Hydroxylysine is a unique amino acid in the human body. Unlike the 20 standard amino acids that directly form most proteins, hydroxylysine is a modified version of lysine. It is not incorporated into proteins during their initial construction but is formed afterward through a specific chemical change. This modified amino acid is primarily recognized for its significance in collagen, the body’s most abundant protein.
The Biological Role of Hydroxylysine
Hydroxylysine contributes to the stability of collagen, which forms the framework for skin, bones, tendons, and ligaments. Collagen molecules are composed of three polypeptide chains that intertwine to form a triple helix. This structure provides connective tissues with their characteristic strength and resilience.
Hydroxylysine serves as an attachment point for sugar molecules, a process known as O-linked glycosylation. These attached sugars are covalently linked to hydroxylysine residues. This glycosylation is a precursor to the formation of strong, stable cross-links between individual collagen fibers.
These cross-links are similar to mortar holding bricks in a wall, providing remarkable tensile strength to tissues. They prevent collagen fibers from sliding past each other, reinforcing the overall structure of connective tissues. The presence and proper modification of hydroxylysine are important for the mechanical properties of tissues like bone and tendon.
How the Body Creates Hydroxylysine
The body does not directly obtain hydroxylysine from the diet; instead, it synthesizes it from the amino acid lysine. This conversion occurs through post-translational modification, meaning the change happens after the initial protein chain containing lysine has been assembled. Within the endoplasmic reticulum, an enzyme called lysyl hydroxylase catalyzes this transformation.
Lysyl hydroxylase adds a hydroxyl group to specific lysine residues, converting them into hydroxylysine. This enzymatic reaction requires vitamin C as an important cofactor. Vitamin C ensures that lysyl hydroxylase functions correctly, allowing the hydroxylation of lysine to proceed efficiently. Without adequate vitamin C, this step in collagen synthesis is impaired, affecting the stability of the collagen structure.
Health Conditions Linked to Hydroxylysine Metabolism
When the body’s ability to create or properly utilize hydroxylysine is impaired, health conditions can arise, primarily affecting connective tissues. One such condition is Kyphoscoliotic Ehlers-Danlos Syndrome (kEDS), caused by a deficiency in lysyl hydroxylase 1. This genetic disorder leads to unstable collagen due to improper cross-linking of collagen fibers.
Individuals with kEDS often experience symptoms such as severe muscle hypotonia at birth, progressive curvature of the spine (kyphoscoliosis), and generalized joint hypermobility that can lead to frequent dislocations. Their skin may also be unusually stretchy, fragile, and prone to easy bruising and poor wound healing. The lack of functional lysyl hydroxylase disrupts the formation of stable collagen, impacting the integrity of various tissues throughout the body.
A classic example of impaired collagen synthesis due to a lack of a necessary cofactor is scurvy, caused by severe vitamin C deficiency. Without sufficient vitamin C, the lysyl hydroxylase enzyme cannot perform its hydroxylation function effectively. This leads to the production of unstable collagen, manifesting in symptoms like bleeding gums, easy bruising, poor wound healing, and weakened blood vessels. Scurvy highlights the direct link between dietary intake of vitamin C and the body’s capacity to synthesize functional collagen through proper hydroxylysine formation.
Dietary Sources and Supplementation
Hydroxylysine is not an amino acid that needs to be consumed directly through diet, as the body produces it internally from lysine. Therefore, supporting the body’s natural synthesis pathways involves ensuring adequate intake of both lysine and vitamin C. Lysine is an essential amino acid, meaning the body cannot produce it and it must be obtained from food.
Good dietary sources of lysine include protein-rich foods such as meat, fish, dairy products like cheese and eggs, and plant-based options like legumes, tofu, and quinoa. Consuming a balanced diet with sufficient protein provides enough lysine for the body’s needs. Vitamin C, a cofactor for hydroxylysine production, is abundant in citrus fruits, bell peppers, broccoli, and various berries.
While some dietary supplements may contain hydroxylysine, these are not common, and scientific evidence supporting their direct effectiveness compared to supporting the body’s own synthesis is limited. The most effective approach for maintaining healthy collagen is to ensure a diet rich in both lysine and vitamin C, allowing the body to naturally produce and utilize hydroxylysine as needed for strong connective tissues.