Proteins-Fibrous (Structural)

The three main types of structural or fibrous protiens fround in the human body are: collagens, elastins and keratins.


~25% of the proteins found in the body are classified as collagens.

structural element of connective tissue

unique type of triple helix, stabilized by crosslinks

Amino Acid Sequence: Glycine at every third position.


Collagen Helix: Only 3.0 residues per turn vs. 3.6 for an average a-helix.

Triple Helix: Three collagen helices wound very tightly together.

Collagen Fibrils: The triple helices are arranged in staggered fashion (covalent crosslinks vital to the structure of mature collagen).

Important specializations seen in collagen:

1) Proline is very abundant and can be hydroxylated by prolyl hydroxylase to form hydroxyproline. Co-factor for prolyl hydroxylase is: ascorbic acid (vitamin C).

Proline --------> Hydroxyproline

2) Lysine is abundant and can be hydroxylated by lysyl hydroxylase to form hydroxylysine. Co-factor for lysyl hydroxylase is: ascorbic acid (vitamin C).

Lysine --------> Hydroxylysine

3) Hydroxylysines can be glycosylated on their newly acquired -OH groups. (sugar attachment)

HO-Lysine ------> "Glycosylation"

4) Cysteine residues at the ends of the collagen helices are oxidized to form disulfide bridges, this places the chains in proper orientation for triple helix formation.

2 Cysteine Residues -------> Disulfide Bridge

5) Covalent crosslinks are formed between allysine and lysine during fibril formation:

(a) Oxidation of certain lysine and hydroxylysine residues converts them from amines to active aldehydes (allysines). The enzyme for this reaction is lysyl oxidase, co-factors: copper, Vitamin B6.

Lysine-NH3+ ------> Allysine-C=O

(b) A reaction between these aldehydes and free amino groups on other lysine residues forms a "Schiff" base, then covalent crosslink.

Lysine-NH3+ + Allysine-C=O ---------> "Schiff" Base

Collagen Maturation:




HO-Pro and HO-Lys

Glycosylation some HO-Lys

Disulfide Formation

Folding "Triple Helix"


Assembly into Fibrils

Lys, HO-Lys to Allysine (active Aldehydes)

"Schiff" Base Formation (Covalent Crosslinks)

Elastin: is another important class of insoluble fibrous proteins.

Elastin lacks the genetic complexity, diversity and helical structure of collagen.

Two features elastin and collagen share:

1) Some prolines are hydroxylated. (not Lysine, so no glycosylation)

2) Some Lysines are oxidized to active aldehydes (allysine).

Unique to elastin:

Four lysines are crosslinked in a unique fashion called desmosine.

Keratins: insoluble structural proteins found in skin, nails and hair

There are two types of keratins:

a-Keratins ( a-helix)

b-Keratins ( b-sheet)

In hair and nails the a-helices of adjacent strands are crosslinked by disulfide bridges.

Nails: there are numerous disulfide bridges making them resistant to chemical alteration ----> "hard"

Hair: there are relatively few disulfide bonds and they can be easily disrupted with "reducing" agents.

Permanent Wave:

1) Disulfides are reduced

2) Cosmetic rearrangement (curlers)

3) Free -SH groups are oxidized to reform disulfides


Some heritable diseases related to collagen:

Osteogenesis Imperfecta

Ehlers-Danlos Syndrome

Menkes' Disease

© Dr. Noel Sturm 2020

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