Amino Acid Properties & Reactions PDF

# Derived or modified amino acids - Non-primary amino acids found in proteins - After the synthesis of proteins, some of the amino acids are modified in post-translation processing. ## 1. 4-Hydroxyproline - found in collagen - formed by hydroxylation of proline. - Hydroxyproline: `HO-C(OH)-C(O)-N-...

# Derived or modified amino acids - Non-primary amino acids found in proteins - After the synthesis of proteins, some of the amino acids are modified in post-translation processing. ## 1. 4-Hydroxyproline - found in collagen - formed by hydroxylation of proline. - Hydroxyproline: `HO-C(OH)-C(O)-N-C(H)-H` - Proline: `C(O)-N-C(H)-H` ## 2. 5-Hydroxylysine - found in collagen - formed by hydroxylation of lysine. - 5-Hydroxylysine: `HO-C(CH2)4-C(OH)(CH2)-(CH2)2-NH2` ## 3. Cystine - formed by conjugation of two cysteines via disulfide linkage - found in high concentrations in digestive enzymes - Cystine: `H2N-C(COOH)-CH2-S-S-CH2-C(COOH)-NH2` # Non protein and Non a-amino acids - These are amino acids that do not occur in proteins - Perform other functions in metabolism ## 1. B-alanine - a component of CoA, carnosine - Catabolic product of some pyrimidine bases. - B-alanine: `NH2-CH2-CH2-COOH` ## 2. g-aminobutyric acid (GABA) - formed from glutamic acid - Neurotransmitter - GABA: `NH2-CH2-CH-(CH2)2-COOH` ## 3. Taurine - occurs in bile combined with bile acids - Taurine: `NH2-CH2-CH2-SO3H` ## 4. DOPA (3,4-dihydroxyphenylalanine) - precursor for melanin pigment, epinephrine and norepinephrine - DOPA: `HO-Ph-CH2-CH(NH2)-COOH` where Ph represents a benzene ring ## 5. Monoiodotyrosine (MIT) and Diiodotyrosine (DIT) - Precursors of thyroid hormones (T3 and T4) - MIT: `HO-Ph-CH2-CH(NH2)-COOH` where Ph represents a benzene ring with one iodine atom - DIT: `HO-Ph-CH2-CH(NH2)-COOH` where Ph represents a benzene ring with two iodine atoms # Properties of amino acids ## I. Physical properties of amino acids 1. **Solubility:** All amino acids are soluble in *water, diluted acids and alkalis*. 2. **Optical activity:** All amino acids *except glycine* are optically active. 3. **Amphoteric property:** All amino acids behave as acids and alkalis (*Zwitter ion*). - **Zwitter ion** is a compound with *no overall electrical charge*, but which contains separate parts which are positively and negatively charged. e.g. amino acids - **Isoelectric point (IEP or IP)**: The pH at which an amino acid carries *equal* positive and negative charge. e.g., IP of glycine is pH 6.07. ## II. Chemical properties of amino acids ### A. Reactions due to presence of *amino group* of amino acids: 1. **Salt formation:** `R-CH(NH2)-COOH + HCl -> R-CH(NH3+Cl)-COOH` 2. **Deamination:** - **Oxidative deamination:** `R-CH(NH2)-COOH + O2 -> R-C(O)-COOH + H2O` forming *a-keto acids* - **Reductive deamination:** `R-CH(NH2)-COOH + H2 -> R-CH2-COOH` forming *fatty acids* - **Hydrolytic deamination:** `R-CH(NH2)-COOH + H2O -> R-CH(OH)-COOH + NH3` forming *hydroxy fatty acids* 3. **Carboxylation:** `R-CH(NH2)-COOH + CO2 -> R-CH(NH-COOH)-COOH` forming *carbamine compounds* - (carbaminohemoglobin) transport of CO2 in the bloodstream 4. **Methylation:** `R-CH(NH2)-COOH + CH3 -> CH3-CH(CH3)-COOH` - Amino acids can be methylated in the presence of *methyl donners* as methionine. - Glycine can be methylated to *betaine* which by reduction gives rise to *choline*. 5. **Action of nitrous acid**: `R-CH(NH2)-COOH + HNO2 -> R-CH(OH)-COOH + N2 + H2O` - Amino acid reacts with nitrous acid give rise to corresponding *fatty acid*, *nitrogen* and *water*. - It can be used for estimation of amino acid by determining the amount of nitrogen elevated and divided by 2, as 1/2 is derived from amino acid and the other 1/2 is derived from nitrous acid. ### B. Reactions due to presence of *carboxylic group* of amino acids 1. **Salt formation**: `R-CH(NH2)-COOH + NaOH -> R-CH(NH2)-COO-Na+ + H2O` 2. **Decarboxylation**: `R-CH(NH2)-COOH -> R-CH2-NH2 + CO2` forming *amine* - *removal of CO2 from amino acids gives rise to the corresponding amines. For example: Histidine gives Histamine and Trytophan gives Tryptamine.