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Amino Acids


Common amino acids have a general structure. They contain a central alpha (aS) carbon atom to which contains

a carboxylic group, an amino group, a hydrogen atom and in addition a side chain R which uniquely defines each

of the 20 common amino acids.

  1.  Side chain R defines the chemical nature & structure of different amino acids.
  2. One of the 20- amino acids Proline is an Imino acid.
  3. Optical Activity – With the exception of glycine, all amino acids have at least one asymmetric carbon atom are therefore optically active. So amino acids acid exist as stereoisomers pair called enantiomers. These amino acid isomers are typically called (laevorotatory) or (dextrorotatory), depending upon direction they rotate plane polarized light.

Absolute Configuration of An Amino Acid

COO—                             COO

•                  •

•                  •

•                  •

NH3+ —C—H                H—C—NH3+

•                  •

•                  •

R                  R

L-configuration                D-configuration


D. if a-NH3+ group is projected to left, amino acid have an L-absolute configuration.

E.  If a-NH3+ group is projected towards the right, D-absolute configuration.


Only L-α-amino acids occur in mammalian proteins

  1. Classification of Amino-acids Based on structure of side chain:
    1. Amino acids with aliphatic side chains e.g. Glycine, Alanine, Valine, Leucine, Isoleucine
    2. Amino acids with side chain containing hydroxyl groups e.g. Serine, Threonine, Tyrosine
    3.  Amino acids with side chain containing sulfur atoms e.g. Cysteine, Methionine
    4. Amino acids with side chain containing acidic groups e.g. Aspartic acid, Glutamic acid
    5. Amino acids with side chain containing basic groups e.g. Arginine, Lysine, Histidine.
    6. Amino acids with containing aromatic rings e.g. Phenylalanine, Tyrosine.
  2. 7 Amino acids with heterocyclic ring e.g. Histidine, Proline, Tryptophan.


Extra Edge

  1. Both D-amino acids and non-?-amino acids occur in nature, but only L-?-amino acids are present in proteins.
  2. The pKa values of all functional groups of an amino acid dictate its net charge at a given pH. pI is the pH at which an amino acid bears no net charge and thus does not move in a direct current electrical field, that is does not move during electrophoresis.


2.   Classification based on polarity of R Groups

Amino acids according to polarity of their side chains (r groups) at ph 7 Total 20 amino acids


Nonpolar R groups

(8 Amino Acids)

Polar but uncharged R groups

(7 Amino Acids)

Charged polar R groups

(5 Amino Acids)



Negatively charged (2)



Aspartic acid



Glutamic acid



Positively charged (3)














3.  Metabolic Classification as Glucogenic, Ketogenic & both Glucogenic and Ketogenic.

  1. Ketogenic amino acids which are degraded to either acetyl coenzyme A (acetyl CoA) or acetoacetyl CoA. Purely ketogenic amino acids are leucine and lysine.
  2. Glucogenic amino acids are degraded to pyruvate or citric acid cycle intermediates, which can give rise to glucose. Purely glucogenic amino acids include alanine, arginine, asparagines, aspartate, cysteine, glutamate, glutamine, glycine, histidine, methionine, proline, serine valine and threonine.
  3. Ketogenic and glucogenic for e.g. isoleucine, phenylalanine, tyrosine and tryptophan.


Note : Total 18 amino acids can be converted to glucose and 6 amino acids can be converted to fats.

Amino Acid & Their Derivatives as Neurotransmitters

  1. Amino acid
    1. Glycine is the major inhibitory neurotransmitter in the brainstem and spinal cord. When released onto a neuron, it hyperpolarizes the neuron and hence decreases its activity.
    2. Glutamic, acid is the major excitatory neurotransmitter. It is released by large number of neurons and is very important in the region of the' brain which influences memory.
  2. Amino acid derivatives:
    1. Dopamine is derived from tyrosine during catecholamine biosynthesis. It acts as a neurotransmitter in brain, where it has many functions, e.g. controlling blood pressure. Increases in dopamine activity cause aggressiveness, and may lead to Schizophrenia or Huntington’s chorea. '­
    2. Norepinephrine, derived from tyrosine, is an important hormone and neurotransmitter. As a hormone, it is a part of the right or flight response. It is the main neurotransmitter of the branch of the autonomic nervous system called the sympathetic nervous system. It plays several roles as neurotransmitter in the brain.
    3. Epinephrine, also derived from tyrosine, is similar to norepinephrine in many of its functions. It is released as a hormone in the fight-or- flight response by the adrenal medulla: It is a major neurotransmitter in certain regions of the brain.  
    4. Serotonin (5-hydroxytryptamine) is a very important neurotransmitter in brain. It is involved with spinal re­flexes, sleep-wake cycle, flow of sensor afferents and habituation.
    5. Gamma-amino butyric acid (GABA), derived by decarboxylation of glutamate is the major inhibitory neuro­transmitter in brain. Most neurons in the brain are continually receiving GABA input that keeps them quiet. Patients with anxiety are prescribed valium (benzodiazepine), which enhances the action of GABA on the brain. Interestingly, the major inhibitory neurotransmitter in the brain-GABA is formed from a major excitatory neurotransmitter, glutamic acid.

  1. Nitrogen Balance:
    If the total daily nitrogen loses in urine, skin and feces are equal to total nitrogen intake, the subject is said to be in Nitrogen equilibrium e.g. healthy adult.
    1. Positive Nitrogen Balance : If nitrogen loses are less than intake, subject is said to be in positive nitrogen balance e.g.  growing children and pregnant woman. For positive nitrogen balance all essential amino acids should be present in dietary proteins.
    2. Negative Nitrogen Balance: If nitrogen losses are greater than the intake, the subject is in negative nitrogen balance e.g.; following surgery, is advanced cancer. Absence of any essential amino Acid in diet leads to negative nitrogen balance.
  2. Protein Turnover:

    Most proteins in body are constant being synthesized & then degraded. In healthy adult, the total amount of protein in the body remains constant. This process, called Protein turnover leads to hydrolysis and resynthesis of 125 to 220 g of body protein each day.  The susceptibility of protein to degradation is expressed as its half life t ½, the time required to reduced its concentration to 50% of its initial value. For eg.; Half life of liver protein range from 30mt – 150 hrs.
  3. Essential Amino Acid:

    1. Amino acid which can not be synthesize by humans and therefore, essential dietary factors.
    2. Total 8 essential amino acids are:-
    3. Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine.
    4. Semiessential amino acid: (synthesized at rates inadequate to support growth of children) for e.g. Arginine & Histidine

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