Methionine Amino Acids: Properties, Function, Benefits, and Sources

What is methionine?

The amino acid methionine, or Met, is essential for human nutrition. It plays an important role in metabolism and the synthesis of proteins. The process of making proteins in eukaryotic organisms begins with the amino acid methionine. In globular proteins, the hydrophobic core is where you'll find most of the methionine residues. Certain methionine residues on protein surfaces can be oxidized to create methionine sulfoxide. They can be transformed back into methionine with the aid of methionine sulfoxide reductase. Making S-adenosylmethionine, a crucial biological methylating agent, is methionine's principal metabolic function. In methylation metabolism, transsulfuration, transmethylation, and methylation can be classified in several ways. Adenosylmethionine regulates these activities via allosteric mechanisms. The methyl groups used extensively in creatine synthesis can be obtained from adenosylmethionine. Piglets consume a significant amount of creatine during their growth. The piglet makes two thirds of it, and the milk provides one third. This can cause the body to produce creatine at excessively high rates and strain its methionine supply.

Methionine structure

L-methionine ((2S)-2-amino-4-(methylsulfanyl)butanoic acid) is an essential amino acid, indicating that it cannot be produced endogenously. The structure of methionine is reflected in its nomenclature, as it has a methyl group covalently bound to a sulfur atom. Both the methyl group and sulfur are essential for its structural and metabolic functions.

The common sulfur-containing amino acids. (Savino R J., et al., 2022)

Methionine pKa

Methionine does not have ionizable groups in its side chain, so its pKa values are associated only with the amino group and carboxyl group of the backbone. It has the α-carboxyl group with the pKa value=2.28, and has the α-amino group with pKa value=9.21.

The pka and isoelectric point values. (Brandão-Lima L C., et al., 2021)

Methionine synthesis

Despite the fact that mammals require supplemental methionine, the majority of fungi are able to effectively absorb sulfates in order to synthesize methionine. Fungi break down sulfate into sulfite and then sulfide through an enzymatic process. It should be noted that two ATP molecules and the reducing equivalents of four NADPH molecules are consumed by each round of this reduction (to sulfide). A precursor for both methionine and cysteine production in yeast and other fungi, this synthesis of homocysteine from sulfide signifies sulfur incorporation into a carbon skeleton. In one-carbon metabolism, specifically the folate cycle, the methyl group of methionine is derived from 5-methyl tetrahydrofolate (5-methyl THF). It is worth mentioning that the production of 5-methyl THF necessitates the addition of NADPH. Taken together, these steps indicate that the synthesis of methionine may be the most significant user of NADPH. When yeasts engage in cysteine biosynthesis, cystathionine plays a key role in the transsulfuration pathway's conversion of homocysteine to cysteine. As a result of an evolutionary difference, most mammalian cells can only convert homocysteine to cysteine in one direction. However, yeasts can use these two amino acids directly as their only supply of sulfur, allowing them to flourish.

In order to biosynthesize methionine, lysine, and threonine, E. coli and C. glutamicum use aspartate as a precursor. Nevertheless, the host microbes make a big impact when it comes to the biochemical intermediates and regulatory mechanisms for methionine synthesis. Homoserine dehydrogenase (hom), aspartate-semialdehyde dehydrogenase (asd), and aspartokinase (lysC; from now on, the gene name is placed in parenthesis to show the protein it encodes in a particular microbe) are all involved in the conversion of aspartate to homoserine in C. glutamicum. The enzyme metX converts homoserine to O-acetyl homoserine (OAH) so that methionine can be synthesized. The acetyl group is donated by acetyl-CoA. Next, C. glutamicum has two different mechanisms for absorbing sulfur. By utilizing cysteine as a sulfur source, the synthesis pathway generates l-homocysteine from OAH through the action of cystathionine-γ-synthase (metB) and cystathionine-β-lyase (metC). Another route involves the enzyme O-acetyl homoserine sulfhydrylase (metY), which converts hydrogen sulfide into l-homocysteine. Methionine synthases that are required or independent of vitamin B12 (metH and metE, respectively) finish the process of methionine biosynthesis. These enzymes use N5-methyl-tetrahydrofolate (CH3-THF) as a methyl donor to add a methyl group to l-homocysteine.

