Structure of amino acid

All the 20 amino acids which are found in proteins have a carboxyl group (-COOH) and amino group (-NH2) bound to a same carbon known as alpha-carbon.

The 20 amino acids of proteins are known as the primary or standard amino acids. These standard amino acids have been given a three letter abbreviation and one letter symbol. The abbreviations of the amino acids are the first 3 letters of the names like Serine (Ser). But 3 amino acids are exceptions which are aspargine (Asn), glutamine (Gln) and tryptophan (Trp). One of the 20 amino acids, proline is an imino acid because it contains (-NH) group and not (-NH2) group.

Classification of amino acids

Amino acids are classified on 5 basis which are as follows-

• Chemical nature of amino acid
• Structure of the side chain of amino acid
• Nature or polarity of the side chain of amino acid
• Nutritional requirement of amino acids
• Metabolic products of amino acids

Classification on the basis of chemical nature:- According to this type, amino acids are classified as-

1. Neutral amino acids- Amino acids which are neutral in nature, that is that these are mono-amino and mon-carboxylic amino acids. Examples include: glycine, alanine, serine, threonine, tyrosine, proline, leucine, isoleucine glutamine etc.
2. Acidic amino acid- These are mono-amino and di-carboxylic amino acids. These include: aspartic acid and glutamic acid
3. Basic amino acid- These are di-amino and mono-carboxylic amino acids. These include: lysine, arginine and histidine.

Classification based on the structure of the side chain of amino acids

1. Aliphatic amino acids:- Amino acids having aliphatic side chains. Examples: glycine, alanine, valine, leucine and isoleucine
2. Hydroxy amino acids: Amino acids having hydroxy group in the side chain. It includes: threonine, serine and tyrosine.
3. Sulfur amino acids:– Amino acids containing sulfur side chains. Examples: cysteine, methionine.
4. Dicarboxylic acids and their amides:– Amino acids having carboxylic acids in the side chain. It includes: glutamic acid, aspartic acid and glutamine.
5. Di-amino Acids:- Amino acids having amino group in the side chain. Examples are: lysine, arginine and histidine.
6. Aromatic amino acids:- Amino acids containing aromatic side chains, examples: phenylalanine, tyrosine and tryptophan.
7. Imino acids:– Amino acids containing secondary amino acids like proline.

Classification based on nature or polarity of amino acids

The below provided flowchart describes this classification.

The above image is taken from wou.edu for education purpose only.

Nutritional Classification of amino acids

1. Essential amino acids:- These amino acids are not synthesized in the body and hence are required to be taken n the diet. Examples are: valine, arginine, lysine, leucine, tryptophan, histidine etc.
2. Non-essential amino acids:- These amino acids are synthesized in the body itself. Eg: glycine, cysteine, glutamine, glutamic acids, proline etc.

Metabolic classification of amino acids

1. Glucogenic amino acids:– Those which can be converted into glucose. Fourteen out of twenty amino acids fall in this category
2. Ketogenic amino acids:- Those which are converted into ketone bodies. Eg: leucine and lysine
3. Both ketogenic as well as glucogenic :- Those which are converted to both glucose as well as ketone bodies. Example: isoleucine, phenylalanine, tryptophan and tyrosine.

Functions of amino acids

• Formation of proteins:- Amino acids are joined to each other to form proteins an peptides
• Formation of glucose:- Glucogenic amino acids are converted into glucose.
• Enzyme activity:- The thiol group of cysteine plays an important role in enzyme activity
• It is the transport and storage form of ammonia
• It acts as buffer solutions
• Amino acids like glycine and cysteine are involved in detoxification reactions

Properties of amino acids

1. Physical properties: Amino acids are
   • Colorless
   • Crystalline
   • Soluble in water
2. Optical properties:- All the naturally occurring amino acids are optically active except glycine. All amino acids have same configuration and hence they are L-alpha amino acids.

