Lipocalin

The lipocalin family of proteins is a functionally and structurally diverse group. Although these proteins are most commonly found in gram negative bacterial cells, vertebrate cells, and invertebrate cells, plant lipocalins have also recently been researched. Lipocalins have been associated with many biological processes, among them immune response, pheromone transport, biological prostaglandin synthesis, retinoid binding, and cancer cell interactions.

Immune Response
Lipocalin proteins are involved in inflammation and detoxification processes caused by immune system activation in mammals. They are known respiratory allergens of mice, cats, dogs, horses, and other animals. Examples of lipocalin proteins involved in immune system responses include alpha-1-microglobulin, alpha-1-acid glycoprotein, and c8gamma. Structural information for many immune system influencing lipocalin proteins is available, while their exact role in biological systems is still somewhat unclear. Human allergenic reactions to lipocalins have not been extensively investigated.

Pheromone Transport
The lipocalin family has been connected with the transport of mammalian pheromones due to easily observable protein-pheromone interactions. Lipocalins are comparatively small in size, and are thus less complicated to study as opposed to large, bulky proteins. They can also bind to various ligands for different biological purposes. Lipocalins have been detected as carrier proteins of important pheromones in the nasal mucus of rodents, as well as mouse and rat urine.

Prostaglandin Synthesis
This family of proteins plays a part in the biological system of terminal prostaglandin synthesis.

Retinoid Binding
Retinol, (vitamin A), is an important micronutrient that affects eyesight, cell differentiation, immune system function, bone growth, and tumor suppression. Retinol absorption and metabolism depends on lipocalins that act as binding proteins. Retinyl esters (present in meats) and beta-carotene (present in plants) are the two main sources of retinoids in the diet. After intake, they are converted to retinol, successively metabolized, and finally bound to retinol binding proteins (lipocalins) in the blood plasma.

Cancer Cell Interactions
Because lipocalins are extracellular proteins, their intracellular effects are not obvious, and demand further study. However, lipophilic ligands, present as substituents to the lipocalins, have the ability to enter the cell, where they can act as tumor protease inhibitors. This research suggests another possible route of protein-tumor investigations.

Structure
Although lipocalins are a broad family of greatly varied proteins, their three-dimensional structure is a unifying characteristic. Lipocalins have an eight-stranded, antiparallel, symmetrical β-barrel fold, which is in essence a beta sheet which has been rolled into a cylindrical shape. Inside this barrel is located a ligand binding site, which plays an important role in the lipocalin classification as a transport protein. Lipocalins have desirable properties in terms of crystallization ability, molecular size, and commercial availability. Structure determination involves such processes as crystallization and multi-dimensional Nuclear Magnetic Resonance spectroscopy (NMR). Specifically, this NMR technique is known as NOESY (Nuclear Overhauser Effect Spectroscopy), and is used to determine structures of otherwise complex macromolecules such as proteins.