From FeliciaRoseLabs:

Collagen formation is vitamin C dependent

Although collagen formation is a complex process, the steps in its synthesis can be summed-up in four pivotal points.

1)      In the first step, the gene that codes for collagen needs to be expressed.  A strand of deoxyribonucleic acid (DNA) opens up to allow a copy to be made of the collagen code. In a process known as transcription, vitamin C enables the code for collagen to be copied to a molecule of messenger RNA (mRNA) that travels out into the extracellular skin matrix to begin the actual coding.

2)      In the second step, mRNA instructs fibroblast skin cells to start producing subunits of collagen known as procollagen. Procollagen will later be assembled into mature collagen.

3)      In the third step, procollagen units are glued together in sets of three by a molecule named hydroxyproline in a process known as hydroxylation. The sets of three form the famous triple helix known as a collagen fibril. Vitamin C is used to form hydroxyproline and without sufficient supply, hydroxylation can’t occur and no collagen is formed.

4)      In the fourth step, hydroxylation again takes place but this time vitamin C forms hydroxylysine which glues the collagen fibrils together into mature collagen, or collagen bundles. Vitamin C is essential for the initiation of transcription, the formation of the procollagen and the extracellular cross-linking of the collagen fibers (Murad 1981).

The image above represents a single subunit of procollagen. Three of these combine to form a triple helix as depicted in the center image. Hydroxyproline does the binding. Finally, scores of collagen fibrils (triple helix) are bound together to form a collagen fiber as depicted in the image on the right. Hydroxylysine binds collagen fibrils into collagen fibers/bundles. Vitamin C enables every step along the way.

NOTE: Hydrolyzed collagen powder supplements have also shown great promise in restoring elasticity to the skin, helping to eliminate cellulite and in maintaining a healthy lipid barrier. When more moisture is retained in the skin, natural repair processes can occur which leads to the formation of more collagen. Translation: Less wrinkling and smoother, firmer skin.