Amino Acids - Structure, Function, Metabolism and Role in Human Health

Introduction to Amino Acids

Amino acids are organic compounds that contain amine (-NH2) and carboxyl (-COOH) functional groups. They serve as the building blocks of proteins, which are essential biomolecules for all living organisms. There are around 500 naturally occurring amino acids, but only 20 are directly encoded by the universal genetic code and incorporated into proteins. These 20 amino acids are called standard or proteinogenic amino acids.

Amino acids are vitally important as they are required to form proteins, peptides, hormones, neurotransmitters, nucleic acids and metabolic intermediates essential for cellular functioning. They play a crucial role in muscle growth and maintenance as well as overall health and wellbeing. Understanding the structure and function of amino acids can provide great insight into human nutrition, metabolism and health.

Key Structural Features of Amino Acids

All amino acids contain an asymmetric carbon atom called the -carbon. This carbon atom has four different substituents - an amino group, a carboxyl group, a hydrogen atom and a variable R group. The R group is also known as the side chain and differs between each of the 20 standard amino acids. This provides the diversity in structure and function seen amongst the amino acids.

Based on the properties of the R group, amino acids can be classified into four categories:

• Nonpolar aliphatic - Glycine, Alanine, Valine, Leucine, Isoleucine
• Polar uncharged - Serine, Cysteine, Threonine, Asparagine, Glutamine
• Positively charged - Lysine, Arginine, Histidine
• Negatively charged - Aspartic acid, Glutamic acid

The sequence and combination of these 20 standard amino acids ultimately determine the structure and function of proteins. The properties of the R groups influence how amino acids interact with each other and their environment.

Key Functions of Amino Acids

Here are some of the key functions of amino acids in the human body:

• Building Blocks of Proteins - Amino acids link together via peptide bonds to form protein chains. The sequence of amino acids determines each protein's unique 3D structure and function.
• Neurotransmitters - Glutamate, aspartate, glycine and GABA are examples of neurotransmitters derived from amino acids.
• Biosynthesis - Amino acids are precursors for synthesis of molecules like nitric oxide, carnitine, heme, nucleotides, creatine, polyamines and hormone peptides.
• Energy Production - Breakdown of amino acids generates glucose intermediates that feed into energy production pathways.
• Nitrogen Transport and Disposal - Amino groups are used to transport nitrogen and excrete nitrogenous waste.

It's clear that amino acids have far reaching effects in human metabolism. They are so fundamentally important that there are multiple intricate mechanisms to regulate their intracellular concentrations, transport and turnover.

Dietary Sources of Amino Acids

Amino acids are primarily obtained through dietary protein which is abundantly found in foods like:

• Meat
• Poultry
• Fish
• Eggs
• Dairy products
• Beans
• Nuts
• Seeds

Animal sources provide complete proteins that contain all nine essential amino acids. Plant sources often have incomplete proteins lacking one or more essential amino acid. By eating a variety of plant proteins, you can ensure adequate intake of all essential amino acids.

Essential amino acids cannot be synthesized in the body so they must be obtained from the diet. The nine essential amino acids are:

• Histidine
• Isoleucine
• Leucine
• Lysine
• Methionine
• Phenylalanine
• Threonine
• Tryptophan
• Valine

The remaining 11 standard amino acids are considered non-essential because they can be synthesized endogenously. However, some sources consider arginine, cysteine, glycine, glutamine, proline and tyrosine to be conditionally essential under certain physiological conditions like illness or growth.

Recommended Daily Intake

There are no Recommended Daily Allowances (RDAs) for individual amino acids. Rather, RDAs exist for protein intake based on body weight and gender. The current RDA for protein is 0.8 grams per kilogram of body weight per day for adults. Endurance athletes or those performing significant strength training may need 1.2-2.0 grams per kilogram per day.

As long as you eat a balanced diet containing a variety of unprocessed whole foods, it's likely you are meeting amino acid requirements from dietary protein intake alone. Those following restrictive diets like veganism may need to pay more attention to ensure adequate essential amino acid intake. In these cases, specific amino acid supplements may be beneficial.

Amino Acid Supplementation

The most popular amino acid supplements include:

• BCAAs (branched-chain amino acids) - Leucine, isoleucine and valine. Primarily used to stimulate muscle protein synthesis.
• EAAs (essential amino acids) - Mixture of all essential amino acids to support protein synthesis and lean mass gains.
• Glutamine - Conditionally essential amino acid. Used for gut health, immune function and as an energy source for cells.
• Carnitine - Derived from lysine and methionine. Used to transport fatty acids for energy production.

These supplements may offer performance and health benefits for athletes, bodybuilders, elderly, vegetarians, vegans and those with certain diseases. However, whole food sources should always be prioritized when possible.

It's important to note that excessive intake of certain amino acid supplements like tryptophan and tyrosine can potentially lead to side effects. Always start with lower doses to assess tolerance.