Above image is taken from easybiologyclass.com for education purpose only

Multiple choice questions (MCQs)

1. Amino acids are building blocks of which biomolecule?
A. Lipids

B. Proteins

C. Carbohydrates

D. Vitamins

2. Which group is present in the structure of amino acids?
A. Hydroxyl group

B. Carboxyl group

C. Amino group

D. Both B and C

3. Which amino acid contains imino group instead of amino group?
A. Glycine

B. Glutamine

C. Proline

D. Aspartic acid

4. Which of the following is the example of neutral amino acid?
A. Aspartic acid

B. Histidine

C. Lysine

D. Valine

5. Which of the following is the example of acidic amino acid?
A. Aspartic acid

B. Histidine

C. Lysine

D. Valine

6. On the basis of polarity of side chain of amino acid, amino acids are classified in how many types?
A. 2

B. 3

C. 5

D. 4

7. Match the following category and its respective example-
A. Hydroxyl amino acid          1. valine
B. Sulfur amino acid               2. phenylalanine
C. Aromatic amino acid          3. cysteine
d. Aliphatic amino acid           4. serine

8. Which of the following amino acid is an exception for the abbreviation rule?
A. Glutamine

B. Proline

C. Glycine

D. Cysteine

9. Which of the following statement is NOT true?
A. Hydrophilic amino acid are serine and tyrosine
B. Hydrophobic amino acid include tryptophan
C. Histidine is both basic as well as aliphatic amino acid
D. Tyrosine is both hydroxylic as well as aromatic amino acid

10. Which of the following is counted as an importance of amino acid?
A. Formation of proteins

B. Forms glucose

C. Act as buffer

D. All of the above

11. Which of the following is NOT the physical property of amino acid?
A. Crystalline

B. Soluble in water

C. Colorless

D. None of the above

12. Which amino acid has ketogenic as well as glucogenic products?
A. Tryptophan

B. Leucine

C. Glycine

D. All of the above

13. Which amino acid is counted as hydroxyl group amino acid?
A. Serine

B. Alanine

C. Both

D. None

14. Which amino acid is counted as aromatic group amino acid?
A. Glutamine

B. Tyrosine

C. Both

D. None

15. Which amino acid is counted as aliphatic group amino acid?
A. Serine

B. Alanine

C. Both

D. None

ANSWERS:-

1. Proteins
2. Both B and C
3. Proline
4. Valine
5. Aspartic acids
6. 2
7. a – 4 b – 3 c – 2 d – 1
8. Glutamine
9. Histidine is both basic as well as aliphatic amino acid
10. All of the above
11. None of the above
12. Leucine
13. Serine
14. Tyrosine
15. Alanine

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REFERENCE:- Pankaja Naik- Biochemistry; 4^th edition; page no:- 56-63

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Glutamic acid is a non-essential glucogenic amino acid.

Synthesis of glutamic acid

Glutamic acid is synthesized from alpha-ketoglutarate  (an intermediate of TCA cycle) by

• Transamination (reaction A from below given picture)
• By reductive animation of ketoglutarate by NH4, catalyzed by glutamate dehydrogenase. (reaction B from below given picture)

Catabolism of glutamic acid

when glutamate is cleaved or broken, it is converted back to alpha-ketoglutarate by either transamination or dehydrogenase reaction.

Significance of glutamic acid

1. Amino acids like glutamine, proline and arginine are derived from glutamate
2. Glutamate is involved in synthesis of glutathione. Glutathione in turn is involved in transport of amino acid into kidney and intestinal cells.
3. Glutamate is decarboxylated to form amine gamma aminobutyric acid (GABA). GABA serves as a neurotransmitter.

Metabolism of Glutamine

synthesis of Glutamine:- Glutamine is the amide of glutamate. It is produced from glutamate from glutamate synthetase . This enzyme adds ammonia to the carboxyl group of the side chain and hence forms the amide.

Degradation of Glutamine:- It is again recovered from glutamate only but using different enzyme. The enzyme used is glutaminase; which is particularly important in the kidney.

