S-Adenosyl-L-homocysteine

S-Adenosyl-L-homocysteine (SAH) is a biochemical compound that plays a significant role in various metabolic processes within the human body. It is a product of transmethylation reactions, which are crucial for the synthesis of proteins, nucleic acids, and lipids.

Structure and Properties

SAH is a homocysteine molecule with an adenosyl group attached to its sulfur atom. The molecule is chiral, meaning it has a non-superimposable mirror image. It exists in two forms, or enantiomers, known as S-Adenosyl-L-homocysteine and R-Adenosyl-L-homocysteine. The "L" and "R" refer to the direction in which the molecule rotates plane-polarized light.

Biological Role

In the body, SAH is produced from S-Adenosyl methionine (SAM) through the process of transmethylation. This reaction, catalyzed by various methyltransferase enzymes, transfers a methyl group from SAM to a substrate molecule, producing SAH as a byproduct.

SAH is then further metabolized by the enzyme S-Adenosylhomocysteine hydrolase into homocysteine and adenosine. Both of these products can be reused in other metabolic pathways: homocysteine can be remethylated to methionine, and adenosine can be phosphorylated to ATP.

Clinical Significance

Abnormal levels of SAH in the body can lead to various health problems. High levels of SAH have been associated with cardiovascular disease, as they can lead to an increase in homocysteine, a known risk factor for heart disease. Additionally, SAH is a potent inhibitor of methyltransferase enzymes, so elevated levels can disrupt normal methylation processes, potentially leading to conditions such as cancer and neurological disorders.

See Also

• S-Adenosyl methionine
• Homocysteine
• Methyltransferase
• Transmethylation