Methionine biosynthesis pathway. (Shim J., et al., 2017)

Methionine metabolism

The transsulfuration route is an enzyme-catalyzed series of two processes that converts homocysteine to cysteine, and it is essential for the breakdown of methionine. A catalytic reaction involving cystathionine beta-synthase converts homocysteine to cystathionine; a further catalytic reaction involving cystathionine gamma-lyase yields cysteine, alpha-ketobutyrate, and ammonia. There is no turning back the clock on homocysteine to cysteine conversion. That is why it is possible to convert methionine to cysteine, but not the other way around. For this reason, it has been noted in nutrition that cysteine can be skipped if enough amounts of methionine are consumed; nonetheless, it is important to note that cysteine cannot compensate for a deficiency in methionine. Very few tissues are involved in the transsulfuration route; its activity is greatest in the pancreas, small intestine, kidneys, and liver. When it comes to glutathione synthesis, it is indispensable because it provides cysteine. Transmethylation may appear to be a mechanism that limits methionine catabolism based on this debate. Would physiological methylation processes be required to catabolize additional methionine, limiting catabolism? Glycine: N-methyltransferase is the key that unlocks this mystery. This enzyme is relatively resistant to inhibition by S-adenosylhomocysteine and has a high Km for S-adenosylmethionine. Using a substrate that is easily accessible, it produces sarcosine, which may be oxidized in mitochondria to replenish the glycine, thereby functioning as a high-capacity transmethylation mechanism.

Methionine metabolism is characterized by its reliance on B vitamin status, which is a crucial aspect. Three vitamins are engaged in the remethylation pathway and one is involved in transsulfuration; a total of four are involved. Folic acid, in its active form as 5-methyltetrahydrofolate, provides the enzyme methionine synthase with methyl groups. Methylenetetrahydrofolate reductase, which is a substrate for 5-methyltetrahydrofolate, uses FAD, an enzyme derived from riboflavin, as a co-factor. Among the few enzymes found in mammals, methionine synthase is unique in that it contains a prosthetic group derived from vitamin B12. The prosthetic groups used by cystathionine beta-synthase and cystathionine gamma-lyase are pyridoxal phosphate. Hyperhomocysteinemia can occur when any of these vitamin-derived cofactors are deficient; this is not surprising given their roles in the elimination of homocysteine by remethylation or transsulfuration.

Metabolic regulation of methionine is intricate. First and foremost, an important event is the division of homocysteine into remethylation and transsulfuration. Remethylation rather than catabolism of homocysteine occurs when a diet low in labile methyl groups is consumed; conversely, a diet high in methyl groups slows down the remethylation rate. It seems that cellular S-adenosylmethionine levels govern this metabolic transition in the liver through allosteric processes. First, S-adenosylmethionine, the enzyme's product, activates the hepatic isoform of methionine adenosyltransferase, which has the peculiar feature of feed-back activation. When there is an excess of methionine, this guarantees that hepatic S-adenosylmethionine levels will be high. The second function of S-adenosylmethionine is to block methylenetetrahydrofolate reductase and activate cystathionine beta-synthase allosterically. The fact that more methionine is available encourages transsulfuration and inhibits remethylation is a result of these processes.

It has only lately been discovered that oxidative stress is another regulatory point. Now we know that cysteine, a building block of glutathione, is produced in large part by the transsulfuration route. The elimination of peroxides relies on glutathione, an essential molecule. Hepatocytes experience an increase in transsulfuration flow and glutathione production in response to oxidative stress caused by oxidants. Cysteinine beta-synthase may undergo this change in response as a result of changes in the redox state of a heme prosthetic group.

Methionine metabolism. (Walvekar A S., et al., 2019)

Sources of methionine

Since it is not possible for the human body to produce its own supply of methionine, this crucial amino acid must be acquired from food. Meat, seafood, eggs, and dairy products are excellent animal-based sources of methionine. Legumes, seeds, nuts, whole grains, and soy products are plant-based sources of methionine that contribute to the diet, although in lesser quantities. Vegetarians and vegans can be sure they obtain enough plant-based foods by eating a variety of them. Another option is to look for L-methionine or other fortified meals and supplements. Methionine is an essential component for proper protein synthesis, methylation, and metabolic function.

