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

2. Structural features of all amino acids
All are a- amino acids
All are zwitterions at physiological pH
each has an
a- amino group
a- carboxyl group
An a- hydrogen (H)
An R - group
The R - group is variable between the 20 amino acids

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

4. 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

5. Aliphatic Nonpolar amino acids

6. 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

7. Aromatic Amino acids

8. 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

9. Sulfur Containing Amino Acids

10. 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

11. Polar Charged Amino Acids

12. Polar Uncharged amino acids
rich in electronegative atoms O & N
Serine - CH2-OH
Threonine - CH(OH)-CH3
Asparagine -amide
Glutamine- amide

13. Polar Uncharged Amino Acids

14. Classification of Amino Acids
Essential OR Non-essential
Acidic
Basic
Aromatic
Sulfur containing

15. 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

16. Acid Base Properties of amino acids
All have two acid-base groups on a carbon
a- amino
a- 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

17. 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 low pH (high [H+]) that dissociate as pH increases

18. 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 a 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

19. Titration of amino acids 3
For amino acids with ionizable side chains there are additional pKa’s observed
The a carboxyl & a 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)

20. Titration Curves Ex glycine
Fig 7.16 p 75
pK1 is for the a 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

21. 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.

22. 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.

23. 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

24. 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.

25. Essential amino acids Adults and children Isoleucine, leucine , valine
Methionine
Lysine
Threonine
Phenylalanine
Tryptophan

26. Essential in addition Growing children also require
Arginine
Histidine
Premature Infants (all of above plus)
Cysteine
Tyrosine

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