Amino Acids Function: Structural Foundations and Biological Roles

Amino acids are the building blocks of life, forming the structural and functional basis of proteins, enzymes and other biomolecules. Amino acids include three types of essential and non-essential, conditionally essential amino acids, with different functional groups, which determine their role in biochemical and physiological processes. By understanding these roles, we can gain insight into cellular mechanisms, metabolic pathways, and potential therapeutic applications.

Amino Acids Functional Groups

At their core, amino acids are organic compounds characterized by amino (-NH₂) and carboxyl (-COOH) groups, which define their essential structural properties. Each amino acid also features a unique side chain (R group), which determines its chemical characteristics and biological functionality. Below is a breakdown of these functional groups:

Amino group (-NH₂): The amino group of amino acids is one of the polar functional groups of amino acids, it reacts with alkalis to make salts and releases ammonia gas in the presence of acid.

Carboxyl group (-COOH): Carboxyl is another polar functional amino acid group that can react with bases to make esters, and can release carbon dioxide under acidic conditions.

Side chain (R group): The side chain is the amino acid molecule without amino and carboxyl group that determines the amino acid's nature and purpose. Depending on their side chain, the following amino acids are in different categories:

• Aromatic amino acids: Phenylalanine , tryptophan, etc. The side chain consists of aromatic rings.
• Acidic amino acids: Like glutamic acid, and aspartic acid, the side chain of which is the carboxyl group.
• Simple amino acids: Examples are lysine, arginine, and the side chain that contains amino groups.
• Polar neutral amino acids: Like serine, and threonine, the side chain with hydroxyl or sulfur atoms.
• Non-polar amino acids: Like valine, isoleucine, and side chains are hydrophobic.

Other functional groups: Other amino acids may have other functional groups such as hydroxyl groups, thiol groups or cyclic groups which might affect the chemical and biological properties of amino acids.

And it is not just their chemical structures that dictate the function of functional groups in amino acids. it is their metabolic and physiological functions in living organisms. For instance, branched-chain amino acids like leucine, isoleucine and valine contribute to the human body as both energy providers and muscle-repairing chemicals. Additionally, amino acids bond with each other via peptide bonds to become proteins, and proteins are peptide bonds between amino and carboxyl groups, amide bonds.

The amino groups, carboxyl groups and side chains (R groups) of amino acids are the functional groups that define amino acids' chemical and biological activity.

Functions of the Essential Amino Acids

Essential amino acids play an irreplaceable role in human growth and development, muscle repair, and metabolic regulation. The functions of several important essential amino acids are detailed below.

Table.1 Function of 9 essential amino acids.

Leucine Amino Acid Function

Leucine is an essential branched-chain amino acid (BCAA) that has a variety of important physiological functions in the human body. Here are the main functions of leucine:

Protein Synthesis and Muscle Growth: Leucine is a key regulator of muscle protein synthesis. It promotes protein synthesis in skeletal muscle and adipose tissue by activating the mammalian target rapamycin (mTOR) signaling pathway. Leucine is unique in that it activates mTOR more than other amino acids, significantly increasing muscle mass.

Metabolic regulation: Leucine plays an important role in metabolic processes, including glucose uptake, mitochondrial biogenesis, and fatty acid oxidation. These processes help to increase the efficiency of energy metabolism in cells and support the energy supply needed for protein synthesis.

Insulin Secretion and Blood Sugar Regulation: Leucine stimulates the secretion of insulin, which helps regulate blood sugar levels. This has potential health benefits for people with diabetes or those who need to control their blood sugar.

Muscle Recovery & Repair: Leucine, along with other BCAAs (isoleucine and valine), aids in the repair and recovery of muscle tissue, especially after exercise. It is also used for postoperative recovery and to speed up wound healing.

Growth Hormone Production: Leucine helps increase the secretion of growth hormone, which is important for growth and development in children and adolescents, as well as muscle maintenance and repair in adults.

Anti-Aging & Health: Studies have shown that leucine may extend longevity by promoting mitochondrial biogenesis and energy metabolism. In addition, it is used to prevent and treat muscle loss (e.g., in the elderly or chronically malnourished).

Energy supply: During high-intensity exercise, leucine can be used as an energy source to reduce central fatigue.

Other potential features: Leucine has also been studied to treat certain diseases, such as cancer (by inhibiting the growth of tumor cells), as well as to improve cognitive function and learning.