Methionine amino acids at Creative Peptides

Methionine pathway

Several metabolic processes are intimately connected to methylation metabolism. A metabolic pathway known as a double ring is comprised of the folate cycle and the methionine cycle. All of these bicyclic processes are known as one-carbon metabolism. Connecting the folate cycle with the transsulfuration pathway, methylation metabolism is the key. The transsulfuration route is linked to the methionine cycle by the sulfur-containing, nonprotein, hazardous AA intermediate Hcy. Protecting genetic material requires Hcy clearance. Mutations at the SAM regulatory region in CBS, the first enzyme in the transsulfuration pathway, cause homocystinuria, a condition strongly linked to cancer. Glutamate metabolism is stimulated by numerous oncogenic insults and mutations, and GSH production connects the transsulfuration route to it. Mutations in the 5-methylthioadenosine phosphorylase (MTAP) gene are common in human malignancies. This enzyme is essential for the methionine salvage pathway. Novel therapeutic targets targeting MTAP-deficient malignancies include protein arginine N-methyltransferase 5 (PRMT5) inhibition. To block PRMT5, MTA binds to the catalytic site and competes with SAM for binding. To eliminate MTA buildup to levels seen in MTAP-expressing cells, methionine restriction (MR) is all that is needed. In addition, the metabolism of methionine is intimately associated with that of PA. Pas may have either pro- or anti-inflammatory effects depending on whether or not ornithine decarboxylase (ODC) activation raises PA levels. Therefore, many cellular metabolisms rely on methionine metabolic.

Response of methionine metabolism in the liver. (Li Z., et al., 2020)

Cross talk between methionine metabolism and the other metabolism. (Li Z., et al., 2020)

Methionine vs cysteine

The sulfur-containing amino acids methionine and cysteine are structurally and functionally distinct from one another. Methionine is hydrophobic and nonpolar due to its stable thioether group, but cysteine is polar due to its reactive thiol group, which can form disulfide bonds that maintain protein structures. Methionine is an essential building block for the methyl donor S-adenosylmethionine (SAM) and the initial codon in protein synthesis. While cysteine aids in protein folding and stability, it also plays an important role in antioxidant defense, mainly through glutathione production. In contrast to cysteine, which is non-essential but may be produced in the body, methionine is considered essential and must be consumed through food. The specific functions they play in living systems are based on their particular chemical characteristics. Methionine resides among a unique class of amino acids due to its incorporation of a sulfur atom. Along with methionine, cysteine and homocysteine share similar chemical structures. Of these three sulfur-containing amino acids, only methionine and cysteine play a role in the synthesis and structure of proteins. Early studies have shown that supplementing the diets of animals with only methionine recorded rates of growth similar to animals fed diets supplemented with both cysteine and methionine while animals fed diets supplemented with exclusively cysteine saw significantly lower rates of growth. In follow-up studies it was shown that, at suboptimal levels of methionine, cysteine was able to compensate for this lack of methionine and normal rates of growth were observed. These early findings, in the context of what is now understood of methionine, suggest that the essential nature of methionine is within its role during protein synthesis as the initiating amino acid.

Structure of the sulfur-containing amino acids. (Savino R J., et al., 2022)

The conversion of methionine to homocysteine and cysteine. (Savino R J., et al., 2022)

Oxidation of methionine

In vivo, under biological oxidative stress conditions, and for protein stability in vitro, methionine oxidation is an important process. The oxidizing species determines whether methionine is oxidized to methionine sulfoxide, a two-electron oxidation, or methionine radical cations, a one-electron oxidation. Catalytic assistance for electron-deficient reaction centers is provided by nearby groups in both reaction processes. The methionine sulfoxide reductase (Msr) mechanism reduces methionine sulfoxide in living organisms, which means that some residues of methionine sulfoxide may have a fleeting role in protein deactivation caused by reactive oxygen species (ROS). Accessibility to Msr determines whether other methionine sulfoxide residues accumulate. In addition, diseases and biological aging can lead to a decrease in the amount of active Msr and/or the necessary cofactors, which can lead to an increase in methionine sulfoxide levels. In contrast, carbon-centered and/or peroxyl radicals are the end products of reactions in which methionine radical cations primarily enter irreversible reaction channels. These could potentially set off a cascade of events involving the oxidation of proteins. In this review, we will go over the comprehensive molecular models of the reactions between methionine and organic model sulfides and a number of well-known ROS that have biological significance. The one-electron oxidation route, together with the physico-chemical parameters that govern it, will be the primary focus, along with its physiological significance, particularly in relation to the oxidation and neurotoxicity of the β-amyloid peptide (βAP) in Alzheimer's disease.

