Amino Acids and Protein Chemistry PDF
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Cairo University Medicine
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This document provides a comprehensive overview of amino acids, their various classifications, and their roles in protein chemistry. It covers chemical classification (aliphatic, aromatic, heterocyclic), classification by polarity (charged, uncharged), nutritional classification (essential, semi-essential, non-essential), and metabolic classification (ketogenic, glucogenic).
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# Amino acids and Protein Chemistry ## Protein and Amino Acid Chemistry Proteins are nitrogenous compounds made of amino acids joined by peptide bonds. * There are 20 amino acids that enter in the structure of proteins. * 19 amino acids * 21st amino acid is selenocysteine. ## General Formula of A...
# Amino acids and Protein Chemistry ## Protein and Amino Acid Chemistry Proteins are nitrogenous compounds made of amino acids joined by peptide bonds. * There are 20 amino acids that enter in the structure of proteins. * 19 amino acids * 21st amino acid is selenocysteine. ## General Formula of Amino Acid Fatty acid: - R - CH<sub>2</sub>-COOH Amino acid: - R - CH - COOH - NH<sub>2</sub> ## Peptide Bond Formation * Peptide bond is formed between the - COOH of 1 amino acid and the NH<sub>2</sub> of another amino acid (by removal of water) ## Classification of Amino Acids 1. Chemical classification 2. Classification according to polarity 3. Nutritional classification 4. Metabolic classification ## 1. Chemical Classification of Amino Acids ### **Aliphatic** * Uncharged, nonpolar * Glycine * Alanine * Valine * Leucine * Isoleucine * Methionine * Proline * Charged * With OH * Serine * Threonine * Homoserine * Homocysteine * With Sulfur * Cysteine * Cystine * With COOH * Aspartic acid * Glutamic acid * With NH<sub>2</sub> * Lysine * Arginine * Histidine ### **Aromatic** * Phenyl alanine * Tyrosine * Tryptophan ### **Heterocyclic** * Tryptophan * Histidine ### **Imino acids** * Proline * Hydroxyproline **NBs:** * Glycine and alanine are considered short chain amino acids. * Homoserine and Homocysteine don't enter in the structure of protein. * Selenocysteine is the 21st amino acid. * Hydroxyproline and hydroxylysine formed by hydroxylation of proline and lysine after protein synthesis. * Tryptophan is considered aromatic and heterocyclic. * Histidine is basic heterocyclic. * Tyrosine is aromatic hydroxyl containing amino acid. ## II. Classification according to Polarity 1. **Uncharged non-polar (hydrophobic) amino acids:** * Glycine - Alanine - Valine - Leucine - Isoleucine. * Methionine - phenylalanine - Tryptophan - Proline. 2. **Uncharged polar amino acids:** * Contain OH: - Serine, Threonine, Tyrosine and Hydroxyproline. * Contain SH: - Cysteine. * Contain amide group: - Asparagine and Glutamine. 3. **Charged amino acids:** * **Basic amino acid:** (Diamino mono carboxylic) * Arginine - Lysine - Hydroxylysine - Histidine. * **Acidic amino acid:** (Mono amino dicarboxylic) * Aspartic acid (Aspartate) - Glutamic acid (Glutamate). ## III. Nutritional Classification ### Amino Acids | | | | |-------------|---------------------|---------------------| | Essential aa | Semi-essential aa | Non-essential aa | | | | | | Not formed in the body and should be supplied in diet. | Formed in the body at a rate enough for adult but not enough for growing | Can be formed in the body at a rate enough for adult and growing | | | | | | VITAL LYMPH | | | | Valine | | | | Isoleucine | Arginine | | | Threonine | | | | Arginine | | | | Leucine | | | | Lysine | | Rest of the amino acids | | Tryptophan | The only semi essential aa | | | Methionine | | | | Phenylalanine | | | | Histidine | | | **Proteins of high biological value:** * Contain all essential amino acids. * Easy to be digested. **Proteins of low biological value:** * Deficient in 1 of the 9 essential amino acids. * Difficult to be digested. ## IV. Metabolic Classification 1. **Ketogenic amino acids:** * Give acetyl CoA and ketone bodies, they are: * Leucine - Lysine. 2. **Mixed glucogenic and ketogenic amino acids:** * Give both glucose and ketone bodies. * Phenylalanine - Tyrosine - Tryptophan - Isoleucine. 3. **Glucogenic amino acids:** * Give glucose. * Includes the rest of amino acids. **NB:** Selenocysteine is 21st amino acid (Like cysteine but instead of sulfur, it has selenium). ## Properties of Amino Acids 1. **Amphoteric properties of amino acids:** * In acidic media they react as bases and carry +ve charges and in alkaline media they react as acids and carry -ve charges. * **Iso Electric point (IEP):** * pH at which the aa. carries both +ve and -ve charges (Dipolar ion or Zwitter ion). * Occurs: at pH 6.02. * At IEP: The amino acid can't migrate in an electric field. | | | | |-------------|---------------|---------------| | R-CH COOH | R CH COO | R-CH COO | | + | + | NH<sub>2</sub> | | NH<sub>3</sub> | NH<sub>3</sub> | | | Acidic media | Zwitter ion | Alkaline media | * Amino acids accordingly may be present in 3 forms according to pH. * The uncharged form is not present at any pH. 2. **Peptide Bond Formation:** * By binding of the a amino group of 1 amino acid with the a carboxyl group of another amino acid. * Binding of 2 amino acids forms **dipeptide** and binding of 3 aa. forms **tripeptide** and so on.
