Amino acids: General Structure, Classification, and Significance

  1. Amino Acid: The basic unit of protein structure. Each amino acid contains both a carbon backbone and a side chain. There are over 20 different amino acids, each with their own chemical makeup.
  2. Protein: A complex aggregate of many small building blocks. Proteins consist of chains of amino acids joined together in specific patterns and linked to other molecules. In humans, the body produces around 100 grams of total protein per day, but most of that (about 90%) is in muscle tissue. Protein is necessary for maintaining and repairing cells. In addition, it helps build hormones and enzymes and is involved in many bodily processes essential to normal functioning.
  3. Amine: An organic compound containing either hydrogen or nitrogen bonded to one or two carboxylic acids. Amines have three distinct functional groups; the primary amine group-NH2 (-NH2), the secondary amine group-NHR (-NR) and the tertiary amine group-NR2 (-NR). Primary and secondary amines can act as bases, while tertiary amines cannot. Tertiary amines can form ionic bonds with metals.
  4. Carboxylic Acid: A polar molecule consisting of a carbonyl group (-CO) and a hydroxyl group (-OH). Two carboxylic acids can link together, forming a peptide bond.
  5. Peptide Bond: a bridge between 2–10 amino acids that forms the basis of protein structure. Peptides can vary greatly in their length and composition.

Introductions of Amino acids

Amino acids are a group of organic compounds that consist of two different functional groups. They are amino (-NH3) and carboxyl (-COOH) groups. The amino group is basic and the carboxyl group is acidic in nature.

Amino acid is a general term used to describe a group of organic compounds containing both carbon and hydrogen atoms. In its simplest form, an amino acid contains two basic chemical groups called carboxylic acids and amines. These two functional groups combine together to give rise to numerous compounds, including amino acids. The following table shows the various amino acids grouped based on their structural features.

  • Ala-Alanine
  • Glu – Glutamic Acid
  • Asp – Aspartic Acid
  • Cys – Cystine
  • Gln – Glutamine
  • His-Histidine
  • Ile-Isoleucine
  • Leu – Leucine
  • Lys-Lysine
  • Met-Methionine
  • Phe-Phenylalanine
  • Pro – Proline
  • Ser-Serine
  • Thr-Threonine
  • Trp-Tryptophan
  • Val – Valine

What is the general structure of Amino acid?

The structure of an amino acid has carboxyl and amino groups attached to an alpha-carbon atom. The alpha side chain is represented by R, which is different for each of the 20 amino acids found in proteins. The amino acids mostly exist in their ionized form in the biological system.

Classification of Amino Acids

The amino acids are classified in different ways based on their structure, chemical properties, nutritional requirements, and metabolic fate.

1. Classification based on Structure

This is the most popular classification based on the structure and chemical nature. Each amino acid is designated with a single letter or three letter symbol. There are 20 different amino acids found in our body as the basic components of protein. These are divided into seven sub categories.

a. Amino acids with aliphatic side chains

  • These are Monoamino monocarboxylic acids.
  • Eg: Glycine, Alanine, Valine, Leucine, isoleucine

b. Hydroxyl group containing amino acids:

  • The amino acids have hydroxyl groups in their side chains.
  • Eg: Serine, Threonine, and Tyrosine (Aromatic amino acid)

c. Sulfur containing amino acids:

  • Cysteine and methionine belong to this category.
  • Sulfhydryl group is present in cysteine.
  • Thioester group is present in Methionine.
  • Cystine is formed by the condensation of two cysteine molecules.

d. Acidic amino acids

  • In this group, amino acids have two carboxyl groups. So these are called dicarboxylic mono amino acids.
  • Ex: Glutamic acid and Aspartic acid
  • Asparagine and glutamine are the respective amide derivatives.

e. Basic amino acids

  • In this group, amino acids have two amino groups. So these are called diamino mono carboxylic amino acids.
  • Eg: Lysine, Arginine, and Histidine

f. Aromatic amino acids:

  • These amino acids have an aromatic (ring) structure in their side chains.         
  • Eg: Phenylalanine, Tyrosine, and Tryptophan
  • In tryptophan, indole rings are present in their side chains.

g.   Imino acids:

  •  The amino acid proline belongs to this group.
  • It contains a pyrrolidine ring and it has an imino group (=NH), instead of an amino group (-NH2).
  • Proline is an alpha-imino acid.

