Structural proteins are required for the formation and support of body tissues. Collagens, characterised by the repeating Gly–X–Y amino acid sequences, are the most abundant structural proteins in the human body. The collagen gene family comprises 45 genes encoding 28 homo- or heterotrimeric collagen types. With some exceptions, most collagen genes are scattered on different chromosomes. All except two of the collagen genes are highly interrupted, with exon numbers up to 118. This is primarily because exons coding for the collagenous domains are relatively small and usually encode a discrete number of Gly–X–Y repeated sequences, and as such the lengths are multiples of 9 base pairs (bp). In the fibrillar collagen genes, exons encoding the Gly–X–Y sequences are most frequently 54 bp or a variation thereof, and start with complete codons for glycine. In the nonfibrillar collagen genes, the most frequent exon sizes are 27, 36 or 63 bp.

Key Concepts:

• Collagens are the most abundant structural proteins in the human body, accounting for approximately 25% of the total protein mass.
  
• Collagens are homo- or heterotrimers, in which three collagen polypeptides, called chains, align in parallel and fold into a unique triple-helical structure, which is based on the repeating Gly–X–Y amino acid sequence of the chains.
  
• The collagens are generally divided into two major classes: fibrillar collagens that form characteristic cross-striated fibrils and nonfibrillar collagens that do not form cross-striated fibers.
  
• To date, 28 vertebrate collagen types, I–XXVIII, encoded by 45 distinct genes, have been described in the literature.
  
• With some exceptions, most collagen genes are scattered on different human chromosomes.
  
• Collagen genes are large and highly interrupted, except for the genes for types VIII and X collagen.
  
• In fibrillar collagen genes, exons coding for the Gly–X–Y sequences are most frequently 54 bp or a variation thereof, and start with complete codons for glycine.
  
• The nonfibrillar collagen genes do not show the 54-bp exon size pattern. The most frequent exon sizes are 27, 36 or 63 bp, and some exons start with a split codon for glycine.