Methionine is an essential amino acid.
It is substrate for other amino acids such as cysteine and taurine, and glutathione.
It is encoded by the codon AUG.
It has an important role in angiogenesis.
Overconsumption of methionine has been shown to be related to cancer growth in a number of studies.
It is an α-amino acid that is used in the biosynthesis of proteins.
It contains an α-amino group, a carboxyl group, and an S-methyl thioether side chain.
It is a nonpolar, aliphatic amino acid.
A proteinogenic amino acid.
Methionine with cysteine, is one of two sulfur-containing proteinogenic amino acids.
Methionine residues generally do not have a catalytic role.
Methionine is one of only two amino acids encoded by a single codon (AUG).
((Tryptophan)), encoded by UGG, is the other.
The methionine codon AUG is also the most common start codon message for a ribosome that signals the initiation of protein translation from mRNA when the AUG codon is in a consensus sequence.
S-Adenosyl-methionine is a cofactor derived from methionine, that serves mainly as a methyl donor.
S-Adenosyl-methionine
SAM is composed of an adenosyl molecule attached to the sulfur of methionine, which allows the S-methyl group to be transferred to an oxygen, nitrogen, or aromatic system, often with the aid of other cofactors such as cobalamin (vitamin B12 ).
As an essential amino acid, methionine is not synthesized de novo in humans.
It must be ingested as methionine or methionine-containing proteins.
If it reacts with cysteine, it produces cystathionine, which is cleaved to yield homocysteine.
If it reacts with methanethiol, it produces methionine.
If homocysteine is produced, the thiol group is methylated, yielding methionine.
Two methionine synthases are known; one is cobalamin dependent and one is independent.
The pathway using cysteine is called the transsulfuration pathway.
The pathway using hydrogen sulfide (or methanethiol) is called direct-sulfurylation pathway.
Cysteine can be made from an activated serine and either from homocysteine by the reverse trans-sulfurylation route or from hydrogen sulfide by the direct sulfurylation route.
Methionine is converted to S-adenosylmethionine (SAM) by methionine adenosyltransferase.
SAM serves as a methyl-donor in many methyltransferase reactions, and is converted to S-adenosylhomocysteine (SAH).
Methionine can be regenerated from homocysteine via methionine synthase in a reaction that requires vitamin B12 as a cofactor.
Homocysteine can be converted to cysteine.
Cystathionine-β-synthase requires the active form of vitamin B6, pyridoxal phosphate, combines homocysteine and serine to produce cystathionine.
Dietary Allowances (RDAs) for essential amino acids methionine combined with cysteine, for adults 19 years and older, is 19 mg/kg body weight/day.
Food sources of Methionine:
Food g/100g
Egg, white, dried, powder, glucose reduced 3.204
Sesame seeds flour, low fat 1.656
Brazil nuts 1.124
Cheese, Parmesan, shredded 1.114
hemp seed, hulled 0.933
Soy protein concentrate 0.814
Chicken, broilers or fryers, roasted 0.801
Fish, tuna, light, canned in water, drained solids 0.755
Beef, cured, dried 0.749
Bacon 0.593
chia seeds 0.588
Beef, ground, 95% lean meat / 5% fat, raw 0.565
Pork, ground, 96% lean / 4% fat, raw 0.564
Wheat germ 0.456
Egg, whole, cooked, hard-boiled 0.392
Oat 0.312
Peanuts 0.309
Chickpea 0.25
Corn, yellow 0.197
Almonds 0.151
Beans, pinto, cooked 0.117
Lentils, cooked 0.077
Rice, brown, medium-grain, cooked 0.052
High levels of methionine can be found in eggs, meat, and fish; sesame seeds, Brazil nuts, and some other plant seeds; and cereal grains.
Most fruits and vegetables and legumes contain very little methionine.
Proteins without adequate methionine are incomplete proteins.
Restricting methionine consumption can increase lifespans in fruit flies, and mice.
Loss of methionine has been associated with senile greying of hair.
Methionine deficiency leads to a buildup of hydrogen peroxide in hair follicles, a reduction in tyrosinase effectiveness, and a gradual loss of hair color.
Methionine raises the intracellular concentration of glutathione (GSH), thereby promoting antioxidant mediated cell defense and redox regulation.
Methionine protects cells against dopamine induced nigral cell loss by binding oxidative metabolites.
Methionine is an intermediate in the biosynthesis of: cysteine, carnitine, taurine, lecithin, phosphatidylcholine, and additional phospholipids.
Abnormal conversion of methionine can lead to atherosclerosis: accumulation of homocysteine.
Methionine may be essential to reversing damaging methylation of glucocorticoid receptors caused by repeated stress exposures, with implications for depression.
It can increase the urinary excretion of quinidine by acidifying the urine.
Aminoglycoside antibiotics used to treat urinary tract infections work best in alkaline conditions, and urinary acidification from using methionine can reduce its effectiveness.