Oxidation pathways of methionine. (Schöneich C. 2005)

Methionine function

It is widely accepted that methionine is frequently added to the expanding peptide chain and is the first amino acid added during protein synthesis. The starting and internal methionines will be discussed individually in order to highlight the importance of methionine in protein construction. Bacteria use N-formylmethionine tRNAs to start translation, whereas eukaryotes and archaea use initiating tRNAs to encode L-methionine. This same methionine is eliminated from the polypeptide chain soon after the start of elongation, despite being essential for the start of protein synthesis. Wingfield showed how the enzyme methionine aminopeptidase (MAP) co-translationally cleaves N-terminal methionine in eukaryotes shortly after protein synthesis begins. Bacteria exhibit a similar mechanism, whereby formylmethionine deformylase must first remove the formyl group, leaving an N-terminal methionine that is prepared for removal by MAP. About two-thirds of the proteins in any given proteome are possible targets for MAP function, and it is fatal to suppress MAP activity in bacteria and yeast. Additional research has demonstrated that the stability and functionality of proteins depend on the elimination of N-terminal methionine or N-formyl methionine. Therefore, N-terminal methionine is not only not necessary for proteins to maintain their structural integrity, but in many situations, it must be removed.

Methionine benefits

(1) Antioxidant Function

Methionine residues in some proteins may be exposed on the surface, rendering them vulnerable to interaction. Research conducted about 40 years ago detailed the oxidation of methionine, resulting in the formation of methionine sulfoxide, which, with further oxidation, yields methionine sulfone. It was noted that both free and protein-bound methionine sulfoxide could be reduced to methionine by methionine sulfoxide reductase, whereas methionine sulfone could not be reduced. The oxidative capacity of methionine led to the suggestion that cells might employ this characteristic as an intrinsic antioxidant for defense. The notion was tested by examining the impact of hydrogen peroxide on glutamine synthetase derived from E. coli. The results indicated that methionine residues on the protein's surface, which were prone to oxidation, were configured to inhibit the oxidation of the protein's active site. Furthermore, it was hypothesized that the capacity of methionine sulfoxide reductase to decrease oxidized methionine residues enables these residues to offer recurrent protection against oxidation. The methionine antioxidant hypothesis was subsequently validated by Shen Luo and Rodney Levine in an experiment in which methionine was replaced with norleucine in E. coli. Although enzymatic activity and growth rates in colonies supplied with norleucine were not markedly different from those with methionine, cells with norleucine substitution exhibited considerably higher mortality rates when exposed to oxidizing agents compared to those receiving methionine. Subsequent examinations of methionine's oxidative role revealed that cells augmented methionine incorporation into proteins during periods of oxidative stress. A review by Rodney Levine and colleagues examined the modified antioxidant action of α-2-macroglobulin, a protein characterized by methionine residues lining its active site as a deliberate protective measure. This suggests that proteins may have evolved to include methionine residues in especially susceptible locations, underscoring methionine's significance as an endogenous antioxidant.

