Questions and Answers
What is the number of amino acids that occur in nature?
What is the physiological pH at which the -COOH group is dissociated?
What type of amino acid is proline?
Which of the following amino acids contains a COOH group?
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How many non-polar amino acids are there?
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What is a common characteristic of all twenty common amino acids?
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Which of the following amino acids is classified as semi-essential?
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What is the function of glucogenic amino acids?
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Which of the following is a neurotransmitter?
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What is the function of catecholamines?
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Study Notes
Amino Acids: Structure and Classification
- Amino acids are the building units of proteins, with around 300 occurring in nature, but only 20 of them are involved in protein synthesis.
- Each amino acid has four different groups attached to the α-carbon (C-atom next to COOH): amino group, COOH group, hydrogen atom, and side chain (R).
- At physiological pH (7.4), the COOH group is dissociated, forming a negatively charged carboxylate ion (COO-), and the amino group is protonated, forming a positively charged ion (NH3+), creating a Zwitter ion.
Classification of Amino Acids
- Amino acids can be classified into four categories:
- By R group (side chain)
- Chemical classification
- Nutritional classification
- Metabolic classification
Classification by R Group (Polarity of Side Chain)
- Polar amino acids: have an R group that contains a polar hydrophilic group, allowing them to form hydrogen bonds with water.
- Examples: serine, threonine, tyrosine, cysteine, glutamine, aspargine, lysine, arginine, histidine, aspartic, and glutamic.
- Non-polar amino acids: have an R group that is an alkyl hydrophobic group, unable to form hydrogen bonds.
- Examples: glycine, alanine, valine, leucine, isoleucine, phenylalanine, tryptophan, proline, and methionine.
Nutritional Classification
- Essential amino acids: 10 amino acids that cannot be synthesized in the body and must be obtained through the diet.
- Deficiency in essential amino acids can affect growth, health, and protein synthesis.
- Semi-essential amino acids: formed in the body but not in sufficient amounts, especially in children.
- Examples: arginine and histidine.
- Non-essential amino acids: can be synthesized in the body.
Non-Standard Amino Acids
- Amino acids that occur naturally in cells but do not participate in peptide synthesis.
- Examples: ornithine and citrulline, which are intermediates in urea biosynthesis.
Amino Acid Derivatives
- Chemical derivatives of amino acids have important biological functions.
- Examples: catecholamines, GABA, dopamine, and histamine.
Functions of Amino Acids
- Amino acids serve various functions beyond being protein constituents.
- Some amino acids are converted to carbohydrates (glucogenic amino acids).
- Specific amino acids give rise to specialized products, such as:
- Tyrosine: thyroid hormones, epinephrine, norepinephrine, and melanin.
- Tryptophan: niacin.
- Glycine, arginine, and methionine: creatine.
- Glycine and cysteine: bile salts.
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Description
Learn about the building blocks of proteins, amino acids, including their structure, classification and properties at physiological pH.