Table.2 Amino acid related services at Creative Peptides.

Lysine Amino Acid Function

Lysine is an essential amino acid that is essential for human health and has a variety of physiological functions. The following are the main functions of lysine:

Protein synthesis: Lysine is one of the essential amino acids for protein synthesis, and it helps build a variety of proteins that the body needs by participating in the formation of polypeptide chains.

Collagen formation: Lysine plays a key role in the formation of collagen, which is an important component of skin, bone, cartilage, and other connective tissues. Lysine maintains tissue integrity and strength by promoting the cross-linking of collagen.

Calcium absorption: Lysine aids in the absorption of calcium, which is essential for bone health. It helps maintain bone strength by facilitating the transport of calcium ions across cell membranes.

Immune function: Lysine is involved in the formation of antibodies that support the normal functioning of the immune system.

Synthesis of hormones and enzymes: Lysine is a precursor to a variety of hormones and enzymes, including growth hormone (GH) and insulin.

Fat metabolism: Lysine is a precursor to carnitine, which plays a key role in the oxidation of fatty acids, helping to convert fatty acids into energy.

Wound healing: Lysine helps speed wound healing, reduces the formation of scar tissue, and promotes the formation of new blood vessels.

Antiviral effects: Lysine has an inhibitory effect on certain viruses, such as herpes virus, and may help reduce the occurrence of viral infections.

Nerve function: Lysine contributes to the normal function of the nervous system by promoting the metabolism of neurotransmitters.

Other functions: Lysine may also be beneficial for lowering blood pressure, improving heart function, and slowing down skin aging.

Valine Amino Acid Function

Valine is an essential amino acid that belongs to the branched-chain amino acid (BCAA) family and works together with leucine and isoleucine to have a variety of important functions for human health. The following are the main functions of valine:

Muscle Growth & Repair: Valine plays a key role in muscle metabolism, promoting muscle growth and repair and preventing muscle breakdown. It protects muscles from damage by providing extra glucose to support their energy needs during strenuous exercise.

Energy Supply: Valine is a glycogen-producing amino acid that can be converted into glucose during high-intensity exercise, providing the body with immediate energy. In addition, it is involved in energy metabolism through the oxidation of branched α-ketoacid dehydrogenase complexes to produce β-hydroxyisobutyric acid, which is an ideal substrate for gluconeogenesis.

Blood Sugar Regulation and Insulin Secretion: Valine helps regulate blood sugar levels and stimulates insulin secretion, thereby maintaining normal blood sugar balance.

Nervous System Support: Valine has a stimulating effect on the central nervous system, supporting brain function and cognition, as well as helping to relieve anxiety and insomnia.

Liver Health: Valine helps remove excess nitrogen (a potential toxin) from the liver and convert it into other parts of the body that it needs. In addition, it may help patients with impaired liver function recover.

The immune system and inflammation: Valine also plays an important role in immune function, strengthening the immune system and reducing inflammation.

Nutrient Balance and Nitrogen Balance: Valine is essential for maintaining nitrogen balance and aids in protein synthesis and tissue repair.

Other health benefits: Valine is also used to treat malnutrition associated with drug addiction and may help alleviate hepatic encephalopathy. 

Function of Non-essential Amino Acids

Although non-essential amino acids can be synthesized in the body, their demand may increase under certain physiological and pathological conditions. For example, it is important to support immune, metabolic, and wound-healing functions. Here we illustrate by listing three major non-essential amino acids.

Table.3 Function of main non-essential amino acids.

Alanine Amino Acid Function

Alanine is a non-essential amino acid that is widely found in proteins and has a variety of important physiological functions. The following are the main functions of alanine:

Protein synthesis: Alanine is one of the basic building blocks of proteins, is involved in the protein synthesis process, and is essential for the growth, repair, and maintenance of muscles, tissues, and organs.

Energy metabolism: Alanine plays a key role in glucose metabolism, particularly in the glucose-alanine cycle. It provides energy to the body by converting amino acids from muscles and other tissues into pyruvate and transporting it to the liver, which converts pyruvate into glucose.

Immune System Support: Alanine boosts immune system function by boosting the production of antibodies.

Nervous System Support: Alanine plays an important role in the central nervous system, helping to maintain cognitive function and overall mental health.

Gluconeogenesis: Alanine is directly involved in the gluconeogenesis process and helps maintain blood sugar levels.

