Types of Collagen: The Collagen Family, the Five Types That Matter Most, and How Collagen Is Made

Collagen is the most abundant protein in animals, but it is not one molecule. Vertebrates make 28 types of collagen, from the rope-like fibrils of tendon to the mesh of basement membranes. Whether a paper concerns a genetic disorder, a bone-turnover marker or a copper peptide in fibroblast culture, the first question is which collagen it measured.

This guide covers how collagen types are classified and where they are found, collagen synthesis step by step, the related disorders and turnover markers, and the evidence on collagen peptides.

What is collagen? A family of 28 proteins

Collagen is a family of proteins rather than a single one. Collagens are the most abundant proteins in mammals, and vertebrates have 28 of them, numbered I to XXVIII and built from alpha chains encoded by more than 40 genes. What makes a protein a collagen is at least one triple-helical domain. Most collagens assemble into larger structures in the extracellular matrix; four are membrane proteins that can also be shed from the cell surface in soluble form.

Because the numerals say little about function, collagen types are grouped by what they build. Fibril-forming collagens make the banded fibrils of skin, bone, tendon and cartilage, which repeat every 67 nanometres (the D-period). Type IV builds the network of basement membranes; FACITs (fibril-associated collagens with interrupted triple helices) sit on fibril surfaces; type VI forms beaded filaments and type VII anchoring fibrils; and cleaving the C-terminal domains of the multiplexins XV and XVIII releases endostatins.

The triple helix every collagen shares

Each collagen molecule has three polypeptide chains, called alpha chains, built on a repeating Gly-X-Y triplet: glycine at every third position, with proline and 4-hydroxyproline often at X and Y. Each chain twists into a left-handed polyproline II helix, and the three wind around one another into a right-handed superhelix. Glycine, the smallest amino acid, sits at the crowded centre of that helix, which lets the chains pack closely; our guide to glycine and proline structure explains both residues.

Hydroxyproline is not specified by the genetic code. Prolyl 4-hydroxylases make it from proline after the chain is built, and it is a major contributor to the triple helix’s thermodynamic stability, which is why vitamin C matters to collagen. Chain make-up varies by type: type I has two α1(I) chains from COL1A1 and one α2(I) chain from COL1A2, while type IV draws on six chains from six genes, assembled into three distinct heterotrimers.

The main types of collagen and where they are found

Type I collagen is the most common collagen in skin, bone, teeth, tendon and ligaments, and the main matrix protein osteoblasts secrete as bone develops. Type II works with types IX and XI in cartilage, and type V, a minor but widely distributed collagen, is needed to nucleate type I fibrils.

Collagen synthesis, step by step

Collagen synthesis starts inside the cell and finishes in the extracellular matrix. For a fibril-forming collagen such as type I:

1. Translation. Procollagen alpha chains, each with a propeptide at both ends, are made on the rough endoplasmic reticulum (ER).
2. Hydroxylation. Prolyl 4-hydroxylases convert selected prolines to 4-hydroxyproline, and lysyl hydroxylases convert selected lysines to 5-hydroxylysine. Prolyl hydroxylation requires ferrous iron (Fe²⁺), 2-oxoglutarate (α-ketoglutarate) and ascorbate, the chemical name for vitamin C.
3. Glycosylation. Some hydroxylysine residues are O-glycosylated.
4. Folding. The C-propeptides of two α1 chains and one α2 chain form the nucleus of the triple helix, which then folds in a zipper-like fashion, assisted by the collagen-specific chaperone HSP47.
5. Secretion. Procollagen moves from the ER to the Golgi by COPII-dependent transport and is secreted.
6. Propeptide removal. Outside the cell, proteinases including BMP1 and ADAMTS-family enzymes cut off the propeptides, leaving tropocollagen.
7. Fibril assembly. Tropocollagen assembles spontaneously into a quarter-staggered array with the 67 nm D-period; type V collagen nucleates type I fibrils.
8. Cross-linking. Lysyl oxidase, a copper-dependent amine oxidase with a lysine tyrosylquinone cofactor, oxidatively deaminates specific lysine and hydroxylysine residues: the first step of the covalent cross-links behind the matrix’s tensile strength.

Hydroxylation is where vitamin C and iron enter the collagen story; cross-linking is where copper does, as an enzyme cofactor. That is not evidence that a copper peptide changes cross-linking in tissue.

When collagen goes wrong: scurvy and inherited disorders

Scurvy

Ascorbate is required for both the hydroxylation and the secretion of procollagen, and that requirement underlies the inhibition of collagen synthesis in scurvy. Canada has a famous case: in the winter of 1535–36 at Stadacona, now Quebec City, scurvy struck Jacques Cartier’s crew, and by mid-February 8 of the 110 men had died. The crew recovered after the Iroquois shared a decoction of a tree they called annedda, usually identified as eastern white cedar; Durzan’s 2009 account credits the decoction’s ascorbic acid.

Osteogenesis imperfecta

In about 85% of people with osteogenesis imperfecta (OI), or brittle bone disease, the cause is a dominant mutation in COL1A1 or COL1A2 that alters the quantity or structure of type I collagen; most other forms are recessive defects in genes involved in making or processing it. In 2000, Francis Glorieux, Frank Rauch and colleagues at McGill University and Montreal’s Shriners Hospital described type V OI, in which no causative mutation was found in the type I collagen genes.

