Introduction to chemistry of proteins

1.9. Classification of Amino acids

Classification of Amino Acid

L-Amino acids are the building blocks of proteins. They are frequently grouped according to the chemical nature of their side chains. Links to individual amino acids are given below:

I. Structural Classification

This classification is based on the side chain radicals (R-groups). Each amino acid is designated by three letter abbreviation e.g. Aspartate as Asp and by one letter symbol D.

II. Electrochemical classification

Amino acids could also be classified based on their acid – base properties

 

 Acidic amino acids (Negatively charged at pH = 6.0)

 Example:

• Aspartic acid               - CH₂ – COO^-
• Glutamic acid              - CH₂ – CH₂ – COO

Basic amino acids (positively charged at PH = 6.0)

Example:

• Lysine                          - CH₂ – CH₂ - CH₂ - CH₂ –NH₃⁺
• Neutral amino acid

 Example:

♦ Serine - CH₂- OH

♦ Threonine - CH₂- OH

                          ↓

                         CH₃

 ♦ Asparagine - CH₂- CO-NH₂

 ♦ Glutamine - CH₂- CH₂ - CO-NH₂

 

Biological/Physiological Classification

This classification is based on the functional property of amino acids for the organism.

1. Essential Amino Acids

Amino acids which are not synthesized in the body and must be provided in the diet to meet an animal’s metabolic needs are called essential amino acids. 

About ten of the amino acids are grouped under this category indicating that mammals require about half of the amino acids in their diet for growth and maintenance of normal nitrogen balance. 

2. Non- Essential Amino Acids

These amino acids are need not be provided through diet, because they can be biosynthesized in adequate amounts within the organism.

Essential Amino Acids in Mammals

Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine

 Non-Essential Amino Acids in Mammals

Alanine, Asparagine, Aspartic Acid, Cysteine, Glutamic Acid, Glutamine, Glycine, Proline, Serine, Tyrosine

3. Semi-essential amino acids

Two amino acids are grouped under semi-essential amino acids since they can be synthesized within the organism but their synthesis is not in sufficient amounts. In that they should also be provided in the diet.

The set of essential amino acids required for each species of an organism can be an indicative of the organism propensity to minimal energetic losses on the synthesis of amino acids. Semi essential amino acids include Arginine and Histidine

IV. Classification Based on the Fate of Each Amino acid in Mammals.

Amino acids can be classified here as Glucogenic (potentially be converted to glucose), ketogenic (potentially be converted to ketone bodies) and both glucogenic and ketogenic.

I. Glucogenic Amino Acids

Those amino acids in which their carbon skeleton gets degraded to pyruvate, α ketoglutarate, succinyl CoA, fumarate and oxaloacetate and then converted to Glucose and Glycogen, are called as Glucogenic amino acids.

They include: -

Alanine, cysteine, glycine, Arginine, glutamine, Isoleucine, tyrosine.

II. Ketogenic Amino Acids

Those amino acids in which their carbon skeleton is degraded to Acetoacetyl CoA, or acetyl CoA. then converted to acetone and β-hydroxy butyrate which are the main ketone bodies are called ketogenic amino acids.

They include: -

Phenylalanine, tyrosine, tryptophan, isoleucine, leucine, and lysine. 

These amino acids have ability to form ketone bodies which is particularly evident in untreated diabetes mellitus in which large amounts of ketone bodies are produced by the liver (i.e. not only from fatty acids but also from ketogenic amino acids)

Degradation of Leucine which is an exclusively ketogenic amino acid makes a substantial contribution to ketone bodies during starvation.

III. Ketogenic and Glucogenic Amino Acids

The division between ketogenic and glucogenic amino acids is not sharp for amino acids (Tryptophan, phenylalanine, tyrosine and Isoleucine are both ketogenic and glucogenic).

Some of the amino acids that can be converted in to pyruvate, particularly (Alanine, Cysteine and serine, can also potentially form acetoacetate via acetyl CoA especially in severe starvation and untreated diabetes mellitus.

Answer this**
A child with tall stature, loose joints, and detached retinas is found to have a mutation in collagen. Which of the following amino acids is the recurring amino acid most likely to be altered in mutations that distort collagen molecules?