Metabolism of Aspartic Acid

Aspartic acid is non-essential glucogenic amino acid Synthesis of aspartic acid. Aspartate is synthesized from oxaloacetate (an intermediate from TCA cycle) by transamination reaction. Catabolism or degradation of aspartic acid Since transamination reaction is easily reversible, so aspartate is converted back to oxaloacetate.

Functions of aspartic acid

• In urea cycle, aspartate reacts with citrulline to form arginosuccinate; which in turn forms arginine and fumerate.
• Aspartate reacts with inosine mono-phosphate (IMP) to form AMP

Metabolism of Aspargine

Synthesis of Aspargine:- Aspargine is an amide of aspartate. It is formed from the aspartate by a reaction in which glutamine provides nitrogen for the formation of amide group.

Degradation of Aspargine:- The amide nitrogen of aspartate is released as ammonia and aspartate by hydrolytic release. This reaction is done by asparginase enzyme.

Multiple choice questions (MCQs)

1. Glutamic acid is nutritionally which type of amino acid?

A. Essential amino acid

B. Non-essential amino acid

C. Semi-essential amino acid

D. Both A and C

2. Glutamic acid is synthesized from which compound?

A. Oxaloacetate

B. Pyruvate

C. Alpha-ketoglutarate

D. Aspartic acid

3. Which enzyme catalyzes the reductive amination of ketoglutarate?

A. Glutamate dehydrogenase

B. Glutamate oxidase

C. Glutamate transferase

D. None of the above

4. Which product is formed after the degradation of glutamic acid? A. Oxaloacetate

B. Pyruvate

C. Alpha-ketoglutarate

D. Aspartic acid

5. Which compound is involved in the synthesis of glutathione?

A. Oxaloacetate

B. Pyruvate

C. Alpha-ketoglutarate

D. Glutamic acid

6. Proline is derived from which compound?

A. Oxaloacetate

B. Glutamic acid

C. Alpha-ketoglutarate

D. Aspartic acid

7. Aspartic acid is nutritionally which type of amino acid?

A. Essential amino acid

B. Non-essential amino acid

C. Semi-essential amino acid

D. Both A and C

8. Match the following enzyme and its substrate-

a. Aspartate transminase 1. aspartate

b. Glutamate dehydrogenase 2. L-glutamate

c. Glutaminase 3. Alpha-ketoglutarate

d. Asparginase 4. Aspartate + alpha-ketoglutarate

9. Glutamine is synthesized from which compound?

A. Aspartic acid

B. Glutamate

C. Lysine

D. Histidine

10. Glutaminase is particularly important in which organ?

A. Kidney

B. Brain

C. Liver

D. Shoulder

11. Which of the following is major transporter of ammonia?

A. Aspartic acid

B. Glutamate

C. Lysine

D. None of the above

12. Which of the following statement is NOT true?

A. Proline is derived from aspartic acid

B. Aspartate reacts with citrulline to form arginosuccinate

C. Glutamine is major transport form of ammonia

D. Aspargine is formed from aspartate

13. Aspartate is synthesized from which compound?

A. Oxaloacetate

B. Pyruvate

C. Alpha-ketoglutarate

D. Glutamic acid

14. Which compound reacts with inosine monophosphate to form AMP?

A. Oxaloacetate

B. Pyruvate

C. Aspartic acid

D. Glutamic acid

15. Which of the following amino acid is glucogenic amino acid?

A. Aspartic acid

B. Glutamic acid

C. Proline

D. All of the above

ANSWERS:-

1. Non-essential amino acid

2. Alpha-ketoglutarate

3. Glutamate dehydrogenase

4. Alpha-ketoglutarate

5. Glutamic acid

6. Glutamic acid

7. Non-essential amino acid

8. a – 4 b – 3 c—2 d – 1

9. Glutamate

10.Kidney

11. None of the above

12. Proline is derived from aspartic acid

13. Oxaloacetate

14. Aspartic acid

15. all of the above

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REFERENCE:– Pankaja Naik- Biochemistry; 4^th edition; page no:- 292-293.

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