Amino acids are organic compounds, consisting of an amino (-NH2) and a carboxylic acid (-COOH) functional group. They play major roles within the human body, producing neurotransmitters and hormones, involvement in cellular processes, muscle protein synthesis (Lopez and Mohiuddin 2020), enzymatic activity and immunity (Maragkoudakis 2017).
Structure
Amino acids are made up of a carbon atom, an amino (NH2) group, carboxyl (COOH) group, a hydrogen (H) atom and the R group (side chain). This R group differs in the tertiary structure of each protein, giving the amino acid its individuality. Amino acids with a polar side chain are soluble in water and make up the surface of the protein, whereas those with a nonpolar side chain will form the proteins core (Lodish and National Library of Medicine 2000).
A condensation reaction results in the formation of a peptide bond between the carboxyl group of one amino to the amino group of another. The DNA of the gene encoding for the protein determines the sequence of amino acids in the polypeptide chain, giving the protein its primary structure.
The secondary structure is determined by the bonds between atoms on the backbone of the polypeptide. For example, an alpha helix is characterised by a hydrogen bond between the carbonyl of one amino acid to the amino group of another, four down the chain. Alternatively, to form a beta pleated sheet, polypeptide chains line up in parallel, allowing hydrogen bonds to form between the carbonyl and amino groups. The protein can be made up of either, or both types of secondary structure (Kahn Academy 2009).
The tertiary structure of the protein is dependent on the R group interactions which fold the protein into a three-dimensional structure. R groups form hydrogen bonds, ionic bonds, sulphur bridges, dipole-dipole interactions or London dispersion forces.
The type of bond made, determines the functionality of the protein. In proteins with more than one polypeptide chain, the interaction between these chains makes up the proteins quaternary structure, forming a closely packed arrangement (Kahn Academy 2009).
Classification
The order of amino acids in the chain determines the structure and function of the protein. Although hundreds of amino acids exist, the human body only utilises 20. These are further categorised into essential, non-essential and conditionally essential amino acids (Lopez and Mohiuddin 2020). The essential amino acids must be consumed through the diet because humans lack the genetic material for their synthesis, whilst the non-essentials can be made by the body (Guedes et al., 2011). The conditionally essential amino acids need to be consumed through dietary means during periods of high metabolic demand, for example during childhood growth or pregnancy, when the body is unable to synthesise these to sufficient amounts (British Nutrition Foundation (BNF) 2020).
| Amino Acid categories (BNF 2020) | ||
| Essential amino acids | Non-essential amino acids | Conditionally essential |
| Isoleucine | Alanine | Cysteine |
| Leucine | Asparagine | Glutamine |
| Valine | Aspartate | Glycine |
| Lysine | Glutamate | Proline |
| Methionine | Glycine | Histidine |
| Phenylalanine | Serine | Arginine |
| Threonine | Tyrosine | |
| Tryptophan |
Degradation of amino acids, following protein digestion, occurs in the liver and skeletal muscle, and is essential as excess amino acids cannot be stored. The amine group of the amino acid is removed by deamination or transamination to begin its catabolism. Deamination is defined as the removal of the amine group (NH3), whereas transamination is the transfer of an amine group from one amino acid to a keto acid, this being an amino acid without the amine group. Upon removal of the amine group, the remaining carbon skeleton is converted for use as a metabolic intermediate or is utilised for the synthesis of non-essential amino acids (Guedes et al., 2011), for which amine groups can bond to.
Muscles are continually being synthesised and broken down. Following protein consumption, digestion and absorption, amino acids are transported to the systemic circulation, where they exist in sufficient quantities to be synthesised into muscle protein. Comparatively, in a fasted state, muscle degradation occurs at a greater rate than synthesis, leading to protein loss. Some amino acids released during muscle degradation are oxidised, meaning they are unable to be reincorporated into muscles, and instead are released into plasma for use by other tissues or cellular processes within the body (Wolfe 2017).
During fasted periods, protein is broken down and the amino acids are utilised for energy, producing glucose, ketone bodies and CO2. If the carbon skeleton of the amino acid is converted to glucose, it is considered glucogenic, or conversely ketogenic, if its carbon skeleton is converted to acetyl coenzyme A or acetoacetate. Therefore, if amino acids are not made into muscle, and are instead broken down into energy, they have a value of approximately 4kcal/ gram (BNF 2018).
