Fiber Formation Induced by Hydrophobicity

blue, pink, green, red, blue and yellow microscopic image of arteries

Biological Strategy

Vertebrates (Mammals, Fish, Birds, Reptiles)

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Chemically Assemble Organic Compounds

Part of the reason that synthesis reactions (chemical assembly) can occur under such mild conditions as ambient temperature and pressure in water is because most often, they occur in a stepwise, enzyme-mediated fashion, sipping or releasing small amounts of energy at each step. For example, the synthesis of glucose from carbon dioxide in the Calvin cycle is a 15-step process, each step regulated by a different enzyme.

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Modify Solubility (Dissolving, Emulsifying, Precipitating, Crystallizing)

Increasing or decreasing the solubility of chemical constituents is one way of managing a host of metabolic processes including material synthesis. For example, organisms that produce calciferous shells utilize dissolved calcium and carbonate ions, but when the time is right, they raise the pH to create alkaline conditions that trigger calcium carbonate to precipitate out of solution as an insoluble solid.

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Vertebrates (Mammals, Fish, Birds, Reptiles)

Subphylum Vertebrata (“jointed”): Mammals, fish, birds, reptiles

Vertebrates are a subgroup of chordates, which all have a flexible, rod-shaped structure that supports the body called a notochord. Non-vertebrate chordates include tunicates, hagfish, and lancelets. In vertebrates the notochord ultimately becomes part of the spine, usually encased in bony joints. All chordates also have a dorsal hollow nerve cord that forms the nervous system and pharyngeal slits that open outside the body during development (and persist to form gills in aquatic animals). Lastly, chordates have a tail at the back of the body––it’s just that sometimes you need an x-ray to see it.

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Elastin fibers in arteries form when tropoelastin molecules come out of solution thanks to solubility-enhancing hydrophobicity.

“Soluble tropoelastin molecules are exported to the extracellular matrix, where they come out of solution and form fibres in all vertebrates. Fibrillogenesis therefore requires that tropoelastin be first soluble and then insoluble. If a tropoelastin molecule is naturally soluble, fibre formation could be initiated by aggregation of large hydrophobic patches, but it may be difficult to collapse the entire molecule. The opposite strategy would be to make tropoelastin itself insoluble, making fibre formation easy, and use chaperone proteins to keep it from coming out of solution, or coacervating, prematurely. Since all elastins are hydrophobic, all should have the potential to coacervate, but the coacervation temperature in some is probably too high for it to occur naturally. The coacervation temperature can be lowered by increasing the hydrophobicity.” (Chalmers et al. 1999)

Image: Kielty et al. 2002 / Journal of Cell Science / CC BY - Creative Commons Attribution alone

Microfibril and elastic fibre formation. Fibrillin is assembled pericellularly into microfibrillar arrays that appear to undergo time-dependent maturation into beaded transglutaminase-crosslinked microfibrils. Mature microfibrils form parallel bundles that may be stabilised at inter-microfibrillar crosslinked regions. In elastic tissues, tropoelastin is deposited on microfibril bundles, and lysyl oxidase-derived crosslinks then stabilise the elastin core.

Image: Kielty et al. 2002 / Journal of Cell Science / CC BY - Creative Commons Attribution alone

Domain structures of fibrillin-1 and elastin, showing molecular interaction sites identified in vitro (see Molecular Interactions). (A) Fibrillin-1 has 47 cbEGF-like domains, interspersed with TB modules. A proline-rich region is towards the N-terminus. N-glycosylation sites are indicated. (B) Elastin contains alternating hydrophobic and crosslinking domains. The C-terminus has two cysteines and a negatively charged pocket.