Article Review Title: Methionine: An Essential Sulfur-Containing Amino Acid
Name: Harriet Kaye A. Reyes, BSED Science 3A
Institution: Isabela State University, Cauayan City Campus
Abstract
Methionine is the initiating amino acid in the synthesis of virtually all eukaryotic proteins. It is one
of the most common sulfur-containing amino acid which is incorporated with proteins. Within
proteins, many of the methionine residues are buried in the hydrophobic core, but some, which are
exposed, are susceptible to oxidative damage. Methionine is crucial for both animal and human
health as it understood to play a fundamental role in various metabolic processes and cellular
functions. This article includes functions of methionine such as methylation processes that begins
with its activation to S-adenosylmethionine, Detoxification and Antioxidant role, and its metabolic
functions. These functions are essential in understanding the role of methionine as a versatile
biomolecule. Methionine is essential for the normal growth and development of mammals. This
essential role derives from the participation of this amino acid, or its derivatives, in several
fundamental biologic processes, including protein synthesis and the numerous S adenosylmethionine. Along with its functions are the importance and applications to several to
medical benefits, societal importance, and practical applications.
Introduction
Methionine is an essential sulfur-containing amino acid crucial for human and animal health.
Being "essential" means the body cannot synthesize methionine and must acquire it through dietary
intake. It plays a fundamental role in various metabolic processes and cellular functions.
Methionine is obtained from animal-based dietary sources such as meat, fish, eggs, and dairy, as
well as certain plant-based proteins like soybeans and lentils. It also studies the role in protein
synthesis that serves as an initiating amino acid, which involves the process in Eukaryotic cells
that begins in Methionine. This positioning is essential for the correct assembly of amino acids
into functional proteins. During the last two decades, work in several laboratories has defined the
potential pathways for methionine metabolism. Concurrently, other studies have established the
existence of regulatory processes and have provided some insight into the individual mechanisms
which may contribute to this metabolic modulation. Methionine plays an essential role in the
immune system through its metabolites. In this regard, Blachier et, al. (2013) found that this amino
acid directly influences the functioning of the immune system because of methionine catabolism
leading to an increase in the production of glutathione, taurine, and other metabolites.
This article provides an insight on the significance of methionine to its role in human and animal
health, coupled with its medical, societal, and industrial significance. It will briefly summarize its
functions, importance and applications to animal and human health.
Structure
Methionine is the most peculiar amino acid in terms of structure. Its molecular formula is
C₅H₁₁NO₂S. The carbon atom at the center is an anchor to which all the functional groups of the
molecule are attached.
This central carbon is attached to an amino group
(-NH₂), which defines methionine as an amino
acid, and a carboxyl group (-COOH), which is
needed for peptide bond formation during
protein synthesis. Methionine is different from
other amino acids because it contains a sulfurcontaining side chain (-CH₂-CH₂-S-CH₃), which
is part of its metabolic processes and contributes
to its special attributes.
Figure 1. 2D Structure of Methionine
The methionine amino acid structure is illustrated in figures 1. The methionine structure shows
that this molecule has the regular amino acid backbone, which is basically:
•
•
A central carbon atom.
An amino group −NH2.
•
•
The carboxyl group −COOH
A unique sulfur-containing side chain (-CH₂-CH₂-S-CH₃), which sets it apart from
other amino acids.
Function of Methionine
Protein Synthesis
Methionine plays a critical role in protein synthesis, serving as the initiating amino acid
during the translation process. In eukaryotic cells, the process begins with methionine
being attached to a special initiator tRNA, known as tRNAᵐᵉᵗᵢ, which pairs with the start
codon (AUG) on mRNA. This positioning is essential for the correct assembly of amino
acids into functional proteins.
As the key amino acid in protein
synthesis, methionine is essential for
translation
across
all
life
domains.
Eukaryotes start synthesizing proteins
from methionine and prokaryotic proteins
based on N-formylmethionate. Why?
Recognition of the start codon during
translation is facilitated by the unique
structure of methionine-specific tRNAᵐᵉᵗᵢ
Figure 2. tRna in Protein Synthesis
(Brosnan & Broslan, 2006; Tharp et al, 2021). Several pathways involved in protein
biosynthesis must be controlled by methionine, which is essential for protein synthesis and
regulation. Methionine deprivation has been shown to impact the S6K1 pathway, which is
critical for protein synthesis regulation in avian myoblasts.
Additionally, methionine has a dual role as initiation and precursor for S-methione (SAM),
which is an essential methyl donor in several phenylation pathways, including those that
affect DNA, RNA, and proteins (Lim et al. (2018)). Furthermore: Methionine's dual
function as a protein and methyl group generator highlights its crucial role in cellular
metabolism and gene expression regulation.
Methylation Processes
Methionine is essential for a number of methylation processes inside cellular metabolism
because it can be converted into S-adenosylmethionine (SAM), the universal methyl donor
in many biological activities. Methionine is essential for methylation, which regulates gene
expression, DNA repair, and the synthesis of other biomolecules, in addition to protein
synthesis.
One of those necessary amino
acids that your body cannot
produce
on
its
own
is
methionine, which you must
obtain
through
diet.
It
functions as the precursor to
S-adenosylmethionine, or
Schematic presentation of DNA methylation. SAM, S-adenosyl
methionine;
SAH,
S-adenosylhomocysteine;
DNMTs,
DNA
methyltransferases; MBPs, methyl-binding proteins; HDACs, histone
deacetylases. Created with BioRender.com
SAM for short, which is crucial for many bodily functions. Do you realize that your body
uses your DNA as a kind of script? SAM aids in DNA methylation, which is similar to
modifying that script to ensure that everything is functioning properly. Additionally, it
plays a role in the synthesis of neurotransmitters, which are the molecules that assist
regulate your mood and include serotonin and dopamine. Do you know what phospholipids
are? They are essential for maintaining the integrity of your cell membranes, and SAM is
also involved in their production. SAM also plays a significant role in other processes that
require the donation of a methyl group. This has an impact on hormone regulation, how
your body handles fat, and even epigenetics. As strange as it may sound, methionine is the
unsung hero that keeps our systems and cells functioning properly.
