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Nutritional, Metabolic and Group based Classification

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Nutritional Classification

Nutritional classification is based on the availability of amino acids.

Essential amino acids

The amino acids which cannot be synthesized in the body must be supplied to the body through diet are called essential amino acids (cannot be synthesized denovo).

Non-essential amino acids

The amino acids which can be synthesized in the body are called non-essential amino acids.

Of the 20 amino acids in proteins, humans are able to synthesize only 11 (non-essential amino acids), the other 9 called essential amino acids, must be obtained in diet. Amino acids are not stored in the body. The division between essential and nonessential amino acids is not clear cut however tryosine for instance is sometimes considered non-essential because humans can produce it from phenylalanine, but phenylalanine itself is essential and must be obtained in the diet.

Arginine can be synthesized by humans, but much of the arginine in proteins also comes from the diet. In addition, the amino acid Arginine and Histidine  are sometimes conditionally essential, meaning they are not normally required in the diet, but must be supplied exogenously to specific population that do not synthesize it in adequate amount.

List-of-essential-and non-essential-amino-acids

Limiting Amino Acid

The net dietary protein utilization is profoundly affected by limiting amino acid.


Metabolic Classification

This classification of amino acids is done on the basis of product form on degradation. Amino acids are used for different purposes in our body. Most of the metabolic pool of amino acids is used as building blocks of proteins, and a smaller proportion is used to synthesize specialized nitrogenated molecules as epinephrine, norepinephrine, neurotransmitters and the precursors of purines and pyrimidines.  Since amino acids cannot be stored in the body for later use, any amino acid not required for immediate biosynthetic needs is deaminated and the carbon skeleton is used as metabolic fuel or converted into fatty acids via acetyl CoA.  The main products of the catabolism of the carbon skeleton of the amino acids are pyruvate, oxalacetate, a-ketoglutarate, succinyl CoA, fumarate, acetyl CoA and acetoacetyl CoA. Amino acids can be classified according to the metabolic fate of the carbon skeleton as: ketogenic,  glucogenic,  ketogenic and glucogenic.


Ketogenic Amino Acids

Lysine and Leucine are the only amino acids that are exclusively ketogenics and on degradation are converted into acetoacetyl-CoA. These can yield ketone bodies in the liver, since acetoacetyl-CoA can be converted into acetoacetate and β-hydroxybutyrate (and also they do not produce metabolites that can be converted in glucose).

Glucogenic Amino Acids

The amino acids that can be converted into pyruvate, α-ketoglutarate, succinate and oxaloacetate, 15 altogether, can be converted into glucose and glycogen. They are called glucogenic amino acids. Glucogenic amino acids  are: Alanine, Arginine, Asparagine, Aspartate, Cysteine, Glutamate, Glycine, Histidine, Methionine, Proline, Serine, Valine and Threonine, Phenylalanine and Tyrosine.

Glucogenic and Ketogenic Amino Acids

Amino acids that yield some products that can become glucose and others that yields acetyl CoA or Acetoacetyl CoA. Amino acids of this kind are Isoleucine, Phenylalanine, Tryptophan, Tyrosine.

Classification on the basis of location of functional groups

Alpha-Amino Acid

  • Alpha-Amino acid is the amino acid having an amine group attached to the alpha carbon atom, i.e. next to the carbonyl group.
  • The general formula of an alpha-amino acid is H2NCHRCOOH, where R is an organic side chain.
  • Glycine and Alanine are examples of an alpha-amino acid.

Beta-Amino Acid

  • Beta-Amino acid is the amino acid having an amine group attached to the beta-carbon atom to a carboxylic acid group.
  • The general formula of a beta-amino acid is H2NCH2CH2COOH, where R is an organic side chain.
  • Lysine and Arginine are examples of beta-amino acid.

Gamma-Amino Acid

  • Gamma-Amino acid is the amino acid having an amine group attached to the gamma carbon atom to a carboxylic acid group.
  • The general formula of a gamma-amino acid is C4H9NO2.
  • 2-aminoisobutyric acid and Aminobutyric acid are examples of gamma-amino acid.

Delta-Amino Acid

  • Delta-Amino acid is the amino acid having an amine group attached to the delta carbon atom to a carboxylic acid group.
  • The general formula of a delta-amino acid is C5H12NO2+

Non-standard Amino Acids

Aside from the 22 standard amino acids, there are many other amino acids that are biologically important. These include the amino acid derivatives found in proteins, non protein amino acids performing specialized functions and the amino acids.

Amino acid derivatives in protein

The 22 standard amino acids can be incorporated into proteins due to presence of universal genetic code. Some of these amino acids undergo specific modification after the protein synthesis occurs.

Some selected examples are:-

  •   Collagen- the most abundant protein in mammals – contains 4-hydroxyproline and 5- hydroxylysine.
  •    Histones – the proteins found in association with DNA – contain many methylated, phosphorylated or acetylated amino acids.
  •    ϒ-Carboxyglutamic acid is found in certain plasma proteins involved in blood clotting.
  •    Cystine is formed by combination of two Cysteines. Cystine is also considered as derived amino acid.

Non-protein amino acids

These amino acids, although never found in proteins , perform several biologically important functions. They may be either α amino acids or non α amino acids. Example of α amino acids ., Ornithine, Citrulline, Thyroxine, Triiodothyronine, S – Adenosyl methionine, Homocysteine, DOPA, Ovothiol, Creatinine. Example of non α amino acids β-Alanine, β-Aminoisobutyric acid, (BAIBA)ϒ-Aminobutyric acid (GABA), δ- Aminolevulinic acid, Taurine

D-Amino acids

The vast majority of amino acids isolated from animals and plants are of L-category. Certain D-amino acids are also found in the antibiotics (actinomycin-D, valinomycin, gramicidin-S). D-serine and D-aspartate are found in brain tissue. D-Glutamic acid and D-alanine are present in bacterial cell walls.

Amino acids useful as drugs

Certain non-standard amino acids that are used as drugs are D-Pencillamine (D-dimethylglycine), a metabolite of penicillin, is employed in the chelation therapy (D-Pencillamine chelate copper) of Wilson’s disease. N-Acetylcysteine can function as an antioxidant and is used in cystic fibrosis and Gabapentin (γ-aminobutyrate linked to cyclohexane) is used as an anticonvulsant.

Protein Turnover

When older proteins are broken down in the body, they must be replaced. This concept is called protein turnover. Different types of proteins have very different turnover rates. Protein synthesis occurs during the process of translation on ribosomes. Protein breakdown occurs generally in two cellular locations:  Lysosomal proteases digest endocytosed proteins.  Cytoplasmic complexes, called proteasomes, digest older or abnormal proteins that have been tagged with ubiquitin for destruction.

Nitrogen Balance

Nitrogen balance is the (normal) condition in which the amount of nitrogen incorporated into the body each day exactly equals the amount excreted.

Negative nitrogen balance occurs when nitrogen loss exceeds incorporation and is associated with protein malnutrition (kwashiorkor), a dietary deficiency disease.Positive nitrogen balance occurs when the amount of nitrogen incorporated exceeds the amount excreted and is associated with growth, pregnancy, recovery phase of injury or surgery, recovery from condition associated with negative nitrogen balance.

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