Ehlers-Danlos syndromes

The classical type of Ehlers-Danlos syndrome is associated with the type V genes COL5A1 and COL5A2. The vascular type results from mutations in COL3A1, the gene for type III procollagen, which is found in blood vessels; in a 2000 study of 220 patients with this type, most deaths were due to arterial rupture.

Alport syndrome

Alport syndrome is caused by variants in COL4A3, COL4A4 or COL4A5, which encode the α3, α4 and α5 chains of type IV collagen. Those chains form the network of the glomerular basement membrane, the cochlear basement membrane and the base of the lens, which is why the kidney, inner ear and eye are involved. About 80% of cases are X-linked, caused by COL4A5 variants.

Measuring collagen turnover: PINP and CTX

Synthesis and breakdown both release fragments, so collagen turnover can be followed in blood. A joint working group of the International Osteoporosis Foundation (IOF) and the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) recommended serum procollagen type I N-propeptide (s-PINP) as the reference marker of bone formation and serum C-terminal telopeptide of type I collagen (s-CTX) as the reference marker of resorption in clinical studies. PINP is the propeptide cut away in step six above; CTX comes from the mature molecule as bone collagen is broken down.

Collagen peptides and hydrolyzed collagen: what the research shows

“Collagen peptides” and “hydrolyzed collagen” describe the same material: low-molecular-weight peptides made by denaturing collagen, which separates the alpha chains into random coils, then cutting the chains with proteolytic enzymes, typically into peptides of about 3–6 kDa. Sources include bovine tendon and skin, porcine skin, chicken, and fish skin or scales; our guide to peptide molecular weight explains how such masses are calculated.

Do collagen peptides reach the blood?

Some fragments do. In a 2005 study, peptide-bound hydroxyproline in volunteers’ blood, negligible beforehand, peaked one to two hours after they ingested gelatin hydrolysates; the main species was the dipeptide prolyl-hydroxyproline (Pro-Hyp). In 2011 the same research group identified hydroxyprolyl-glycine (Hyp-Gly) in human blood. These studies show what appears in plasma, not where the peptides go next or what they do in skin, cartilage or bone.

What the randomized trials measured

The trials pooled in two recent meta-analyses measured skin hydration, elasticity and wrinkles. A 2021 meta-analysis of 19 randomized controlled trials (1,125 participants, 95% of them women) reported results favouring hydrolyzed collagen over placebo on all three. A 2025 meta-analysis of 23 trials (1,474 participants) found the same pooled direction, but trials without pharmaceutical-company funding showed no significant effect on any outcome, and neither did the high-quality trials. Its authors concluded that there is currently no clinical evidence supporting collagen supplements for preventing skin aging.

Matrikines: collagen fragments as signals

Peptides derived from extracellular-matrix proteins that regulate cell activity are called matrikines. Collagen-linked examples sit at very different evidence levels.

A palmitoylated form of KTTKS, whose fatty-acid tail aids delivery across the skin and resistance to skin proteases, is a well-known cosmetic “signal peptide”. Cosmetic peptide classes include signal peptides like it; carrier peptides such as GHK-Cu, which complexes copper; and neurotransmitter-inhibitor peptides such as acetyl hexapeptide-3 and SNAP-8, which interfere with SNARE-complex assembly in vitro. Only the first two classes connect to collagen biology; see our guides to peptides in skincare and acetyl hexapeptide-3 and SNAP-8. Nothing here is a claim about how skin looks or ages.

The 1988 GHK-Cu result is also cell work: Maquart, Loren Pickart and colleagues reported that the complex stimulated collagen synthesis in fibroblast cultures, which describes cells in a dish, not living tissue. Our GHK-Cu research guide covers the wider literature, and GHK-Cu vs AHK-Cu compares its alanine analogue.

What’s established and what isn’t

Sourcing collagen-research peptides in Canada

Copper peptides such as GHK-Cu and AHK-Cu, and cosmetic-class peptides such as SNAP-8, raise the usual synthetic-peptide documentation questions, plus one for metal complexes.

• Check identity and purity separately. Mass spectrometry confirms the expected mass; HPLC estimates what share of the sample is the target peptide. See our guides to HPLC testing and the peptide COA.
• For copper complexes, ask what was characterized: the tripeptide, the copper complex, or both.
• Match the batch number on the certificate to your vial.
• Ask how peptide content was determined. Lyophilized peptides carry counter-ions and water; our guide to amino acid analysis explains how content is measured.
• Store cold, dry and dark, sealed until needed; see the peptide storage guide and our overview of research peptides in Canada.

GHK-CuThe copper tripeptide reported in 1988 to stimulate collagen synthesis in fibroblast culturesView product AHK-CuThe alanine analogue of GHK-Cu, compared in our GHK-Cu vs AHK-Cu guideView product SNAP-8A neurotransmitter-inhibitor peptide acting on SNARE assembly in vitro, the contrast class to collagen signalsView product

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