 10 and 50 amino acids produces a peptide. Whereas proteins are larger, more complex and may consist of multiple peptides.
Similar to stacking cups, these building blocks fit together in only one orientation: The amino group of one is always attached to the carboxylic acid group of the other. And just like creating color patterns in a stack of cups, the order of amino acids is a defining characteristic of peptides and proteins.
Although there are hundreds of types of amino acids, only 22 are naturally used by life forms to make proteins. These include 20 normal tastes encoded in our DNA and two produced by other mechanisms. “Nature uses them to great effect,” Zhang said.
Scientists can already produce natural amino acids cheaply. “But we have developed an efficient chemical synthesis to produce unnatural or non-canonical amino acids in a way that they can be used directly for peptide synthesis,” Zhang said.
A two-step technique
The newly published paper details a new technique for synthesizing amino acids and then attaching them to peptides using a resin scaffold. The team uses gold catalysis to create amino acids from cheap, readily available chemical components. The technique is highly stereoselective, meaning it can produce amino acids with a specific handedness rather than an undesirable mixture of right- and left-handed ones.
Linking amino acids together requires exposure and initiation of reactive sites. This fact is an advantage for chemists, because it allows them to connect the molecules in the proper order for the peptide they intend to make. Current synthetic techniques require removal of the component shielding the amino group as well as activation of the acid group during peptide synthesis. However, their method produces amino acids where the acid group is already prepared to react. only the amino group requires disclosure.
Similar to the Zhang lab’s recent work on oligosaccharides, the team used a resin scaffold to assemble peptides from amino acids. The frame attaches to one side of the growing peptide, allowing them to add amino acids one by one to the molecule in a rinse-and-repeat process. “We’re basically attaching things to the resin and then just growing the chain,” he said.
This technique is popular in industry because it greatly simplifies the cleaning process. Instead of going through the tedious effort of purifying the peptides from a solution, the molecules can be detached from the scaffold and washed away. “Our method can be transferred to this process with very little friction or adaptation,” Kohnke added.
Expanding availability and opening up opportunities
Access to more amino acids opens up entirely new possibilities for biochemists, medical researchers and materials scientists. It’s like trading a box of 22 colored crayons for a palate of 500 different shades.
But the production of unnatural amino acids is often difficult, expensive or impractical. “Many existing methods either involve many time-consuming steps, work only for a narrow set of molecules, or require further manipulations before they are ready for peptide synthesis,” Kohnke said. The new technique largely solves these problems by easily and cheaply producing amino acids that are immediately useful for peptide synthesis.
Zhang is particularly interested in the development of novel peptide therapeutics. Peptides have found use in more than 80 medicines worldwide since insulin was first synthesized in the 1920s, which changed type 1 diabetes from a death sentence to a completely manageable condition.
While natural peptides are effective, they are fragile – enzymes in the body can quickly break them down. “By incorporating unnatural amino acids, drug designers can ‘shield’ the peptide against enzymes or force it into a specific shape to better lock onto a receptor,” Zhang explained. Ozempic itself is a particular success of this approach, as it contains an unnatural amino acid in addition to a fatty acid side chain.
The Zhang lab is currently working to automate the process. Realizing the full potential of non-natural amino acids will require their immediate availability to non-chemists. At this point, they are actively seeking to collaborate with other research groups to make the technique more accessible to drug development and materials research.
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