Amino Acids Building Blocks of Proteins and The end-products of Protein digestion.
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Transcript of Amino Acids Building Blocks of Proteins and The end-products of Protein digestion.
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Amino Acids
Building Blocks of Proteins and The end-products of Protein digestion.
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Structural features of all amino acids All are - amino acids All are zwitterions at physiological pH each has an
- amino group - carboxyl group– An - hydrogen (H)– An R - group
The R - group is variable between the 20 amino acids
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Glycine the simplest amino acid Not required in the diet Important in various biosynthetic paths Abundant in collagen
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Nonpolar aliphatic amino acids Glycine - R group H Alanine - R group -CH3
Leucine R group is -CH2- CH(2 CH3)
Isoleucine R group -CH (CH3)- CH2-CH3
Proline - R group is unusual ; the only imino acid
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Aliphatic Nonpolar amino acids
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Aromatic Amino Acids
Largely non-polar; exception tyr is polar All absorb strongly in UV spectrum
(280nm) Phenylalanine - phenyl ring Tyrosine - phenyl ring with -OH Tryptophan -indole ring
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Aromatic Amino acids
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Sulfur containing amino acids
Cysteine - R group is -CH2-SH ; reduced
– Highly reactive capable of reacting to become oxidized
– Two cysteines joined by sulfhydrl bridge is cystine; oxidized
Methionine- R- group -CH2-CH2-S-CH
– Sulfur in a thioether linkage
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Sulfur Containing Amino Acids
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Polar Charged amino acids
Rich in electronegative atoms O & N Arginine +1 net charge; guanidino Lysine +1 net charge Histidine + 1 net charge; imidazole ring Aspartate - 1 net charge Glutamate - 1 net charge Arg, Lys and His are basic amino acids Asp and Glu are acidic amino acids
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Polar Charged Amino Acids
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Polar Uncharged amino acids
rich in electronegative atoms O & N Serine - CH2-OH
Threonine - CH(OH)-CH3
Asparagine -amide Glutamine- amide
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Polar Uncharged Amino Acids
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Classification of Amino Acids
Essential OR Non-essential Acidic Basic Aromatic Sulfur containing
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Things you should already know!
Which amino acids are hydrophobic. Which amino acids are hydrophilic. What aliphatic means. What aromatic means. You should be able to :
– Recognize the structure of amino acids– Know the basic chemistry of each– Know the particulars of any interactions
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Acid Base Properties of amino acids All have two acid-base groups on a carbon
– - amino– - carboxyl
In addition those amino acids with ionizable side chains have an additional acid base group
Amino acids can have a significant buffering effect on solutions
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Titration of Amino Acids
Just as a weak acid like acetic acid can be titrated with base (NaOH), an amino acid can be titrated
Ionizable groups carry protons @ low pH (high [H+]) that dissociate as pH increases
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Titration of amino acids 2
For amino acids with no ionizable side chain groups there are only 2 pka’s
– 1 The first pka corresponds to the carboxyl group (pka 2.0); releases a proton and becomes - charged
– 2 The second pka corresponds to the a amino group (pKa 9.0); it takes on a proton & becomes + charged
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Titration of amino acids 3
For amino acids with ionizable side chains there are additional pKa’s observed– The carboxyl & amino groups same
pKa’s 2 and 9 respectively
– The third pKa varies with each aa bearing an ionizable side chain (See Fig 7.15 p73)
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Titration Curves Ex glycine
Fig 7.16 p 75 pK1 is for the carboxyl group (just
above 2.0) the pI is next around pH 6 pK2 is for the a amino group (almost pH
10.0) See also fig 7.17 p 75 Histidine
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pK values and pI
pK is that point on the pH scale where a particular acid and its conjugate base are at a concentration of 50:50 concentration of each.
Phosphoric acid example; has 3 pK’s pI is the isoelectric point; used to describe
the point on the pH scale where an amino acid or a protein is electrically neutral.
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Henderson-Hasselbach equation pH = pK + log [base]/[acid] This equation is useful in the discussion
of weak acid and their conjugate base in buffering.
Buffers are acid base combinations which tend to keep the pH within a pK value. They work best in a range of +1 or -1 pH unit.
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Physiologically important Buffers Carbonic acid / bicarbonate (most
important buffer in extracellular fluids) Phosphate buffer (important buffer in
cells and plays a role in the kidneys in achieving acid base balance)
Hemoglobin (buffer in RBC’s) Plasma proteins such as albumin Ammonia buffer system of kidneys
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Histidine as a Buffer
The only amino acid with an ionizable side chain that does have buffering capacity at physiological pH
The imidazole ring ionization pK 2 is approximately 6.5
This is indeed close enough to the physiologically pH 7.4 to have some buffering capacity.
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Essential amino acids
Adults and children– Isoleucine, leucine , valine– Methionine– Lysine– Threonine– Phenylalanine– Tryptophan
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Essential in addition
Growing children also require– Arginine– Histidine
Premature Infants (all of above plus)– Cysteine– Tyrosine
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Amino Acids not found in proteins Beta alanine Ornithine - urea cycle Citrulline - urea cycle Gama carboxyglutamate - Vit K