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Amino Acids are the building units of proteins
Amino Acids are the building units of proteins. Proteins are polymers of amino acids linked together by what is called “ Peptide bond” (see latter). There are about 300 amino acids occur in nature. Only 20 of them occur in proteins. Structure of amino acids: Each amino acid has 4 different groups attached to α- carbon ( which is C-atom next to COOH). These 4 groups are : α-amino group, COOH group, Hydrogen atom and side Chain (R) Amino Acids R
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Classification of amino acids
1- Chemical classification: According to number of COOH and NH2 groups i.e. according to net charge on amino acid and A- Monobasic, monocarboxylic amino acids i.e. neutral or uncharged: R
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According to chemical structure of R group, neutral amino acids are subclassified into:
I- Aliphatic, neutral amino acids: (6 amino acids) 1- Glycine (Gly): R= H 2- Alanine (Ala) R= CH3 3, 4, 5-Branched chain amino acids: valine, leucine and isoleucine R is branched Valine (Val): R= isopropyl group
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Leucine (Leu): R= isobutyl gp
Isoleucine (Ile) R = is isobutyl R is isobutyl in both leucine and isoleucine but branching is different: in leucine → branching occurs on γ carbon in isoleucine→ branching occurs on β- carbon 6- Proline: In proline, amino group enters in the ring formation being α-imino group so proline is an α-imino acid rather than α-amino acid
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Valine Leucine Isoleucine
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II- Neutral Sulfur containing amino acids: 2 amino acids
1- Cysteine (Cys): R= CH2SH What is cystine? 2- Methionine (Met): III- Neutral, hydroxy amino acids: 2 amino acids 1- Serine (Ser): R= CH2OH 2- Threonine (Thr):
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Formation of cystine The bond S-S in cystine is called: disulfide bond
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Serine Threonine
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Cysteine Methionine
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VI- Neutral aromatic amino acids: includes 3 amino acids
1- Phenyl alanine (Phe) : It’s alanine in which one hydrogen of CH3 is substituted with phenyl group. So it’s called phenyl alanine. 2- Tyrosine (Tyr): - it is P- hydroxy phenyl alanine - it is classified as phenolic amino acid
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3- Tryptophan: contains indol ring
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Phenylalanine Tyrosine Tryptophan
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VI- Neutral amino acids with amide group in R (2):
1- Aspargine (Asn): 2- Glutamine (Gln):
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At physiological PH (7.4), in neutral amino acids, -COOH group is dissociated forming a negatively charged carboxylate ion (COO-) and amino group is protonated forming positively charged ion (NH3+) forming Zwitterion
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B- Basic amino acids: contain two or more NH2 groups or nitrogen atoms that act as base i.e. can bind proton. At physiological pH, basic amino acids will be positively charged. a- Lysine( Lys) b- Arginine(Arg): contains guanido group: c- Histidine (His): contains imidazole ring. guanido
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C- Acidic Amino acids: e.g. a- Aspartic acid (Asp) (aspartate), b- Glutamic acid (Glu) (glutamate) at physiological pH, acidic amino acids will carry negative charge
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A- Neutral amino acids (15 amino acids)
Summary: A- Neutral amino acids (15 amino acids) Classified according to R into - Aliphatic amino acids including glycine, alanine, valine, leucine, isoleucine and proline Sulfur containing amino acids including: cysteine and methionine Hydroxy amino acids including serine and threonine Aromatic amino acids including phenylalanine, tyrosine and tryptophan Amino acids containing amide group including aspargine and glutamine
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Basic amino acids (3) Lysine Arginine Histidine
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Acidic amino acids (2) Aspartic acid Glutamic acid
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At physiologic pH, neutral amino acids are present as Zwitterion (carry equal positive and negative charges) so it is neutral (not charged). - At physiological pH, basic amino acids will be positively charged. While acidic amino acids are negatively charged So, Basic and acidic amino acids are called charged amino acids
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