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Questions and Answers
What is the main role of amino acids in biological systems?
What is the main role of amino acids in biological systems?
Which group is not attached to the α-carbon atom of an amino acid?
Which group is not attached to the α-carbon atom of an amino acid?
What is the significance of the isoelectric point (IEP) of an amino acid?
What is the significance of the isoelectric point (IEP) of an amino acid?
Which of the following statements about zwitter ions is true?
Which of the following statements about zwitter ions is true?
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Which of these classifications does not apply to amino acids?
Which of these classifications does not apply to amino acids?
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Which amino acids are classified as aliphatic amino acids?
Which amino acids are classified as aliphatic amino acids?
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Which of the following groups of amino acids contains a hydroxyl group (-OH) in their R group?
Which of the following groups of amino acids contains a hydroxyl group (-OH) in their R group?
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Which amino acids are considered basic because they contain an amino group (-NH2) in their R group?
Which amino acids are considered basic because they contain an amino group (-NH2) in their R group?
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Which amino acids contain sulfur in their R group?
Which amino acids contain sulfur in their R group?
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Which of the following amino acids forms a ring structure due to its side chain?
Which of the following amino acids forms a ring structure due to its side chain?
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Study Notes
Amino Acids Overview
- Approximately 300 amino acids exist in nature
- Only 20 (or 22) occur in proteins
Importance
- Amino acids are the fundamental building blocks of proteins
Structure
- Each amino acid has four distinct groups attached to a central carbon atom (α-carbon), adjacent to the carboxyl group
- These groups are:
- Amino group (-NH₂)
- Carboxyl group (-COOH)
- Hydrogen atom (-H)
- Side chain (R-group)
General Formula
- A central carbon atom (α-carbon) bonded with:
- Carboxyl group (-COOH)
- Amino group (-NH₂)
- Hydrogen atom (-H)
- Variable side chain (R-group)
Physical Properties
- All amino acids are soluble in water, dilute acids, dilute alkalis, and ethanol.
- Amino acids possess high melting points (above 200°C), indicating substantial stability
Amphoteric Properties
- Amino acids exhibit amphoteric (acid-base) behavior
- In neutral aqueous solutions, a proton typically shifts from the carboxyl group to the amino group, resulting in a zwitterion
- Zwitterions bear both positive and negative charges
Zwitterion
- A zwitterion is electrically neutral (net charge zero)
- It cannot migrate in an electric field
Isoelectric Point (IEP)
- The pH at which the zwitterion forms
- Example: Alanine's IEP is 6
Classification of Amino Acids
- Based on side chain properties:
- Nature of side chain ("R-group")
- Polarity of side chain ("R-group")
- Nutritional classification
- Metabolic classification
1/ Aliphatic Amino Acids
- Includes Glycine (Gly) and Alanine (Ala), Valine (Val), Leucine (Leu), and Isoleucine (Ile)
2/ Hydroxyl Amino Acids
- Includes Serine (Ser), Threonine (Thr), and Tyrosine (Tyr)
3/ Sulfur Amino Acids
- Includes Methionine (Met) and Cysteine (Cys)
- Cystine is formed by two cysteine molecules linked by a disulfide bond
4/ Acidic Amino Acids
- Includes Glutamic Acid (Glu) and Aspartic Acid (Asp)
5/ Basic Amino Acids
- Includes Histidine (His), Lysine (Lys), and Arginine (Arg)
6/ Aromatic Amino Acids
- Includes Phenylalanine (Phe), Tyrosine (Tyr), and Tryptophan (Trp)
7/ Imino Acids
- Includes Proline (Pro)
Classification by Polarity
-
Polar Amino Acids:
- Side chains contain polar hydrophilic groups
- Form hydrogen bonds with water
- Subtypes: Hydroxyl groups (OH), SH, Amide group (NH₂) (basic amino acids: lysine, arginine, histidine) or COOH (acidic amino acids)
-
Non-Polar Amino Acids:
- Side chains are alkyl hydrophobic groups
- Cannot form hydrogen bonds with water
- Examples: Glycine, Alanine, Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, Proline, Methionine
Nutritional Classification
-
Essential Amino Acids:
- Cannot be synthesized by the body and must be obtained from the diet
- Examples: Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine
-
Semi-essential Amino Acids:
- Can be synthesized by the body but not in sufficient amounts, especially for children
- Examples: Arginine, Histidine
-
Non-essential Amino Acids:
- Can be synthesized by the body in sufficient amounts
- Remaining amino acids
Metabolic Classification
-
Ketogenic Amino Acids:
- Metabolized to form ketone bodies
- Examples: Lysine, Leucine
-
Mixed Ketogenic & Glucogenic Amino Acids:
- Metabolized to form both ketone bodies and glucose
- Examples: Isoleucine, Phenylalanine, Tyrosine, Tryptophan
-
Glucogenic Amino Acids:
- Metabolized to form glucose
- Examples: Rest of the amino acids
Deficiency in Selected Foods
- Certain foods lack specific amino acids
- Wheat/Rice: Lysine deficiency
- Corn: Lysine, Tryptophan deficiency
- Beans: Methionine, Tryptophan deficiency
- Peas: Methionine deficiency
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