Nitrogen transfer: Alanine transfers nitrogen from peripheral tissues to the liver for processing and excretion in the body through the action of alanine aminotransferase (ALT).

Antioxidant Effects: Alanine helps remove toxins from the body and protects cells from oxidative damage by boosting collagen synthesis.

Other Health Benefits: Alanine has also been studied to support liver health, promote wound healing, and reduce muscle breakdown.

As a versatile non-essential amino acid, alanine plays an important role in the body's energy metabolism, protein synthesis, immune support, and nervous system function.

Serine Amino Acid Function

Serine is a non-essential amino acid that, although nutritionally considered replaceable, plays an integral role in metabolic processes. Serine plays an important role in a variety of biochemical processes, including protein synthesis, nucleic acid synthesis, lipid metabolism, and nervous system function.

Protein and nucleic acid synthesis: Serine is an important component of protein synthesis and is involved in the biosynthesis of purines and pyrimidines, which are the basic units of DNA and RNA. In addition, serine is also involved in the synthesis of nucleotides, such as thymine and purine by providing a carbon backbone.

Lipid metabolism: Serine is a precursor to phospholipids and sphingolipids such as phosphatidylserine and sphingomyelin, which play a key role in the structure and function of cell membranes. Phosphatidylserine also plays an important role in the process of apoptosis.

Function of the nervous system: Serine has an important role in the central nervous system, where it is involved in the synthesis of neurotransmitters such as dopamine and serotonin, which regulate mood and behavior. In addition, D-serine, as a neurotransmitter, can regulate the activity of NMDA receptors, which is of great significance for the research of neurological diseases such as Alzheimer's disease.

Immune system: Serine is involved in the synthesis of immunoglobulins and antibodies, thereby enhancing the function of the immune system. It also supports muscle growth and maintenance, including heart muscle, by promoting creatine absorption.

Methylation: Serine is the primary carbon source for methylation reactions, supporting the methylation of DNA and RNA through the production of S-adenosylmethionine (SAMe), which is essential for gene expression regulation.

Other metabolic pathways: Serine is also involved in the production of NADPH, an important component of antioxidant defense. In addition, it also plays a role in fatty acid metabolism and muscle growth.

Disease associations: Disorders of serine metabolism are associated with a variety of diseases, including metabolic syndrome, schizophrenia, depression, fatty liver, type I diabetes, hypertensive nephrirosis, primary biliary cirrhosis, and cancer.

Tyrosine Amino Acid Function

Tyrosine is a non-essential amino acid synthesized in the body from another amino acid, phenylalanine. Tyrosine has a variety of important functions in living organisms, including the following:

Precursor to neurotransmitters: Tyrosine is a precursor to several neurotransmitters, including norepinephrine, epinephrine, and dopamine. These neurotransmitters play a key role in regulating mood, attention, and nervous system function. For example, dopamine is an important neurotransmitter that is involved in many functions of the brain, while a lack of dopamine may lead to diseases such as Parkinson's disease.

Synthesis of hormones: Tyrosine is an important precursor to thyroid hormones such as T3 and T4, which are essential for maintaining a normal metabolic rate and body temperature. In addition, tyrosine is involved in the synthesis of epinephrine and norepinephrine, which play an important role in the stress response.

Melanin production: Tyrosine is a precursor to melanin, which is responsible for the color of skin, hair and eyes. Therefore, tyrosine also plays an important role in skin protection and pigmentation.

Components of protein structure: Tyrosine is one of the basic building blocks of proteins and is involved in the structural formation of almost all proteins.

Energy metabolism: Both tyrosine and phenylalanine are precursors to gluconeogenesis and ketone production, which means that they can be converted into glucose or ketone bodies in the body for energy.

Other functions: Tyrosine is also involved in the catalysis of certain enzymes and plays a regulatory role in signaling processes. For example, tyrosine residues act as phosphorylation sites in the active sites of some enzymes, influencing metabolic pathways.

Although tyrosine can be synthesized from phenylalanine in the body, it may become a conditionally essential amino acid in certain circumstances, such as when phenylalanine is deficient or in metabolic disorders. Additionally, tyrosine supplements are sometimes used to improve mood, enhance concentration, and increase mental alertness due to their role in neurotransmitter and hormone synthesis.