2. Classification based on Polarity

The amino acids are classified into 4 groups based on their polarity, which are present on their side chain. It reflects the functional role of proteins.

a. Non-Polar amino acids:

  • These amino acids are hydrophobic in nature.
  • They have no charge on their side chains.
  • Eg: Alanine, Leucine, Isoleucine, Valine, Methionine, Phenylalanine, Tryptophan, and Proline

b. Polar amino acids without charge:

  • These amino acids do not carry any charges on their ‘R’-side chain.
  • They have hydroxyl, sulfhydryl and amide groups which participate in the hydrogen bonding formation in protein structure.
  • Eg: Glycine, Serine, Threonine, Cysteine, glutamate, Asparagine and Tyrosine.

c. Polar amino acids with positive charge on their ‘R’ side chain

  • These are dibasic monocarboxylic acids.
  • Lysine, arginine, and histidine belong to this group.

d. Polar amino acids with a negative charge on their ‘R’ side chain

  • These are dicarboxylic monoamines.
  • Aspartic and glutamic acids belong to this group.

3. Classification based on the Nutritional requirements

Twenty amino acids are required for protein synthesis in our body. It should not come with your regular diet. Based on the nutritional requirements, these amino acids are divided into two classes.

a. Essential Amino Acids

  • The amino acids, which are not synthesized in our body and, therefore, need to be supplied through the diet. These are called essential amino acids.
  • These amino acids are required for proper growth and maintenance of the individual.
  • Here are the essential amino acids listed. (PVT TIM HALL)
    •  P – Phenyl Alanine
    •  V – Valine
    •  T – Threonine
    •  T – Threonine
    •  I – Isoleucine
    •  M – Methionine
    •  H – Histidine
    •  A – Arginine
    •  L – Leucine
    •  L – Lysine
  •  Arginine and histidine—these two amino acids are partially synthesized by human adults. These are considered as semi essential amino acids.
  • The remaining amino acids are essential amino acids.

b. Non-essential amino acids:

  • The body can synthesize about 10 amino acids to meet biological needs, so they do not need to be consumed in the diet.
  •  Eg: Glycine, Alanine, Serine, Cysteine, Aspartate, Asparagine, Glutamate, Glutamine,Tyrosine, and Proline.

4. Classification based on their metabolic fate

The carbon skeleton of amino acids can serve as a precursor for the synthesis of glucose (glycogenic) or fat (ketogenic) or both. In this classification, the amino acids can be divided into three groups.

a.   Glucogenic amino acids:

  •  The amino acids can serve as precursors for the formation of glucose or glycogen.
  • Eg: Alanine, Aspartate, Glycine, Methionine, etc.

b.  Ketogenic Amino Acids:

  • These amino acids helped in the formation of fat molecules.
  • Eg: Leucine, and Lysine

Significance of Amino acids

  • Amino acids are the building blocks of protein. These compounds help build our muscles, bones, hair, skin cells, etc. If we don’t have enough amino acids then our body cannot function properly.
  • As human beings, we need 18 different types of amino acids to survive. Each type of amino acid contributes differently towards the formation of protein and the maintenance of the body’s structure.

There are two categories of amino acids, the primary and the non-primary.

  • Primary amino acids are those that do not occur in nature and must be obtained through diet.
  • Non-primary amino acids are naturally occurring substances found in food and are added directly to the diets of animals. We can get these from many foods including meat, dairy products, eggs, legumes, grains, and vegetables.
  • Amino acids play a major role in muscle tissue production. In fact, some people say that protein consumption helps to maintain muscle mass. Muscle is responsible for movement and respiration.
  • It also provides structural integrity for the body and aids in keeping organs within place. When muscle tissue is damaged, amino acids are released from the body and are converted back into glucose in order to heal the damage.
  • Glucose is what gives us energy. Most of the time this conversion takes place without any problems. However, if the level of protein consumed is reduced or inadequate, then the body may convert less glucose into usable energy.
  • This causes the person to become weak and lose their muscle mass.
  • This condition is called malnutrition and it is dangerous for the body. Malnutrition can lead to many serious illnesses like cancer, diabetes, heart attacks, strokes, kidney disease, osteoporosis, gout, rickets, and arthritis.

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