Overview of Hydroxyproline (Hyp)

Structure, Metabolism, and Functions of Hydroxyproline

The non-protein amino acid hydroxyproline (Hyp) has a molecular weight of 131.13 g/mol. It is a byproduct of proline hydroxylation after post-translational protein modification, most commonly detected in collagen. Prolyl hydroxylase hydroxylates proline residues during collagen manufacture, resulting in two isomeric forms: trans-4-hydroxy-L-proline (4-Hyp) and trans-3-hydroxy-L-proline (3-Hyp), with 4-Hyp being more common in collagen.

Proline is a precursor for the synthesis of hydroxyproline, and the two are very closely related. In the endoplasmic reticulum, prolyl hydroxylase converts proline to hydroxyproline. Cofactors required for this activity include 2-oxoglutarate, molecular oxygen, ascorbic acid, and Fe2+. The hydroxyproline produced is critical to collagen's structural stability and function.

Figure 1. Chemical structures of proline and hydroxyproline.
(Source: Gabr SA, et al. 2016)

Collagen is the most prevalent protein in the body, comprising approximately one-third of all proteins. Collagen's distinctive features, including as thermal and mechanical stability, stem from its triple helix structure, which is made up of repeating Gly-X-Y motifs, with Pro and 4-Hyp occupying the X and Y positions, respectively. 4-Hyp contributes significantly to collagen's mechanical and thermal stability. Collagen's melting point is inversely correlated with its 4-Hyp content, which preserves the structural integrity of the collagen by creating a web of hydrogen bonds between collagen strands. The large content of 4-Hyp in collagen, which accounts for almost one-third of all proteins in humans and other animals, demonstrates its relevance. Although the body normally has a low amount of Hyp, alterations in this level can be indicative of pathological changes in specific disorders as well as the metabolic condition of tissues. Changes in Hyp concentration can be a significant signal of the metabolic condition of tissues like skin, bones, and muscles because collagen is widely distributed in these tissues.

The liver and kidneys are where hydroxyproline is primarily metabolized. 4-hydroxy-L-proline oxidase converts 4-Hyp to glyoxylate, which is subsequently broken down into glycine. In contrast, the 3-hydroxy-L-proline dehydratase pathway breaks down 3-Hyp to produce glutamate and ornithine. To prevent cellular damage, the intermediate hydroxyproline metabolites, such as hydrogen peroxide and glyoxylate, must be transformed quickly.

Figure 2. The metabolic pathway of 4-hydroxyproline in different vertebrate species.
(Source: Belostotsky R, et al. 2022)

Hyp serves a variety of vital functions in organisms. To begin with, as a collagen component, it is critical for tissue structural stability and mechanical strength. Second, Hyp plays important functions in physiological processes such as cell signaling, hypoxia response, and antioxidant reactions. Hyp, for example, phosphorylates protein kinase B and DYRK1A to modulate cell signaling. Additionally, the metabolism of Hyp is related with antioxidant reactions, which assist to remove reactive oxygen species in the body.

Applications of Hyp in Regenerative Medicine and Nutritional Cosmetics

Hyp has important implications in regenerative medicine, particularly in maintaining protein structure and function. Hyp is a fundamental component of collagen, which is commonly employed as a biomaterial in tissue engineering and regenerative medicine. Because the high concentration of Hyp in collagen improves its structural stability and mechanical strength, it is widely used in the production of artificial tissue scaffolds for tissue repair.

Collagen is utilized as a scaffold material in tissue engineering to produce a variety of tissues. Hyp's presence in collagen contributes to the structural stability and function of these scaffolds. For example, Hyp can increase the mechanical strength of scaffolds, allowing them to better support cell development and tissue regeneration following implantation in the body. Hyp is also utilized to create polymers that restore injured tissues. Hyp, which contains collagen, can aid in wound healing and skin renewal. Hyp may also improve the mechanical strength of freshly produced tissue, bringing it closer to genuine tissue. In addition to being utilized as scaffold and repair materials, Hyp is actively being explored for its applications in bone regeneration and cartilage repair, where it provides the mechanical strength and biocompatibility required to support tissue regeneration and repair.

Due to its part in preserving the structure and function of collagen, Hyp is extremely important in the nutrition and cosmetics sectors. Hyp is commonly utilized in skincare products since it aids in the structural function of collagen in the skin. Hyp slows skin aging by increasing collagen stability, promoting suppleness, and firmness. Patients with disorders such as systemic lupus erythematosus report improved skin health after using Hyp.

Hyp is also commonly utilized in a variety of cosmetic products, including lotions, serums, and masks, due to its particular action on collagen. Hypnosis can boost the anti-aging effects of these products, improving skin texture and luminosity. Hyp is used in nutritional supplements to help the body produce and maintain collagen. Supplementing with Hyp promotes skin, bone, and joint health while reducing tissue deterioration caused with aging. Hyp is also utilized in some functional foods and beverages to boost their health effects.