What is an amino acid?

Amino Acids: The Building Blocks of Proteins

Amino acids are the central units in protein metabolism, serving as the building blocks for proteins and as precursors for metabolically essential compounds in the human body. 1 They consist of chains of amino acid subunits joined together by peptide bonds, with chain lengths ranging from two amino acids to thousands, resulting in molecular weights from hundreds to hundreds of thousands of Daltons. 1

Structure and Classification

Amino acids are organic compounds containing both amino and acid groups. From a nutritional perspective, they can be classified into three categories:

• Essential (Indispensable) Amino Acids: These cannot be synthesized by humans and must be provided through diet or parenteral nutrition. The 8 essential amino acids include:

  • Isoleucine
  • Leucine
  • Lysine
  • Methionine
  • Phenylalanine
  • Threonine
  • Tryptophan
  • Valine
  • (Histidine is sometimes included as the 9th essential amino acid) 1, 2

• Non-essential Amino Acids: These can be synthesized by the body from other amino acids or precursors. 1

• Semi-essential (Conditionally Essential) Amino Acids: These can be synthesized from other amino acids, but their synthesis may be limited under certain circumstances. For example, cysteine is synthesized from methionine, and tyrosine from phenylalanine. 1, 3

Physiological Functions

Amino acids serve multiple critical functions in the human body:

1. Structural Components: They form the building blocks of proteins, which are essential for the body's structure and proper function. 1

2. Functional Roles: Proteins function as enzymes, hormones, antibodies, and transport and structural components. 1

3. Metabolic Precursors: Amino acids are converted to metabolically essential compounds such as nucleic acids, creatine, and porphyrins. 1

4. Cell Signaling: Many amino acids participate in cell signaling pathways involving protein kinases and G protein-coupled receptors. 4

5. Regulatory Functions: Some amino acids regulate key metabolic pathways affecting health, growth, development, and reproduction. 4, 5

Nutritional Quality of Proteins

The nutritional quality of food proteins varies based on their essential amino acid composition:

• Complete Proteins: Foods containing essential amino acids at levels that facilitate tissue growth and repair. These have high biological value, with a large proportion of protein being absorbed and retained. Animal sources like eggs, milk, meat, poultry, and fish typically provide complete proteins. 1

• Incomplete Proteins: Foods deficient in one or more essential amino acids. Most plant proteins fall into this category but can be combined with complementary proteins to improve nutritional quality (protein complementarity). For example, combining corn (limited in lysine) with beans (limited in methionine). 1

Protein Requirements and Intake

The recommended daily allowance (RDA) for protein of high biological value for adults is 0.8 g/kg body weight. Actual protein consumption in the US ranges from 88-92g for men and 63-66g for women. 1

Proper protein nutrition requires:

• Balanced and sufficient intake of essential amino acids
• Adequate nitrogen intake for the body to produce non-essential amino acids 1

Clinical Implications

Amino acids play crucial roles in:

• Growth and development, particularly in infants and children
• Tissue repair and maintenance
• Immune function
• Prevention of metabolic diseases
• Supporting reproductive health 4

In clinical settings, particularly for parenteral nutrition, amino acid solutions should be formulated with consideration for both essential and non-essential amino acids to support optimal health outcomes. 1

Absorption and Metabolism

Dietary proteins are digested in the gastrointestinal tract and absorbed as small peptides (di- and tripeptides) and free amino acids. These are then used for protein synthesis in cells or for the synthesis of specific non-protein products. 2, 6

For individuals in nitrogen balance, the amount of protein equal to daily protein intake is degraded each day, with nitrogen being excreted primarily as urea and ammonia. The carbon skeletons of degraded amino acids are recovered through gluconeogenesis, ketone synthesis, or oxidized to carbon dioxide. 2

Human Genetics: Genetics, Molecular Biology, and Developmental Biology

AI:

What is an Amino Acid

Amino acids are the central units in protein metabolism, serving as the building blocks for proteins and as precursors for metabolically essential compounds in the human body. 1

Structure and Classification

Amino acids are organic compounds containing both amino and acid groups 3. They consist of chains joined together by peptide bonds, with lengths ranging from two amino acids to thousands, resulting in molecular weights from hundreds to hundreds of thousands of Daltons 1.

From a nutritional perspective, amino acids are classified into three categories:

• Essential (Indispensable) Amino Acids:

  • Cannot be synthesized by humans and must be provided through diet
  • The 8 essential amino acids are: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine 1
  • Histidine is sometimes included as the 9th essential amino acid 2

• Non-essential Amino Acids:

  • Can be synthesized by the body from other amino acids or precursors
  • Include glutamate, glutamine, aspartate, asparagine, serine, glycine, proline, and alanine 2

• Semi-essential (Conditionally Essential) Amino Acids:

  • Can be synthesized from other amino acids but synthesis may be limited under certain circumstances
  • Examples: cysteine (from methionine) and tyrosine (from phenylalanine) 1, 2

Physiological Functions

Amino acids serve multiple critical functions:

1. Protein Building Blocks: Form the structural and functional components of all cells 1

2. Enzymatic and Regulatory Roles: Function as enzymes, hormones, antibodies, and transport components 1

3. Metabolic Precursors: Converted to compounds like nucleic acids, creatine, glutathione, and porphyrins 1, 2

4. Cell Signaling: Participate in signaling pathways involving protein kinases and G protein-coupled receptors 4

5. Gene Expression: Modulate gene expression affecting growth, development, and metabolism 4, 5

Nutritional Considerations

The nutritional quality of proteins depends on their amino acid composition:

• Complete Proteins: Contain all essential amino acids in proper proportions (mostly animal sources)
• Incomplete Proteins: Deficient in one or more essential amino acids (most plant proteins)
• Protein Complementarity: Combining complementary incomplete proteins to achieve complete amino acid profiles 1

Absorption and Metabolism

Dietary proteins are digested in the gastrointestinal tract and absorbed primarily as:

• Small peptides (di- and tripeptides)
• Free amino acids 6

For individuals in nitrogen balance, protein equal to daily intake is degraded with nitrogen excreted as urea and ammonia, while carbon skeletons are recovered through gluconeogenesis or ketone synthesis 2.

Health Implications

Amino acids are essential for:

• Growth and development
• Tissue repair and maintenance
• Immune function
• Prevention of metabolic diseases
• Supporting reproductive health 4

Proper balance of amino acids is critical for optimal health outcomes and prevention of various disorders related to protein metabolism.