(2) Regulate Autophagy

Although the availability of methionine and its influence on growth have been extensively researched, particularly in mammalian cells, a recent investigation in yeast has demonstrated that methionine functions as an anabolic signal and modulates metabolic transformation. This study delineates the fundamental metabolic pathway regulated by methionine, based on longstanding studies in yeast indicating that methionine enhances proliferation and suppresses autophagy. Excess methionine induces widespread alterations in gene expression, characterized by a distinct anabolic transformation signature. This anabolic program delineates a distinct hierarchy of genes controlled by methionine that are implicated in metabolism. In this hierarchy, methionine concurrently enhances the expression of genes associated with three critical metabolic pathways: the pentose phosphate pathway (PPP), glutamate synthesis, and pyridoxal phosphate synthesis. The metabolites and co-factors generated at these three nodes were essential for the functionality of a group of metabolic proteins, which were transcriptionally activated by methionine. Significantly, these proteins participated in the synthesis of almost all other amino acids and nucleotides. This study revealed that the presence of methionine, even in the absence of other amino acids, initiates the synthesis of all other amino acids and nucleotides necessary for sustaining anabolism. The essence of this anabolic program depends on the generation of metabolites and co-factors that facilitate reductive biosynthesis. In accordance with the findings of this work, the methionine-sensing tRNA thiolation modification route induces a comprehensive metabolic reconfiguration by modulating flux through the PPP, unexpectedly by regulating phosphate availability. Under conditions of methionine sufficiency, cells lacking tRNA thiolation redirect carbon flux towards trehalose synthesis, therefore liberating and recycling the sequestered phosphate. By facilitating this, methionine availability maintains equilibrium in carbon and nitrogen metabolism within cells. The significance of elevated PPP activity in economic growth is now broadly recognized. Nonetheless, the often-overlooked association of methionine with the PPP was initially identified in yeast, and subsequent research offers a comprehensive understanding of the metabolic regulation influenced by methionine. Ultimately, there exists a well-established association between methionine metabolism and the one-carbon folate cycle, which seems to be ubiquitous. Research on yeast has demonstrated that methionine functions as a potent growth signal, initiating a coordinated anabolic program within cells.

(3) As a signal for growth

To review the previous part, it is important to understand that cell destiny decisions are multi-step procedures that take into account the cellular requirements in addition to the various food inputs. These decisions, especially commitments toward cell development, are heavily influenced by sentinel metabolites like acetyl CoA. Among these sentinel metabolites, methionine has recently come to light. Earlier, we covered how research in S. cerevisiae showed that an excess of methionine can prevent autophagy, boost cellular anabolism and proliferation, and do this even when amino acids are scarce overall. Not only is this proliferative response noticeable even when no other free amino acids are present, but cells induce a robust anabolic program when methionine is added alone. It seems that this basic idea of methionine signaling growth is universal across all kinds of creatures. The survival and metastasis of some cancers depend on methionine. Fertility and growth are both enhanced when Drosophila are given supplemental methionine. In contrast, methylation restriction has been linked to an extended life span in fruit fly models and others. These findings provide more evidence that methionine is a powerful growth signal with a special role in determining cell fate.

Methionine uses

(1) Regulate immune system

Methionine is crucial for the immune system via its metabolites. Blachier discovered that this amino acid directly affects immune system functionality due to methionine catabolism, which enhances the formation of glutathione, taurine, and other metabolites. Methionine is efficiently utilized by hepatocytes for the direct production of glutathione, a low-molecular-weight antioxidant. Simultaneously, methionine has demonstrated the ability to chelate lead and facilitate its removal from tissues, hence reducing oxidative stress. A reduced concentration of methionine can induce transsulfuration. An increase in methionine consumption results in a reduction in substrate flux through the transmethylation pathway and an elevation of flux through the transsulfuration pathway.

An effect of methionine limitation on mammalian immune system function and oxidative stress is being studied by certain researchers at the moment. Evidence suggests that oxidative stress might be mitigated and glutathione production enhanced by cutting back on this amino acid. Increased methionine supplementation changed the oxidative activity in one branch of the pentose phosphate pathway (PPP), according to Campbell's research. Additionally, in comparison to oxidative pentose phosphate, they discovered that cells pre-incubated with methionine were more resistant to the thiol oxidizing agent diamide. Nevertheless, research conducted by Maddineni's group found that mice whose methionine intake was limited showed less oxidative stress without alterations to the activity of their antioxidant enzymes. This indicates that additional research is required to ascertain how reducing methionine intake impacts antioxidant activity.