Timing and Dosage Recommendations

Here are some general supplement timing and dosage recommendations:

• BCAAs - 5-20 grams before, during and/or after workouts
• EAAs - 10-40 grams between meals or pre/post workout
• Glutamine - 5-10 grams between meals, before bed or after workouts
• Carnitine - 1-3 grams once or twice daily with food

However, needs are individualized and depend on factors like diet quality, protein intake, training status, fitness goals, age and health conditions. It's best to experiment under guidance of a sports dietician or nutritionist.

Metabolism of Amino Acids

Amino acids are metabolized in several ways depending on bodily requirements at the time. The three major metabolic fates are:

1. Incorporation into Proteins
2. Catabolism for Energy
3. Conversion to Specialized Products

Metabolism is a highly complex and coordinated process optimizing amino acid utilization for each circumstance. Here is a brief overview of the major pathways:

1. Incorporation into Proteins

During protein synthesis, amino acids are sequentially linked by peptide bonds to form polypeptide chains. This process is directed by codon sequences on mRNA transcripts based on the genetic code. Growing children have particularly high protein synthesis requirements.

The cellular concentration of amino acids regulates global protein synthesis rates. Excesses activate the mechanistic target of rapamycin (mTOR) pathway to upregulate protein synthesis. BCAAs like leucine are particularly potent activators.

2. Catabolism for Energy

Amino acids can be broken down into intermediate compounds and fed into metabolic pathways to generate energy. The first step is called transamination where the amine group is transferred to a ketoacid converting the amino acid into an -ketoacid. Glucogenic amino acids can be converted into pyruvate, acetyl CoA or intermediates of the citric acid cycle.

Ketogenic amino acids are converted into acetyl CoA or acetoacetyl CoA which enter the ketogenesis pathway. BCAAs like leucine are exclusively ketogenic. Overall, amino acids contribute up to 15% of the body's total energy production needs.

3. Conversion to Specialized Products

Specific amino acids serve as precursors for crucial biological compounds like heme, purines, creatine, nitric oxide, polyamines, hormones, neurotransmitters and more. These compounds perform highly specialized functions in the body.

For example, tyrosine is converted into dopamine, norepinephrine and epinephrine for neurological and endocrine signaling. Tryptophan is converted to serotonin which regulates mood and appetite. Sulfur amino acids like cysteine contribute to antioxidant glutathione synthesis.

Such amino acid conversions are tightly regulated to ensure synthesized levels match physiological requirements. Imbalances can affect neurological function, redox status, energy levels and more.

Amino Acid Deficiencies

Amino acid deficiencies are rare in developed nations except in specific medical conditions or restrictive diets. Signs and symptoms include:

• Loss of lean muscle mass
• Fatigue
• Impaired immunity
• Skin, hair, nail and vision problems
• Neurological symptoms like anxiety, depression and memory loss
• Hormonal imbalances
• Slow growth and development in children

However, isolated deficiencies of specific essential amino acids are possible even with adequate total protein intake. For example, lysine deficiency can occur when diets are predominantly wheat-based.

Deficiencies are more likely in developing countries where protein intake is lower or access to complete proteins limited. Assessment of amino acid status involves measuring plasma levels or urinary excretion profiles.

Treatment and Supplementation

Treatment simply involves increased dietary intake of high quality, complete protein sources. In severe cases, essential amino acid supplements may be used under medical supervision. Lysine, tryptophan, threonine and sulfur amino acids like methionine are commonly supplemented.

If protein deficiency is due to an absorption disorder or liver disease, then special medical formulas for IV or enteral feeding would be utilized containing free amino acids that are easy to digest and absorb.

Toxicity and Side Effects

Amino acids are rather safe with little toxicity when consumed from natural food sources. However, high dose supplementation may cause adverse effects in sensitive individuals.

Possible side effects include:

• Gastrointestinal distress like nausea, heartburn, diarrhea
• Nervous system overstimulation, headaches, fatigue, irritability
• Kidney problems
• Nutrient imbalances
• Changes to endocrine and metabolic regulation

Those with liver or kidney disease need to be especially cautious with amino acid supplements due to impaired excretion and detoxification mechanisms.

Excessive intake of branched-chain amino acids may result in a deficiency of other essential amino acids. High tryptophan doses can cause serotonin syndrome characterized by heart palpitations, agitation, headache and nausea.

Consult a doctor before taking high dose amino acid supplements, especially if you have an underlying health condition or take medications. Start with low doses and monitor for any adverse effects.

Conclusions

Amino acids are fundamental organic compounds that link together to form proteins essential for human life. They play a central role in growth, development, immunity, neurotransmitter synthesis and energy production. The 20 standard amino acids can be classified as essential or nonessential based on if they can be synthesized in the body.

Complete proteins containing all essential amino acids are found in foods like meat, eggs, dairy and certain plants. Meeting daily protein intake recommendations ensures adequate amino acid intake for most healthy people. Those with greater needs or on restrictive diets may benefit from targeted amino acid supplementation.

Amino acids are primarily metabolized for protein synthesis, catabolized for energy or converted to specialized products like hormones. Deficiencies are rare but can impair many bodily functions. While generally safe, high dose supplementation can potentially cause toxicity in vulnerable groups. Amino acids are essential compounds that must be adequately obtained from the diet or supplements only when truly required.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult with a healthcare professional before starting any new treatment regimen.