Multifarious Beneficial Effect of Nonessential Amino Acid, Glycine: A Review

Abstract

Glycine is most important and simple, nonessential amino acid in humans, animals, and many mammals. Generally, glycine is synthesized from choline, serine, hydroxyproline, and threonine through interorgan metabolism in which kidneys and liver are the primarily involved. Generally in common feeding conditions, glycine is not sufficiently synthesized in humans, animals, and birds. Glycine acts as precursor for several key metabolites of low molecular weight such as creatine, glutathione, haem, purines, and porphyrins. Glycine is very effective in improving the health and supports the growth and well-being of humans and animals. There are overwhelming reports supporting the role of supplementary glycine in prevention of many diseases and disorders including cancer. Dietary supplementation of proper dose of glycine is effectual in treating metabolic disorders in patients with cardiovascular diseases, several inflammatory diseases, obesity, cancers, and diabetes. Glycine also has the property to enhance the quality of sleep and neurological functions. In this review we will focus on the metabolism of glycine in humans and animals and the recent findings and advances about the beneficial effects and protection of glycine in different disease states.

Figure 1

Functions and metabolic fate. Glycine has multiple roles in many reactions such as gluconeogenesis, purine, haem, and chlorophyll synthesis and bile acid conjugation. Glycine is also used in the formation of many biologically important molecules. The sarcosine component of creatine is derived from glycine and S-adenosylmethionine. The nitrogen and α-carbon of the pyrrole rings and the methylene bridge carbons of haem are derived from glycine. The entire glycine molecule becomes atoms 4, 5, and 7 or purines.

Figure 2

Sequential reactions of enzymes in the glycine cleavage system (GCS) in animal cells. The glycine cleavage system (GCS) is also known as the glycine decarboxylase complex or GDC. The system is a series of enzymes that are triggered in response to high concentrations of the amino acid glycine. The same set of enzymes is sometimes referred to as glycine synthase when it runs in the reverse direction to form glycine. The glycine cleavage system is composed of four proteins: the T-protein, P-protein, L-protein, and H-protein. They do not form a stable complex, so it is more appropriate to call it a “system” instead of a “complex.” The H-protein is responsible for interacting with the three other proteins and acts as a shuttle for some of the intermediate products in glycine decarboxylation. In both animals and plants the GCS is loosely attached to the inner membrane of the mitochondria [1].