Conditionally Essential Amino Acids Function

Conditionally essential amino acids can become essential during periods of stress, disease, or growth. They play crucial roles in immune regulation, wound healing, and energy metabolism. Below, we highlight the primary functions of these amino acids, using two typical examples of conditionally essential amino acids.

Table.4 Function of main conditionally essential amino acids.

Arginine Amino Acid Function

Arginine is a conditionally essential amino acid that plays a multifaceted role in the human body, with functions ranging from basic metabolic processes to complex physiological and pathological reactions. Here are the main functions of arginine:

Protein synthesis: Arginine is a substrate for protein synthesis and is involved in the synthesis and breakdown of proteins in the body.

Synthesis of nitric oxide (NO): Arginine is an important precursor to NO, a powerful vasodilator that regulates blood pressure, blood flow, and cardiovascular function. In addition, NO is also involved in immunomodulatory and antimicrobial effects.

Immune function: Arginine has an important effect on the immune system, it promotes the production of white blood cells, enhances immune function, and becomes an essential amino acid in certain situations, such as infections or stress states.

Wound healing: Arginine aids in wound healing, speeding up the healing process by promoting the production of new skin cells and reducing the risk of infection.

Hormone secretion: Arginine is a synthetic precursor of a variety of hormones (such as growth hormone, insulin-like growth factor, etc.), which play an important role in growth and development, metabolic regulation, etc.

Metabolic regulation: Arginine participates in the urea cycle to help remove excess ammonia from the body and prevent ammonia poisoning. It is also involved in the synthesis of substances such as polyamines, creatine, proline, etc.

Cardiovascular Health: Arginine helps lower blood pressure, improves blood vessel function, and may have a preventive effect on cardiovascular disease through the action of NO.

Muscle function: Arginine promotes muscle protein synthesis by regulating the mTOR pathway, which contributes to muscle growth and repair.

Anti-tumor effects: Arginine can inhibit the proliferation of tumor cells in some cases, achieving this effect by modulating immune responses and metabolic pathways.

Other functions: Arginine is also involved in regulating energy balance, promoting bone repair, improving sexual function, etc.

Cysteine Amino Acid Function

Cysteine plays an important role in several biological processes, including antioxidant, protein structure stabilization, detoxification, collagen synthesis, neurotransmitter synthesis, and DNA repair. These functions make it one of the key amino acids for maintaining human health. Here is a detailed description of its key features:

Antioxidant effects: Cysteine is an important precursor to glutathione (GSH), a powerful antioxidant that neutralizes free radicals and reactive oxygen species and protects cells from oxidative damage. In addition, cysteine itself has antioxidant properties, scavenging free radicals and neutralizing toxins.

Stability of protein structure: The thiol group (-SH) of cysteine allows it to form disulfide bonds with other cysteine residues, which play a crucial role in the tertiary and quaternary structure of proteins, especially in complexly folded proteins such as enzymes and antibodies.

Detoxification: Cysteine is involved in the detoxification process in the liver and is a component of several detoxification enzymes. It is also able to help transport and remove toxic heavy metals from the body.

Collagen synthesis: Cysteine is an important component of collagen, the main structural protein of skin, hair, and nails. It helps maintain the elasticity and texture of the skin.

Synthesis of neurotransmitters: Cysteine can be involved in nervous system function as an inhibitory neurotransmitter.

DNA repair: Cysteine is involved in the DNA repair process and helps maintain the normal function of cells.

Regulation of enzyme activity: Cysteine plays a key role in the catalytic reactions of many enzymes, especially those that rely on their thiol groups for redox reactions.

Nutritional supplementation: In some cases, such as premature infants or older adults, cysteine may be considered an essential amino acid and needs to be consumed through the diet.

Synthesis of other biological compounds: Cysteine can also be involved in the synthesis of non-protein compounds such as sulfuric acid and taurine.

Summary

Understanding the functions of amino acids is essential for scientific research, particularly in fields like biochemistry, nutrition, and medicine. Amino acids play critical roles in processes such as protein synthesis, enzyme function, immune response, and metabolic regulation. Research into their diverse functions helps unravel the mechanisms behind health and disease, advancing our ability to design targeted treatments and interventions. This knowledge is pivotal for developing new therapeutic strategies, improving drug efficacy, and enhancing human health, thereby contributing significantly to the scientific and medical communities.

Reference

1. Choi, Bo-Hyun, and Jonathan L. Coloff. The diverse functions of non-essential amino acids in cancer. Cancers 11.5 (2019): 675.