Proteins/Amino Acids PDF

PROTEINS/AMINO ACIDS By INONDA RN.  Proteins Are among the most abundant biological macromolecules Are polymers of aminoacids The sequence of amino acids in a protein is dictated by the sequence of nucleotides in a segment of the DNA in the chromosomes, and the...

PROTEINS/AMINO ACIDS By INONDA RN.  Proteins Are among the most abundant biological macromolecules Are polymers of aminoacids The sequence of amino acids in a protein is dictated by the sequence of nucleotides in a segment of the DNA in the chromosomes, and the uniqueness of each living organism is due to its endowment of specific proteins. Amino acid general formula. For all 20 amino acids except glycine, the α carbon is bonded to 4 different groups:- -A carboxyl group, an amino group, An R group and a hydrogen atom. Alanine  More than 200 aminoacids identified in nature Most amino acid residues are L- isomers. 20 serve as building blocks of proteins Known as common a acids. Selenocysteine found in most enzymes where it reduces oxidative stress. It is the 21st amino acid.  Classification 1. Are classified into 5 groups on the basis of the charge and polarity of the R group. 2. Are also classified according to nutritional importance. 3. Are classified depending on the metabolic fate of the carbon skeleton. Non polar aliphatic R gps  R groups are non polar and hydrophobic Side chains of amino acids tend to cluster together within proteins thus stabilizing proteins by hydrophobic interactions. Include glycine, alanine, proline, valine, leucine, isoleucine and methionine. Glycine is partially polar. Proline is an imino acid - Relatively non polar -Participate in hydrophobic interactions though tyrosine can form hydrogen bonds -Tryptophan and tyrosine can absorb UV light at 280 nm. Phenylalanine- sparingly. -Tryptophan has an indole ring. - R groups are soluble in water thus are hydrophilic  Formation of cystine from cysteine  Positively charged amino- acids Are hydrophilic Have a positive charge at Ph 7 Include lysine, histidine and arginine Histidine is the only a.acid having an ionizable side chain with a pKa near neutrality(6.1) Also has an imidazole ring. Plays a role in enzyme catalysed reactions where it serves as a buffer.  Negatively charged amino- acids Are hydrophilic Have a net negative charge at Ph 7 Include aspartate and glutamate  Uncommon amino acids 5- Hydroxylysine, 4- Hydroxyproline- Collagen γ Carboxyglutamate – Blood clotting proteins Phosphorylated amino acids – signalling Desmosine- elastin Ornithine and citrulline- urea cycle  Classification- nutritional importance Essential amino-acids: Are those that cannot be synthesized in the body thus they are obtained from diet. Include Phenylalanine, Valine, threonine, Tryptophan, isoleucine, methionine, Histidine, Arginine, Lysine and Leucine. Non-essential: Are those the body can synthesize.  Fate of carbon skeleton Glucogenic amino-acids: Are converted to glucose or TCA intermediates. Include glycine, alanine, valine, serine, threonine etc. Ketogenic amino-acids: Converted to aceto acetyl CoA or acetyl CoA ( Ketone bodies). Include leucine and lysine. Ketogenic/Glucogenic amino acids: isoleucine, phenylalanine, tyrosine and tryptophan.  Nomenclature Are designated a name depending on either a 3 letter abbreviation or a one letter symbol. Abbreviations are the first 3 letters of their names except for Asparagine (Asn), Glutamine ( Gln), Isoleucine (Ile) and Tryptophan (Trp) This is the most conventionally used method.  Properties of amino-acids 1. Show optical isomerism except glycine. All have a chiral centre thus exist as D/L isomers. 2. Are dipolar ions (zwitterions) 3. Have acid base properties. Can act as acids or bases (ampholytes). 4. Have buffering action thus are buffers in the body 5. Aromatic amino acids absorb at 280nm ( ultra violet). This property used to quantify the amount of these a.acids in a protein.  Properties of amino-acids 6) All amino acids absorb in infrared region 7) They react with ninhydrin to form a purple colored solution. For this reaction to occur, the amino acid should have a free amino group. Proline does not thus it will not form the purple solution with ninhydrin. 8) They are soluble in water  Properties 9. Have high melting points 10. Are colorless and crystalline 11. Taste varies: Some are tasteless eg tyrosine some are sweet eg glycine and alanine while some are bitter eg arginine and phenylalanine.  Reactions of carboxyl groups Carboxyl groups form amides and esters They react with alcohols to form esters React with ammonia to form amides React with amines to form amides React with alkali to form salts eg monosodium glutamate is key in flavoring food Reactions of amino group 2. React with nitrous acid to yield nitrogen gas 3. React with mineral acids to form salts 4. React with sanger’s reagent ( 2,4 dinitroflorobenzene) to give a yellow precipitate  Acid base property Amino acids exist as dipolar ions in neutral aqueous solutions known as zwitterions. When in water, they can act as proton donors or acceptors Are amphoteric. When titrated, all monoamino carboxylic acids show the same pattern of titration curve.  Acid base properties of amino acids They are used to:- 1. Separate aminoacids 2. Identify amino acids 3. Quantify different amino acids 4. Determine the sequence of amino acids in a protein.  Acid base properties of amino- acids Common aminoacids are weak polyprotic acids. All contain at least 2 dissociable hydrogens….COOH and NH3  Glycine Glycine changes from positively charged ion at low Ph to a negatively charged ion at high Ph. Titration curve for glycine  General points Carboxyl group of aminoacids is the first to loose the H. All aminoacids have a pK1 value between 2.0 to 2.4. Pk2 values range between 9.0 – 9.8 Those with ionisable side chains eg glu, Asp, His, Lys, Arg have a Pk3 Value.  Iso electric point. A point where the aminoacid has no charge is known as the isoelectric point Isoelectric Ph is an average of the 2 pKa values except for cases where the amino acids have a charged R gp. Titration curve for glutamate Titration curve for lysine  Further reading 1. Acetic acid has a pka of 4.8 while most amino acids have a pka ( associated with the carboxylic group) ranging 2.1- 2.4. Explain this difference. 2. How many chiral centers are in isoleucine.