Protein sources
The Protein Digestibility Corrected Amino Acid Score (PDCAAS) indicates protein quality of foods. A score of 100%, being the highest quality protein as it meets all of the body’s amino acid requirements. The Digestible Amino Acid Score (DIAAS) is a more recent criteria, also measuring protein quality. Although all plant and animal foods contain amino acids, animal sources tend to be of better quality, being richer in the essentials, as well as being more easily absorbed (Berrazaga et al., 2019). Plant based proteins, with exception of soya and quinoa tend to have incomplete amino profiles. For example, legumes are limited in methionine, so should be combined with grains which are limited in lysine and threonine, to ensure a full amino acid profile is obtained.
Isolated amino acids
Consumption of isolated amino acids has become popular, following evidence suggesting their intake, particularly branched chain amino acids (BCAAs), to maximise muscle protein synthesis. For example, leucine supplementation following resistance exercise, is often used to increase the synthesis of muscle. However, the extent to which this is effective is dependent on the presence of other essential amino acids existing in quantities that do not compete for cell carriers, which would inhibit leucine uptake (Santos and Nascimento 2019). Jackman et al., (2017) investigated the effects of 5.6g isolated BCAA intake following resistance exercise in comparison to a placebo. The treated group experienced a 22% increase in muscle protein synthesis compared to controls, however this was even greater when a whey protein supplement was consumed, containing BCAAs in similar quantities (Churchward-Venne et al., 2012). Consistent findings suggest that BCAAs do not significantly enhance muscle protein synthesis, independent of other essential amino acids.
How do I declare amino acids on pack?
According to the US Food and Drug Administration (FDA), if a product contains only individual amino acids, it cannot be declared as protein (FDA 2019). Nevertheless, amino acids have an energy value of ~4kcal/ gram, the same as protein, if they are not made into muscle, but instead are utilised as energy (BNF 2018).
Isabella Cotton – Oxford Brookes University BSc Nutrition Student 2020
References
Berrazaga I, Micard V, Gueugneau M and Walrand S (2019) The Role of the Anabolic Properties of Plant- versus Animal-Based Protein Sources in Supporting Muscle Mass Maintenance: A Critical Review. Nutrients 11(8): 1825.
British Nutrition Foundation (BNF) (2018) Protein. Available at: https://www.nutrition.org.uk/nutritionscience/nutrients-food-and-ingredients/protein.html (Accessed 05/12/2020).
Churchward-Venne TA, Burd NA, Mitchell CJ, West DWD, Philp A, Marcotte GR, Baker SK, Baar K and Phillips SM (2012) Supplementation of a suboptimal protein dose with leucine or essential amino acids: effects on myofibrillar protein synthesis at rest and following resistance exercise in men. The Journal of Physiology 590(11): 2751–2765.
Guedes R, Prosdocimi F, Fernandes G, Moura L, Ribeiro H and Ortega J (2011) Amino acids biosynthesis and nitrogen assimilation pathways: a great genomic deletion during eukaryotes evolution. BMC Genomics 12(Suppl 4): S2.
Jackman SR, Witard OC, Philp A, Wallis GA, Baar K and Tipton KD (2017) Branched-Chain Amino Acid Ingestion Stimulates Muscle Myofibrillar Protein Synthesis following Resistance Exercise in Humans. Frontiers in Physiology 8.
Khan Academy (2009) Orders of protein structure. Khan Academy. Available at: https://www.khanacademy.org/science/biology/macromolecules/proteins-and-amino-acids/a/orders-of-protein-structure (Accessed 05/12/2020).
Lodish HF and National Library Of Medicine (2000) Molecular cell biology. New York: W.H. Freeman; Basingstoke.
Lopez M and Mohiuddin S (2020) Biochemistry, Essential Amino Acids. StatPearls, Treasure Island (FL)
Maragkoudakis P (2017) Dietary Protein. EU Science Hub – European Commission. Available at: https://ec.europa.eu/jrc/en/health-knowledge-gateway/promotion-prevention/nutrition/protein (Accessed 03/12/20).
Santos C de S and Nascimento FEL (2019) Isolated branched-chain amino acid intake and muscle protein synthesis in humans: a biochemical review. Einstein (São Paulo) 17(3).
The Astrophysics & Astrochemistry Laboratory (2020) Amino Acids and Their Production during the Photolysis of Astrophysically Relevant Ices. Available at: http://www.astrochem.org/sci/Amino_Akahcids.php (Accessed 04/12/2020).
US Food and Drug Administration (FDA) (2019) Dietary Supplement Labeling Guide: Chapter IV. Nutrition Labeling. FDA. Available at: https://www.fda.gov/food/dietary-supplements-guidance-documents-regulatory-information/dietary-supplement-labeling-guide-chapter-iv-nutrition-labeling#4-12 (Accessed 04/12/20).
Wolfe RR (2017) Branched-chain amino acids and muscle protein synthesis in humans: myth or reality? Journal of the International Society of Sports Nutrition 14(1).