Methionine, along with cysteine, is the only sulfur-containing proteinogenic amino acid. In protein synthesis, L-methionine – its natural and biochemically active form – occupies a special position because it is always the first amino acid, the starter substance from which a protein is assembled. L-methionine is essential and serves primarily as a supplier of methyl groups (-CH3) for important hormones such as choline, epinephrine, creatine and many more.
What is methionine?
L-methionine (M or Met), the natural and bioactive form of methionine, is an essential proteinogenic amino acid. Along with cysteine, which in turn is synthesized from methionine, it is the only sulfur-containing amino acid. For the synthesis of proteins, L-methionine occupies a special position because it is always the first amino acid, the starter amino acid, for the construction of any protein. Methionine is encoded on the mRNA (messenger RNA) by the nucleic base triplet adenine-uracil-guanine (AUG), which is also known as the start codon. This means that every mRNA starts with the starter triplet AUG. In order to start protein synthesis, the tRNA (transport RNA) must first provide L-methionine before the next amino acid can be attached. Proteins consist of a string of at least 100 proteinogenic amino acids, each linked together by a peptide bond. In addition to its role as a component of many proteins, L-methionine is considered the most important methyl group supplier for the synthesis of hormones such as adrenaline, choline, creatine, histidine and many more. In addition, L-methionine is also considered a sulfur supplier for the synthesis of certain compounds in the body.
Function, effect and tasks
Methionine, in its biochemically reactive L-form, performs higher-level functions in the body’s metabolism as well as specific functions. One higher-level function is to basically form the initial amino acid of a protein. This means that protein synthesis comes to a halt if not enough L-methionine is available in the body. However, in many cases, once protein synthesis has been initiated, the methionine is split off again and recycled, so that it is then available again for the next protein synthesis. Particularly in some structural proteins, L-methionine is an important component that influences the structural strength of ligaments, tendons and fasciae. The hardness of finger and toenails and the strength of hair also depend on the number of sulfur bridges in keratin, so methionine is of great importance here. Methionine can cross the blood–brain barrier relatively easily and is involved in the formation of the myelin sheaths of the nerves – also in the CNS. Excess methionine that is not directly needed can convert to S-adenosylmethionine (SAM) by attaching to ATP (adenosine triphosphate) and act as a methyl group donor (-CH3). After releasing the methyl group, methionine can be recycled again and is available for further metabolic processes. Excess methionine can be degraded and metabolized to some extent through several steps. Additional administrations of methionine lead via the physiological degradation process to a deliberate acidification of the urine, which in the case of urinary tract infections inhibits bacterial growth and supports the effect of the administered antibiotics. Also, the low pH of the urine can dissolve calcium phosphate and magnesium ammonium phosphate stones that have begun to form.
Formation, occurrence, properties, and optimal values
Methionine is an essential amino acid, so it must be supplied externally through the diet. Many foods, both of animal and plant origin, contain methionine, but not in free form, always bound to proteins. Foods with appreciable amounts of bound methionine include raw beef, raw salmon, sesame seeds, dried soybeans and many other foods, including plant foods. Brazil nuts, with over 1,000 mg of methionine per 100 g, even have a content almost twice as high as raw salmon. The digestion of proteins takes place in the small intestine. The proteins are largely broken down into smaller pieces (polypeptides) by specialized peptidases and absorbed via the villi of the small intestine. In a balanced diet, it can be assumed that sufficient methionine is absorbed. The indications for optimal amounts vary somewhat.As a reference value, a requirement of approx. 13 to 16 mg per kilogram of body mass can be assumed for humans. A normal-weight person with a body mass of 75 kg is therefore dependent on a daily supply of methionine in the order of 975 to 1,200 mg.
Diseases and disorders
The essential amino acid methionine serves as a starting material for numerous complex metabolic processes, so that disturbances of certain conversion processes can lead to sometimes serious symptoms due to the absence of certain enzymes. A deficiency of methionine also leads to a deficiency of S-adenosylmethionine (SAM). SAM deficiency has been linked to the development of fatty liver and promotion of depression, among other things. Some disorders of methionine-cysteine metabolism, triggered by a deficiency of certain enzymes, lead to a strong accumulation of the intermediate homocysteine. The best-known cause of homocystinuria, as the accumulation of homocysteine is called, is a genetic defect that causes a deficiency in cystathionine beta-synthase. The excess homocysteine promotes the formation of thromboses and has negative effects on the connective tissue, mainly of the skeleton and the eyes, so that there is a risk of a change in the position of the eye lenses (lens ectopy). Homocystinuria also affects mental processes. When the methionine metabolic disorder leads to a deficiency of cysteine, there is also a deficiency of glutathione and taurine, which has important protective functions on the nerves. An association cysteine deficiency with the progression of Alzheimer’s disease and Parkinson’s disease has been noted.
