1. Amino Acids and Proteins
Biology 12
E. McIntyre

2. Where do we find proteins?
What do gelatin desserts, hair, antibodies, spider webs, blood clots, egg whites, tofu, and fingernails all have in common? They are all made of protein.

3. Proteins
genetic information in DNA codes specifically for the production of proteins
50% of the dry mass of most cells
As with carbohydrates and lipids, their three-dimensional structure is directly related to their function
enzymes are proteins

4. Proteins: Basic Characteristics
Proteins are amino acid polymers
central carbon atom surrounded by an amino group, a carboxyl group, a hydrogen atom, and a variable group of atoms called a side chain, usually symbolized by the letter ‘R’. Remember that carboxyl and amino groups are functional groups.
There are 20 different R groups commonly found in living organisms and, so there are 20 different amino acids.
General structure of an amino acid

5. There are 20 amino acids.
Note all are the same except for the side chains

6. Amino acids: General Properties
Amino acids are amphiprotic (which means that they possess both acidic (carboxyl) and basic (amino) functional groups.)
When dissolved in water, the carboxyl group donates an H ion to the amino group, causing the carboxyl group to become negatively charged and the amino group to possess an extra hydrogen and a net positive charge.

7. …Amino acids: General Properties
Amino acids may be polar (hydrophilic), nonpolar (hydrophobic), or charged (acidic or basic)
Acidic amino acids possess a carboxyl group on their side chains, and basic amino acids contain amino groups on their side chains.
These characteristics are determined by an amino acid’s side chain.

8. Examples of Amino Acids

9. Types of Amino Acids Nonpolar R = H, CH3 O Polar ll
R = –CH2OH, –CH2SH, –CH2C–NH2,
(polar groups with –O-, -SH, -N-)
Acidic
R = –CH2COOH, or -COOH
Basic
R = –CH2CH2NH2

10. Learning Check AA1
Identify each as polar or nonpolar A. B.

11. Solution AA1
Identify each as polar or nonpolar A B
Non-polar
polar

12. Essential Amino Acids 10 amino acids not synthesized by the body
Arginine (arg) histadine (his) isoleucine (ile)
leucine (leu) threonine (thr) lysine (lys), methionine (met) phenylalanine (phe) trytophan (trp)
valine (val)
Must obtain from the diet
All in diary products
1 or more missing in grains and vegetables
Want to remember the essential amino acids?
Any Help In Learning These Little Molecules Proves Truly Valuable

13. Amino Acids as Acids and Bases
Ionization of the –NH2 (amino) and the –COOH (carboxyl) group
An amphiprotic molecule has both a + and – charge
An amphiprotic molecule is neutral overall
Zwitterion of alanine
In water, an amino acid loses a H+ ion from the carboxyl group and
donates it to the amino group.

14. pH and ionization H+ OH– + + Positive ion zwitterion Negative ion
H3N–CH2–COOH H3N–CH2–COO– H2N–CH2–COO–
Positive ion zwitterion Negative ion
Low pH neutral pH High pH

15. Learning Check AA2 CH3 CH3 + H3N–CH–COOH H2N–CH2–COO– (1) (2)
(1) (2)
Select from the above structures
A. Alanine in base.
B. Alanine in acid.

16. Solution AA2 CH3 CH3 + H3N–CH–COOH H2N–CH2–COO– (1) (2)
(1) (2)
Select from the above structures
(2) Alanine in base.
(1) Alanine in acid.

17. Stop & Think
Remember that carboxyl and amino groups are functional groups.
Think: What are functional groups?
How are polymers of amino acids made?

18. Solution to Stop & Think
Carboxyl groups and amino groups react together to link amino acids.

19. The Peptide Bond
Amide bond formed by the –COOH of an amino acid and the –NH2 of the next amino acid
O CH3
| | |
NH3–CH2–COH H3N–CH–COO–
O CH3
| | |
NH3–CH2–C – N–CH–COO–
| peptide bond H

20. Peptides Amino acids linked by amide (peptide) bonds
Gly Lys Phe Arg Ser
H2N COOH
end Peptide bonds end
Glycyllysylphenylalanylarginylserine

22. Learning Check AA3
What are the possible tripeptides formed from one each of leucine, glycine, and alanine?

23. Solution AA3
Tripeptides possible from one each of leucine, glycine, and alanine
Leu-Gly-Ala
Leu-Ala-Gly
Ala-Leu-Gly
Ala-Gly-Leu
Gly-Ala-Leu
Gly-Leu-Ala

24. Learning Check AA4
Write the three-letter abbreviations for the following tetrapeptide:

25. Solution AA4
Ala-Leu-Cys-Met

26. Types of Proteins Type Examples
Structural tendons, cartilage, hair, nails
Contractile muscles
Transport hemoglobin
Storage milk
Hormonal insulin, growth hormone
Enzyme catalyzes reactions in cells
Protection immune response

27. Protein Structure