What are polypeptides and proteins?
Polypeptides and proteins are very important to us as they help with several vital functions in the body. Both are made up of amino acids, the building blocks of life, but they’re not exactly the same. Polypeptides are smaller and more simple, while proteins are bigger and more complex. Polypeptides can form parts of proteins, but proteins take on way more roles in the body, like acting as enzymes or building muscles.
A polypeptide is a chain of amino acids linked by peptide bonds. According to the International Union of Pure and Applied Chemistry, polypeptides typically have more than 20 amino acids [1]. They can vary in size and structure, sometimes forming complex molecular shapes [2]. Thanks to the diversity of amino acids, polypeptides can fold into secondary structures and have a wide range of physicochemical and biological properties [3].
Proteins on the other hand are usually made up of over 50 amino acids and often contain multiple peptide subunits [1]. Proteins carry out a wide variety of functions in living organisms, including DNA replication, transporting molecules, catalysing metabolic reactions, and providing structural support to cells [4]. They can have a primary, secondary, and tertiary structure; some proteins even have a quaternary structure [4].
In summary, while polypeptides and proteins are chains of amino acids, proteins are larger and have a more nuanced structure. They often contain multiple peptide subunits and can perform a wider variety of bodily functions.
When does a polypeptide become a protein?
A polypeptide turns into a protein when it folds into a specific 3D configuration and takes on a job in the body. This isn’t a simple process ,it goes through several stages to get the right structure and function.
A basic chain of amino acids is the first step on the path from polypeptide to protein. Similar like stringing beads together, but with amino acids as the "beads" connected by covalent bonds created by a condensation reaction. A short chain of two to fifty amino acids is referred to as a peptide. However, it becomes a polypeptide, an unbranched chain that is one step closer to becoming a fully functioning protein, once it reaches 20 amino acids [5].
The ribosome, a small but vital component of the cell, is where the polypeptide is made. Here's how it works: with a little help from the enzyme aminoacyl-tRNA synthetase, each amino acid uses ATP to link with its appropriate tRNA. Every time a condensation reaction takes place as the amino acids join together via peptide bonds, a water molecule is expelled. A polypeptide chain will eventually emerge if you continue to add amino acids [5].
A polypeptide is typically regarded as a small protein if it has 100 amino acids or less. However, a polypeptide's ability to fold into a precise three-dimensional form is what truly makes it a functional protein and is essential to its function in the body [6].
A polypeptide does not fold into a protein in a straightforward, one-step manner. Many factors affect it, such as interactions with ligands, other proteins, or even the ribosome. To get to its ultimate shape, it occasionally even needs to go through membrane pores [7].
When a polypeptide folds into a certain three-dimensional shape and assumes a physiological role, it ultimately becomes a protein. During this process, a peptide chain is formed, additional amino acids are added, and the chain then folds into the precise shape needed to perform its function. Although it involves several steps, proteins couldn't perform all the various functions for which they are obligated without it.
Functions of polypeptides vs. fully formed proteins in the body
Even though polypeptides and proteins are made of the same building blocks, their roles depend on their size and structure.
With over 20 amino acids, polypeptides perform a variety of vital functions in the body. They aid in the regulation of immunological responses, muscle regeneration, gene expression, and DNA repair [8]. By promoting several physiological processes, they also significantly contribute to general health [9]. Furthermore, a range of biological effects might result from the interaction of polypeptides with receptors, which sets off intracellular signals [10].
On the other hand, fully formed proteins, usually made up of more than 50 amino acids, carry out various organism functions. They are involved in DNA replication, transporting molecules, catalysing metabolic reactions, and providing structural support to cells.
Peptide-based drugs vs. protein therapies
Two categories of biologics that are causing a stir in the pharmaceutical industry are peptide-based medications and protein therapeutics. They are a good substitute for conventional small-molecule medications because they are very selective, efficient, and often have low toxicity [11].
The smaller of the two, peptide-based medications are typically created from brief segments of naturally occurring proteins or peptides. They have well-understood processes, are generally harmless in the body, and are excellent at focusing on specific molecules.
The drawback? They have a short half-life and are less effective because of their small size and simple structure, which make them easier to break down. In order to circumvent this, researchers employ strategies such as chemical changes, encapsulation, and non-natural amino acids to keep them stable and functional for longer [12].
In contrast, protein treatments contain considerably larger molecules that are typically administered by injection rather than pill form. They are extremely accurate because of their size and structure, which enable highly exact interactions with their targets. However, there are drawbacks associated with that precision, such as poor bioavailability, trouble passing across membranes, and unstable metabolism. Despite these challenges, protein treatments continue to play an important role in medicine, frequently serving as the sole viable treatment for specific ailments [13].
There are benefits and drawbacks to both protein treatments and peptide-based medications. The ailment, the target of the therapy, and the patient's requirements determine the most suitable option. To make these medicines even more effective, researchers are always trying to enhance their stability, bioavailability, and delivery systems [14, 15].