Structure
Below, we discuss some of the key structures of collagen and elastin. Because both are proteins, they not only have repeating units of multiple types of amino acids, but they also have chains, helices, multiple strands, and cross-linking. Further, collagen and elastin are found in a vast array of body structures; therefore, our discussion will center around the most common structures - Type 1 collagen/collagen fibrils for collagen, and the more generalized structure for elastin.
Collagen
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Collagen is a protein, therefore it is made up of amino acids, in a repeating pattern. “The primary amino acid chain for collagen is of glycine-proline-X or glycine-X-hydroxyproline” where X is any other amino acid (Wu et al 2020; Szulc 2018). The individual monomers: glycine, proline, and hydroxyproline (note that amino acid X is not listed below; for more information on all 28 subtypes of collagen and their accompanying amino acids, refer to Dong & Lv, 2016):
From left to right: glycine (G-Biosciences, n.d.), proline (TCI Chemicals, n.d.), and hydroxyproline ("Hydroxyproline," 2021).
In their triple-helical domains, collagens have 3 chains, called alpha chains, wound in a tight, stress-resistant triple helix
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(Wu et al., 2020; Granta 2021). Sometimes those three chains are the same, called homotrimers, or different, called heterotrimers, a defining property of each type of collagen. Each of those triple-helical domains are shaped like rods, flexible yet stiff, due to their amino acid imperfections and helical nature, respectively (Ricard-Blum, 2011). A typical triple helix can be seen to the right:
Right: "Collagen Triple Helix," with alanine irregularity where there should be glycine. Recall the typical structure is glycine-proline-X or glycine-X-hydroxyproline (Goodsell 2020).
Collagens are “multi-domain proteins,” characterized by different sections with different properties. All have triple-helical domains, as described above, but there is also wide variation in the other types and locations of domain, dependent on type of collagen (e.g. non-collagenous domain, C-terminal propeptide, membrane domains, thrombospondin domains, etc.) (Ricard-Blum, 2011). Different local domains are associated with slight variations in chemical and material properties, such as hydrophilicity, plasticity, flexibility, and molecular recognition (Ricard-Blum, 2011; Bella et al., 2006). Extensive diagramming of the different domains of collagen fibers is described by (Ricard-Blum, 2011, Figure 1). One such example of collagen with its many domains can be seen below:
Above: Alpha 1 & alpha 2 strands of type 1 collagen, a fibril forming collagen. (Ricard-Blum, 2011, Figure 1). Below, left: collagen fibrils from tendon, pictured from an atomic force micrograph. Shows axial alignment (Fratzl, 2008, pp. 50). Below, right: collagen fibrils from tendon, pictured from an electron micrograph. Shows 67 nm repetition of strands.
Typically, collagen triple helices in fibrils are cross-linked, and some level of crystalline. Typically, collagen fibrils (the most common, type 1), have a high degree of crystallinity, although they are thought to still contain areas of amorphous structures. Electron micrographs most agree with fibril models that place crystals in concentric circles, with periods of order and disorder along the grain boundaries. When packed densely, type 1 collagens (among others) even exhibit liquid crystal properties, where their behavior is dependent on only relatively nearby molecular interactions. This type of semi-crystallinity, a balance between order and disorder is common in many biological polymers (Fratzl, 2008, pp. 50-61). The micrographs to the left show some of the repetition of strands within fibrils.
Below: "Supramolecular assemblies formed by collagens," (Ricard-Blum 2011, Figure 2.
Macrostructures of collagen also vary widely: again, because collagens are found most everywhere in the body, they serve a wide array of functions and therefore macrostructures. An illustration of some of the most common macrostructures of collagen can be seen to the right. The most abundant macrostructure of collagen is the fibril, at the top left of the image. Type 1 collagen fibers form fibrils, basically tubes of fibers with a periodicity of 64-67 nm. In general, these collagen fibrils will range between 15 and 500 nm in diameter and allow the collagen strands to align, increa-
-sing/improving their structural properties (Ricard-Blum, 2011; Orgel et al., 2006). Typical fibril collagen molecules have a length of about 300 nm, but the length of the molecule varies widely based off of the subtype and alpha strand; Type VII collagen has one of the longest triple-helical regions of vertebrate collegens at 420 nm (Fratzl, 2008). Due to the wide variation in types of collagen, the molecular weight is quite inconsistent, however some (mayhap unreliable) sources have said that a typical Type 1 collagen fibril is about 300,000 g/mol (AAT Bioquest 2018). The fact that this number exists is a testament to its lack of reliability, as collagen is really, again, 28 subtypes, in multiple configurations and strands and cross-links. Hydrolysed collagen can range in the hundreds to tens of thousands g/mol in molecular weight (Hong et al., 2019). Other types are almost certainly different weights due to the variation in amino acid type, etc.
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Elastin
Elastin at its base macrostructure is a combination of tropoelastin structures combined into one structure. Macrostructure shows that oftentimes the elastin is crosslinked with many smaller pieces of tropoelastin so the structure is very open (Daamen et al., 2007). This open structure leads to more elasticity, but can easily break down due to the small diameter of the individual links. This is part of the reason why elastin is so sensitive to any environmental change including sunlight and water where the material
Above: Image of Elastin's macrostructure with additional how the 3D helical structure is created with the monomer and repeating units (Research Gate 2020)
becomes very brittle (Debelle & Alix 1999).
In addition, elastin is also protein-based which allows for it to be created from many amino acid monomers. Bovine elastin has been shown to have a “higher entropy than its monomer” (Debelle & Alix 1999) which determines that even though the tropoelastin is soluble whereas elastin is insoluble in water. This determines that the helical cross linked structure of elastin is able to stabilize the instability of the tropoelastin and overall create a structure that is able to be used in other applications while staying reliable.
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Left: The elongation of elastin from its helical macrostructure (Look for Diagnosis 2020)
In its microstructure, elastin is made up of a combination of carbon, hydrogen, oxygen, and nitrogen. Inside, these atoms are able to form hexagonal amino acid structures. These molecules combine to form “tropoelastin molecule [that] typically consists of two types of domains encoded for by separate exons: hydrophobic domains with many Gly, Val, Ala and Pro residues which often occur in repeats of several amino acids, like Gly-Val-Gly-Val-Pro, Gly-Val-Pro-Gly-Val and Gly-Val-Gly-Val-Ala-Pro, and hydrophilic domains with many Lys and Ala residues which are important in crosslinking.” (Daamen et al., 2007). The hydrophobic and hydrophilic domains allow for a van der waals connection between atoms and allow for cross-linking to become more reliable in the helix structure.
Above: The repeating unit of elastin in the elastin microstructure, these are amino acids; also indicated by (Daamen et al., 2007) referenced by "Gly-Val-Gly-Val-Pro" (Peptide Bonds)
Above: Overview of elastin's uses and how it is created in the body; both micro and macrostructure (Animated Biology with Arpan, 2020)