1. Amino acids

2. Amino Acid Monomers
Amino acids are organic molecules with carboxyl and amino groups
Amino acids are the building blocks of proteins.
Contain a carboxylic acid group and an amino group on the alpha () carbon.
Amino acids differ in their properties due to differing side chains, called R groups

3. Amino group Carboxyl group
-Carbon
Amino
group
Carboxyl
group

4. 20 amino acids make up protein
Amino Acid Monomers
20 amino acids make up protein
Dipolar (Have both charges)
+ve end (NH3+)
-ve end (COO-)

5. The 20 amino acids of proteins
Nonpolar
Glycine
(Gly or G)
Alanine
(Ala or A)
Valine
(Val or V)
Leucine
(Leu or L)
Isoleucine
(Ile or I)
Methionine
(Met or M)
Phenylalanine
(Phe or F)
Trypotphan
(Trp or W)
Proline
(Pro or P)
Polar
Serine
(Ser or S)
Threonine
(Thr or T)
Cysteine
(Cys or C)
Tyrosine
(Tyr or Y)
Asparagine
(Asn or N)
Glutamine
(Gln or Q)
Electrically
charged
Acidic
Basic
Aspartic acid
(Asp or D)
Glutamic acid
(Glu or E)
Lysine
(Lys or K)
Arginine
(Arg or R)
Histidine
(His or H)
The 20 amino acids of proteins

6. Nonpolar Glycine (Gly or G) Alanine (Ala or A) Valine (Val or V)
Leucine
(Leu or L)
Isoleucine
(Ile or I)
Figure 5.17 The 20 amino acids of proteins
Methionine
(Met or M)
Phenylalanine
(Phe or F)
Tryptophan
(Trp or W)
Proline
(Pro or P)

7. Polar Asparagine (Asn or N) Glutamine (Gln or Q) Serine (Ser or S)
Threonine
(Thr or T)
Cysteine
(Cys or C)
Tyrosine
(Tyr or Y)
Figure 5.17 The 20 amino acids of proteins

8. Electrically charged Acidic Basic Aspartic acid (Asp or D)
Figure 5.17 The 20 amino acids of proteins
Aspartic acid
(Asp or D)
Glutamic acid
(Glu or E)
Lysine
(Lys or K)
Arginine
(Arg or R)
Histidine
(His or H)

9. Essential Amino Acids
Group of amino acids not synthesized by the body
arg, his, ile, leu, lys, met, phe, thr, trp, val
Must obtain from the diet
All in diary products
1 or more missing in grains and vegetables

10. Complete protein
Proteins in diet comes from both animal and vegetable sources.
Most animal sources (meat, milk, eggs) provide complete protein (They contain all of the essential amino acids.
Vegetable sources usually low on or missing some essential amino acids.

11. Amino acids stereoisomers
All amino acids except glycine are chiral.
Amino acids have stereoisomers.
In biological systems, only L-amino acids are used in proteins.
L-Alanine D-Alanine L-Cysteine D-Cysteine

12. Zwitter ion
Both the –NH2 and the –COOH groups in an amino acid undergo ionization in water.
Zwitter ion = double ion – when both NH3+ and COO- are ionized
COO- can accept a proton, NH3+ can accept electron, so can act as acid and base (amphoteric)
At the isoelectric point (pI), the + and – charges in the zwitterion are equal.

13. Amino Acids as Acids
In solutions more basic than the pI, the —NH3+ in the amino acid donates a proton.
+ OH–
H3N—CH2—COO– H2N—CH2—COO–
Zwitter ion at pI Negative ion

14. Amino Acids as Bases
In solution more acidic than the pI, the COO- in the amino acid accepts a proton. H
H3N—CH2—COO– H3N—CH2—COOH
Zwitterion at pI Positive ion

15. pH and Ionization
Acidic amino acids such as aspartic acid have a second carboxyl group that can donate and accept protons.
The pI for aspartic acid occurs at a pH of 2.8.

16. Amino Acid Polymers
Amino acids are linked by peptide bonds
A polypeptide is a polymer of amino acids
Polypeptides range in length from a few to more than a thousand monomers
Each polypeptide has a unique linear sequence of amino acids

17. Making a polypeptide chain
bond
Amino end
(N-terminus)
Side chains
Backbone
Carboxyl end
(C-terminus)
(a)
(b)
Making a polypeptide chain
Figure 5.18

19. If only one a.a. begins with a certain letter, that letter is used
Cysteine = Cys = C
Histidine = His = H
Isoleucine = Ile = I
Methionine = Met = M
Serine = Ser = S
Valine = Val = V

20. If more than one a.a. begins with a certain letter, that letter is assigned to the most commonly occurring one
Alanine = Ala = A
Glycine = Gly = G
Leucine = Leu = L
Proline = Pro = P
Threonine = Thr = T

21. Phonetically suggestive
Aspartic acid (“asparDic acid”) = Asp = D
Phenylalanine (“Fenylalanine”) = Phe = F
Arginine (“aRginine”) = Arg = R
Tyrosine (“tYrosine”) = Tyr = Y
Tryptophan (double ring in the molecule) = Trp = W

22. A letter close to the initial is used
Asparagine (contains N) = Asn = N
Glutamic acid (near D) = Glu = E
Glutamine (“Q-tamine”) = Gln = Q
Lysine (near L) = Lys = K