Fibrous Proteins: Definition, Collagen, Function and Studies of This Protein

It is the type of protein that is only found in animals and has a rod-like shape, which can look like a wire around a structure.

Another name used for such types includes scleroproteins and is used primarily as a storage protein that becomes useful when there is a lack of such nutrition in the body.

Fibrous proteins are long filamentous protein atoms. Fibrous proteins frame “pole” or “wire” shapes, and are latent capacity or auxiliary proteins. They are insoluble in water.

A fibrous protein is a protein with an elongated shape. Fibrous proteins provide auxiliary support to cells and tissues.

There are extraordinary types of helices present in two fibrous proteins α-keratin and collagen. These proteins frame long filaments that serve a fundamental part of the human body.

Fibrous proteins consist of elongated polypeptide chains that run parallel to each other and are stabilized by cross-linking. In humans, its main function is to provide structure, support and aid in biomechanics.

They are not found in differentiated plants. The most commonly found fibrous protein (and protein in general) is collagen, which makes up about 30% or more of the total protein in the body.

Fibrous protein and collagen

The basic structure of all collagens is a triple helix, however collagen can be grouped into at least 16 types based on the three-dimensional structures formed at the points where the helix breaks.

The basic triple helix comes from a great abundance of glycine, proline, and hydroxyproline in its primary structure. Each of these amino acids contributes to the formation and stability of the triple helix.

Glycine fits into the crowded center of the helix, where a hydrogen bond can take place between its peptidyl group and another peptidyl group on an adjacent polypeptide chain and hold the structure together.

Proline and hydroxyproline allow true twisting of the chains resulting in the characteristic helical formation.

Collagens are major components of bone (type I collagen), cartilage (type II collagen), blood vessels, tendons, and other components of the body. Like most fibrous proteins, collagen is insoluble in an aqueous medium.

Function of fibrous proteins

Fibrous proteins are often specifically designed to aggregate and form stringy bundles. Examples of these include fibrous proteins in the collagen and α-fibrous classes.

Filament formation requires that the complementary groups in two or more molecules be in appropriate axial and azimuthal orientations to facilitate interaction.

For example, the presence of periodic groups of acidic residues in one molecule could be close to periodic groups of basic residues in another. This could facilitate the formation of a network of stabilizing intermolecular ionic interactions and therefore specify a unique mode of aggregation.

Similarly, apolar residue patches in different molecules could be in positions where the two areas could join and protect both regions from the aqueous environment, stabilizing and specifying the mode of assembly.

Complementarity of form and hydrogen bonding potential are other means used in vivo to provide assembly and interaction specificity.

However, for these possibilities to be realized for filament-favoring molecules, two special sequence-related characteristics are required.

First and by definition, a filamentous structure implies the presence of a contiguously repeated relatively short-range motif. Such a motif will adopt a particular conformation, often one of the well-known elements of the secondary structure (α-helical, β-chain, or similar to collagen).

As such residues or groups of residues would naturally favor similar environments, it is extremely likely that the elements are helically related to each other, and that an elongated molecular structure is formed.

Second, a regular pattern of intermolecular interactions is required for filamentous assembly to occur; this also infers a corresponding regularity in the underlying amino acid sequences of the interacting molecules.

The sequence repeats in proteins vary greatly. Some are very short and some are extremely long.

The repeats can be exact or approximate, they can contain residues that are absolutely conserved in some positions but not others, and they can be extremely imprecise in a general character that is not conserved.

Some reps are set in length and some are not. Some are primarily functional and, in an apparent contradiction in terms, can also vary in length. Some repeats occur many times contiguously, while others meet only a small number of consecutive times.

Still others are individually distributed in what appears to be (but clearly is not) randomly throughout the entire protein chain. Many motifs are now well established and these have been recognized in quite diverse proteins.

Fibrous Protein Studies

Fibrous proteins provide a number of challenges to those seeking to understand them in detail at the molecular level. No less important of these challenges is trying to crystallize fibrous proteins in a form suitable for structural research using X-ray diffraction techniques.

Even fragment crystallization has proven problematic, although real progress has been made and some informative results have been obtained for intermediate filament proteins, muscle proteins, fibrinogen, collagen, and many other fibrous molecules.

In some cases, nuclear magnetic resonance methods have made it possible to avoid the crystallization stage, which makes it possible to study proteins in solution.

Of course, nuclear magnetic resonance methods have their particular problems and most of the structural data have been obtained using X-ray diffraction.

Despite these difficulties (or perhaps because of them), a set of chemical and physical methods has been devised to facilitate the research community in their quest to gain a deep understanding of fibrous proteins.

Some of these are theoretical and some are experimental. Among the above methods are those based on bioinformatics. These methods use modeling and pattern recognition techniques to identify sequences and structural motifs previously discovered and characterized in detail.

These motifs are often short in length and repeat consecutively many times. In fact, these two characteristics are characteristic of fibrous proteins.

The repeats are not limited to fibrous proteins and many are found in globular proteins (although these are usually quite long compared to those seen in fibrous proteins).

Consequently, they lead to domains of reasonable size (perhaps 20-50 residues but sometimes much larger).

An appreciable number of α-fibrous structures have now been resolved at atomic resolution, allowing some of the key structural principles to be recognized and incorporated into de novo design methods.

Much progress has also been made on specific members of the class of α-fibrous structures.

These include the intermediate double-stranded (double helix) filamentous molecules and some of their associated proteins, the single-stranded (but triple helix) spectrin / α-actinin / dystrophin molecules, and also the three-stranded (triple helical) fibrin-fibrinogen molecule.