Amino Acids

Building Blocks of Proteins and The end-products of Protein digestion.

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

Glycine the simplest amino acid Not required in the diet Important in various biosynthetic paths Abundant in collagen

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

Aliphatic Nonpolar amino acids

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

Aromatic Amino acids

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

Sulfur Containing Amino Acids

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

Polar Charged Amino Acids

Polar Uncharged amino acids

rich in electronegative atoms O & N Serine - CH2-OH

Threonine - CH(OH)-CH3

Asparagine -amide Glutamine- amide

Polar Uncharged Amino Acids

Classification of Amino Acids

Essential OR Non-essential Acidic Basic Aromatic Sulfur containing

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

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

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

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

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)

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

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.

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.

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

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.

Essential amino acids

Adults and children– Isoleucine, leucine , valine– Methionine– Lysine– Threonine– Phenylalanine– Tryptophan

Essential in addition

Growing children also require– Arginine– Histidine

Premature Infants (all of above plus)– Cysteine– Tyrosine

Amino Acids not found in proteins Beta alanine Ornithine - urea cycle Citrulline - urea cycle Gama carboxyglutamate - Vit K