Biological activity of the Met. (Martínez Y., et al., 2017)

(2) Promote animal growth

A chicken's immune system is mostly composed of the bone marrow, tonsils, lymph nodes, thymus, spleen, and bursa of Fabricius. The effects of methionine on the immune system have been found to vary. Studies have shown that chickens with an adequate amount of methionine in their bodies have larger spleens and bursa of Fabricius relative weights, whereas chickens with an inadequate amount of methionine develop immune organ dysplasia and have smaller thymus, spleen, and bursa of Fabricius relative weights. While the spleen is mostly unaffected by methionine supplementation, the thymus and bursa of Fabricius of layers might gain weight during brood rearing. The results suggest that methionine may have a more pronounced impact on the maturation of the thymus and bursa of Fabricius, which are major immune organs, rather than the spleen, which is a secondary organ. In addition to increasing the head kidney and spleen index in juvenile Jian carp, methylation may influence the thymus weight of early weaned piglets and meat rabbits.

(3) As a protecting ligand

There is a strong correlation between early cancer lesions and the Met transport rate being drastically elevated due to the tumor cells' excessive Met demand. Additionally, Met-dependent tumor cells transport Met in huge quantities by overexpression of the membrane-spanning protein L-type amino acid transporter 1 (LAT1). As a result, Met has become a valuable tool for tumor diagnosis by linking with LAT1. The methionine-functionalized mesoporous silica nanoparticles (DTX/MSN-Met) developed by Khosravian and colleagues in 2016 have a high drug loading capacity and are pH responsive. Amido-bond conjugation of Met to MSNs produced MSN-Met. After that, MSNs with a positive charge were adsorbing docetaxel (DTX), which has a negative charge, due to an electrostatic contact force. Results from cell uptake experiments showed that MCF-7 cells overexpressing LAT1 were able to successfully absorb DTX/MSN-Met. DTX/MSN-Met appeared to have a remarkable capacity to selectively target tumors, as it primarily accumulated at the tumor site in tumor-bearing animals.

FAQ

Is Methionine polar or nonpolar?

The side chain of methionine is –CH₂–CH₂–S–CH₃, characterized by a linear structure terminating in a methyl group bonded to a sulfur atom. Sulfur possesses a somewhat greater electronegativity than carbon; nevertheless, the linked methyl group mitigates the overall polarity. Methionine is a nonpolar amino acid characterized by its hydrophobic properties, despite the presence of a sulfur atom in its side chain.

Is Methionine hydrophobic or hydrophilic?

Methionine is one of the most hydrophobic amino acids due to the extremely hydrophobic nature of its side chain, which is imparted by the terminal methyl group. This causes the hydrophobic core of globular proteins to encase almost two-thirds of the methionine residues. On the other hand, the surface of the protein might include as many as a third of them. This makes them easy targets for reactive oxygen species, which can oxidize the sulfur atom to sulfoxide.

References

1. Savino R J., et al., The potential of a protein model synthesized absent of methionine, Molecules, 2022, 27(12): 3679.
2. Shim J., et al., L-Methionine production, Amino acid fermentation, 2017: 153-177.
3. Walvekar A S., et al., Methionine at the heart of anabolism and signaling: perspectives from budding yeast, Frontiers in microbiology, 2019, 10: 2624.
4. Li Z., et al., Methionine metabolism in chronic liver diseases: an update on molecular mechanism and therapeutic implication, Signal transduction and targeted therapy, 2020, 5(1): 280.
5. Brandão-Lima L C., et al., Clay mineral minerals as a strategy for biomolecule incorporation: Amino acids approach, Materials, 2021, 15(1): 64.
6. Schöneich C. Methionine oxidation by reactive oxygen species: reaction mechanisms and relevance to Alzheimer's disease, Biochimica et Biophysica Acta (BBA)-Proteins and Proteomics, 2005, 1703(2): 111-119.
7. Martínez Y., et al., The role of methionine on metabolism, oxidative stress, and diseases, Amino acids, 2017, 49: 2091-2098.
8. Ruan T., et al., Effects of methionine on the immune function in animals, Health, 2017, 9(5): 857-869.
9. Liu J., et al., Biomedical applications of methionine-based systems, Biomaterials Science, 2021, 9(6): 1961-1973.