* 3. **Esterification:** `R-CH(NH2)-COOH + R-OH -> R-CH(NH2)-COOR + H2O` - The last -COOH group in *dicarboxylic amino acids* can combine with ammonia to form the corresponding *amide*, i.e., glutamic acid forms glutamine and aspartic acid forms aspargine. - Glutamine: `H2N-C(O)-CH2-CH2-CH(NH2)-COOH` - Aspargine: `H2N-C(O)-CH2-CH(NH2)-COOH` ### C. Reactions due to presence of *side chain* of amino acids - (*Color reactions of proteins*) | Reaction name | Principle | Resulting color | |---|---|---| | Rosenheim | Indole group of tryptophan + conc. sulfuric acid + Rosenheim's reagent | Purple color | | Millon | Phenolic group of tyrosine + Millon's reagent | Red color after boiling. | | Xanthoproteic | Phenyl group of phenylalanine + conc. sulfuric acid | Orange color | | Sulfur | Sulfur of cysteine or cystine + lead acetate | Black color | - **Sulfur reaction**: It is the reaction between sulfur of *cysteine or cystine* and *lead acetate* giving *black color*. - **N.B.:** Methionine does not give a positive sulfur reaction test because its sulfur is masked by a methyl group. ## III. Chemical properties of amino acids: ### C. Reactions due to presence of both COOH and NH2 groups 1. **Zwitterion formation:** - `NH3+-R-CH-COO- ` 2. **Reaction with ninhydrin**: - `Amino acids + Ninhydrin + NH3 -> Blue color complex + CO2 + H2O` - The intensity of the blue color produced is used as a measure of the amount of amino acid present. - Therefore, Ninhydrin is used in *amino acid analysis of proteins*. - All amino acids, *except proline*, are hydrolyzed and react with ninhydrin. 3. **Formation of peptide bonds**: - `R1-CH(NH3+)-C(O)-OH + H3N+-CH(R2)-C(O)-OH -> H2O + R1-CH(NH3+)-C(O)-NH-CH(R2)-C(O)-OH` - Peptide bond formation is a *dehydration synthesis reaction* (*also known as a condensation reaction*). # Peptides - A peptide is short chains of amino acids linked by peptide (amide) bonds. - If 2 amino acids are linked together, they form a *dipeptide*. - If 3 amino acids are together linked, they form a *tripeptide*. - *Polypeptides* may contain more than 10 and up to 100 amino acids. ## What is the Difference Between a Peptide and a Protein? - Peptides are *smaller* than proteins. - Peptides are defined as molecules that consist of between 2 and 50 amino acids. - Whereas proteins are made up of 50 or more amino acids. ## Examples of physiologically active peptides ### Peptide hormones - insulin (51), glucagon (29), angiotensin I (10), angiotensin II (8) and gastrin (17). ### Antithrombotic peptides - Inhibit the blood platelet aggregation, - i.e. Casoplatelins (bioactive peptides from bovine casein). ### Antioxidative peptides - Inhibit lipid autooxidation process, - i.e. L-carnosine, Glutathione. ### Neuropeptides - Act as neurotransmitters, - i.e. Acetylcholine, and GABA. # Insulin - Is a peptide hormone produced by beta cells of the pancreatic islets. - Function is to maintain normal blood glucose level (keeps your blood sugar level from getting too high (hyperglycemia) or too low (hypoglycemia)). - Insulin is described as a "key," which unlocks the cell to allow sugar to enter the cell and be used for energy. # L-carnosine - Naturally occurring dipeptide made by chemical combination of b-alanine and histidine. - Present in meat, fish, and poultry. - Can be synthesized by carnosine synthetase enzyme in the *brain and muscle*. ## Biological Importance of L-carnosine: 1. Buffering function 2. Antioxidant function: inactivate reactive oxygen species and scavenge free radicals. 3. Therapeutic uses: prevent signs of aging, improve muscle strength and exercise performance # Glutathione (GSH) - A *tripeptide* comprised of three amino acids (cysteine, glutamic acid, and glycine). - Produced naturally by the liver. - Found in fruits, vegetables, and meats - Can be present in *reduced (GSH)* and *oxidized (GSSG)* forms. ## Biological Importance of Glutathione (GSH): 1. Powerful *antioxidant* (reducing) agent: preventing damage to important cellular components caused by reactive oxygen species. 2. It activates many enzymes e.g., Glutathione peroxidase. 3. Prevents hemolysis of the RBCs. 4. It inactivates insulin hormone. 5. Prevent rancidity of fat. 6. Help in amino acids absorption.