A peptide is typically defined as a chain of amino acid monomers (that is, a molecule that may chemically attach to other molecules to form a polymer) which are linked by covalent bonds. They are typically split into two groups: Long chain peptides, also known as polypeptides or proteins; and short chain peptides, sometimes referred to as ogliopeptides.
Peptides play a large role within a certain subset of biomimetic materials, which are substances developed using elements that naturally occur in nature. The particular subset of biomimetic materials that pertains to peptides relates to tissue engineering; the process in which cells, materials, and engineering is combined with appropriate biochemical and physio-chemical aspects to enhance, repair, or replace biological functions of tissues found in living organisms (M. Radisic, et al, 1357-1368).
The primary use of peptides in tissue engineering results from their ability to elicit responses by mimicking extracellular matrix, or ECM, proteins. In essence, by replicating the functionality of these proteins, peptides can manipulate various biological functions, outside the realm of the normal physiological regulations, as dictated by an organism’s typical processes. (Shin, H, et al, 4353-5364)
While both forms of peptides share some similar traits that make them essential study components, other aspects of their behavior are unique. (Shin, H, et al, 4353-5364)
The Functionality of Long Chain Peptides
A long chain peptide is referred to as a polypeptide because it is a lengthy, continual chain of polymers made up of many amino acids – typically between 10 and 100 –linked together chemically. Long chain peptides have a molecular weight of up to about 10,000 grams per mole, and can fold into specific three-dimensional configurations. These types of peptides are commonly referred to as proteins. They are largely considered to be the core components of basic biological processes found within living organisms. Some of the functions that proteins perform include:
- DNA replication – The process of making two identical copies out of one original DNA molecule. It is the foundation for biological inheritance for all living organisms. In essence, this is the code that determines an organism’s physical makeup.
- Cell signaling – The complex system of communication that controls and regulates essential cellular activities and coordinates a cell’s action. This sometimes falls under the parameters of responding to stimuli. This communication is responsible for maintaining an organism’s overall systematic functionality.
- Enzyme catalysis – The process of controlling the rate of a chemical reaction by specialized proteins as a means to maintain biological efficiency. (G. Chaturvedi, 1-7)
Polypeptides regulate or trigger a significant amount of organic functions, either acting near to or distance from the site where they are produced and released. Their manner of regulation can be traced back to the hormones that they help to express within the organism. These hormones help to regulate various essential functions that enable an organism to operate normally, from antidiuretic (that is, water retaining) action in the kidneys to blood sugar control in the pancreas. (G. Chaturvedi, 1-7)
Because of their lengthy size, long chain peptides have higher instability than their short chain counterparts. While a polypeptide contains a wealth of information within its construct, its physical structure allows for more potential manifestations of degradation through processes such as:
- Hydrolysis – The process in which chemical bonds experience cleavage, otherwise known as division, due to the addition of water.
- Racemiztion – The process in which a conversion of an enatiomerically pure mixture (that is, where only one enantiomer is present) into some sort of mixture where more than one enantiomers are present. (G. Chaturvedi, 1-7)
Polypeptides can also experience issues pertaining to physical degradation depending upon their molecular weight, such as:
- Denaturation –The process in which proteins or nucleic acids lose the structure that is present in their native state due to external stress, resulting in the disruption of cell activity or even cell death. Some of these external stresses include a concentrated inorganic salt, a strong base, a strong acid, heat, or an organic solvent such as alcohol.
- Self Association – The process in which a long chain peptide interacts selectively and in a non-covalent way within itself.
- Gelation – The process where a fluid solution is converted into a semi-solid mass composed of multiple peptide fibrils and tangled within a complex mesh.
- Adsoprtion – The process in which atoms or ions form a gas, liquid, or a dissolved solid to adhere to a molecule’s surface. This process is a consequence of surface energy, which disrupts intermolecular bonds. This process is sometimes marked with the polypeptide collapsing.
- Aggregation – The process in which a protein structure creates its shape is known as protein folding. Occasionally, these proteins get mis-folded, which inhibits their proper functionality. The process of aggregation relates to the erroneous process that occurs when misfolded proteins accumulate at either an intracellular level or an extracellular level. (G. Chaturvedi, 1-7)
This high level of chemical instability makes the study of polypeptides a difficult challenge, as numerous paths of disruption makes it difficult to obtain and determine proper and consistent clinical results.
The Functionality of Short Chain Peptides
By contrast, short chain peptides (which are sometimes known as oligopeptides) are significantly smaller chains of amino acids. These amino acids are sequentially bonded together to form peptide bonds. These short peptide chains do not contain proteins. While there is no definitive maximum number of amino acids a chain can include and still be considered a short chain peptide, the minimum number of amino acids that must be present to be considered a short chain peptide is only two. (Scitable)
Short chain peptides, however, can share similar traits with long chain peptides or polypeptides. For example, they both can secrete hormones, which are essential components for the overall mechanics and operations of an organism (Scitable). However, short chain peptides do have an advantage over long chain peptides in that the smaller chain size is more stable. Small chain peptides do not experience some of the issues that long chain peptides do because of their shorter length—most notably collapsing during absorption. Additionally, short chain peptides can be replicated much more efficiently (Shin, H, et al, 4353-5364). This makes them significantly more attractive for research. The stability and efficiency of short chain peptides results in a more stable base of research metrics from which a valid library of case studies can be created. Because short chain peptides won’t collapse as a result of their length, research done with short chain peptides provide more consistent, stable data and results. This, in turn, provides researchers with more accurate, definitive set of data overall. (Shin, H, et al, 4353-5364)
Peptides and Tissue Engineering
The primary focus of short chain peptide research on tissue engineering is to discover and develop biocompatible materials to help combat natural degradation that is part of the aging process.
Specificallyamino acids located within the peptides are used as building blocks by other biological. These peptides are often referred to as “self-assembling peptides,” because they can be modified to contain biologically active motifs (Holmes, 1). This enables them to replicate information derived from tissue and to reproduce the same information independently. Thus, these peptides act as building blocks capable of conducting multiple biochemical activities—up to and including tissue engineering.
It should be noted that tissue engineering research currently being performed on both short chain and long chain peptides is still in early stages. As such, any research data or metrics relative to the peptides’ effects on tissue engineering should only be considered as preliminary.
Radisic, Milica; Park, Hyoungshin; Grecht, Sharon; Cannizzaro, Christopher; Langer, R.; Vunjak-Novkovic, Gordana; “Biometric Approach to Cardiac Tissue Engineering,” Philosophical Transactions: Biological Sciences Vol. 362, No. 1484, Bioengineering the Heart (August 29, 2007; pg. 1357-1368)
Shin, H; S. Jo, and A.G. Mikos; “Biomemetic Materials for Tissue Engineering,” Biomaterials, 2003.24; p. 4353-5364
Chaturvedi, Gunja; “A Report on Stability of Polypeptides and Proteins,” Birla Institute of Technology and Science, Pilani (Rajasthan), August 2009, p. 1-7
Kyle, Stuart; Aggeli, Amalia; Ingham, Eileen; McPherson, Michael J; “Recombinant Self-Assembling Peptides as Biomaterials for Tissue Engineering”; Biomaterials, 31(36); December 2010, p. 9395-9405
Scitable, “Peptide Definition”, www.nature.com/scitable/definition/peptide-317
Holmes, Todd C. “Novel peptide-based biomaterial scaffolds for tissue engineering; Trends in Biotechnology, Vol. 20 No. 1 January 2002