Detoxification and Antioxidant Role
Methionine is integral to the synthesis of glutathione, a tripeptide that serves as one of the
body's most powerful antioxidants.
Methionine (Met) is a sulfur-containing amino acid that is present in peptides and proteins.
The most important common characteristic of Met residues in proteins is that they are
subject to reversible oxidation and reduction, mediated either enzymatically or
nonenzymatically (Kim et al. 2014). Met residues of proteins are especially susceptible to
oxidation. Methionine neutralizes reactive oxygen species (ROS) and protects cells from
oxidative damage and by some other biological oxidants. The susceptibility of proteinbound methionine to oxidation generally correlates with the surface exposure of a
methionine residue. While surface-exposed Met residues are the most readily oxidized with
negligible consequence for protein activity, Met residues buried within protein core, which
are vital for function of a protein, are less prone to oxidation (Grune et al. 2013). The
oxidation of protein-bound methionine to methionine sulfoxide (MetO) is considered as an
undesirable and harmful consequence of oxidative stress, linked to age-related
malfunctions (Stadtman et al. 2005; Ruiz et al. 2005; Pamplona and Barja 2006; Reeg and
Grune 2015; Aledo 2021). Methionine also regenerates other antioxidants, such as
vitamins C and E. Supports the detoxification of harmful substances in the liver and
prevents the accumulation of toxic metabolites by enhancing liver function and metabolic
detox pathways.
Metabolic Functions
In metabolic functions, it includes lipid metabolism which methionine supports the
breakdown of lipids, helping to reduce fat deposits in the liver and promoting overall lipid
balance. Methionine's role in the methionine cycle and methylation is associated with lipid
regulation and reduced risks of non-alcoholic fatty liver disease (NAFLD) when in balance.
Homocysteine regulation; through its conversion to cysteine and glutathione, methionine
helps regulate homocysteine levels, which helps lower cardiovascular disease risks by
reducing oxidative stress and inflammation. Energy production which methionine
contributes to energy metabolism by participating in the synthesis of creatine, a compound
critical for energy storage in muscles.
The methionine – homocysteine cycle contains re-methylation and transculturation
components. The enzyme S-adenosyl homocysteine hydrolase (SAH) contributes to
methylation of DNA and RNA. Other abbreviations: CBS = cystathionine–b–synthase;
MTR = 5-methylterathydrofolate-homocyteine
methyltransferase;
MTHFR = methyl
tetrahydrofolate reductase; THF-tetrahydrofolate; MTHF = methyltetrahydrofolate.
These diverse roles illustrate methionine’s importance not only in fundamental biological
processes but also in maintaining cellular health and metabolic homeostasis.
Importance
Medical Importance/Benefits
Methionine offers significant medical benefits, particularly for liver health, where it aids
in detoxifying harmful substances and supports liver regeneration. Its derivative, S-
adenosylmethionine (SAM), plays a vital role in neurochemical synthesis, facilitating the
production of essential molecules like creatine and epinephrine. Additionally, methionine
supplementation has shown promise in disease management, particularly in treating liver
disorders and depression through SAM-related pathways, highlighting its therapeutic
potential in maintaining and restoring overall health.
Societal Importance
Methionine holds profound societal importance, particularly in agriculture and human
nutrition. In animal feed, it is an indispensable additive, enhancing livestock growth,
improving meat quality, and boosting agricultural productivity by optimizing protein
synthesis in animals. This use contributes to sustainable food production and economic
efficiency in the farming sector. For human nutrition, methionine is crucial for maintaining
dietary balance, especially for individuals following vegan or vegetarian diets, which often
lack sulfur-containing amino acids. By preventing deficiencies, methionine supports
overall health and nutritional adequacy. These applications highlight its pivotal role in
supporting global food security and public health.
Practical Applications
Methionine has wide-ranging practical applications across industries due to its
biochemical properties and versatility. As a dietary supplement, it addresses amino acid
deficiencies, particularly in populations with dietary restrictions like vegans and
vegetarians. Its role in detoxification supports liver health, particularly in conditions
involving fatty liver and toxin overload. In biotechnology, methionine is an essential
amino acid in microbial and cell culture media, optimizing cell growth and protein
production for research and industrial use, such as in biopharmaceutical manufacturing.
In pharmaceuticals, methionine is leveraged for its detoxifying and hepatoprotective
properties, assisting in treatments for liver diseases and as part of formulations to support
overall metabolic health. The cosmetic industry uses methionine in skin care products,
promoting skin repair and collagen synthesis due to its sulfur content, which is vital for
keratin production. These diverse applications underscore methionine’s critical role in
advancing health, science, and industrial innovation.
Conclusion
Methionine is an amino acid which is essential for protein synthesis, methylation, and
antioxidant protection. Its role in human and animal health, coupled with its medical,
societal, and industrial significance, underscores as a versatile biomolecule.
In conclusion, methionine is an essential sulfur-containing amino acid for human and
anima health which is acquired through dietary intake. Its functions are crucial for protein
synthesis by being the start codon in the transcription process in cells to form a polypeptide
chain from the template of DNA strands. It is a precursor involved in methylation
processes, crucial for DNA repair, gene expression, and the synthesis of neurotransmitters.
Methionine's diverse roles make it indispensable for both health and economic
productivity.
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