1. Chapter 19: Proteins

2. AMINO ACIDS The building blocks of proteins
Only 20 amino acids are common in proteins Called -amino acids because amino group is attached to the -carbon, next to the carboxylate group

3. AMINO ACID “R” GROUPS
Amino acids are categorized into four groups based on the “R” group characteristics. The “R” group can be:
1. neutral and nonpolar (e.g. -CH3)
2. neutral and polar (e.g. -CH2-OH)
3. basic (e.g. -CH2CH2CH2CH2-NH3+)
4. acidic (e.g.-CH2-COO-)

4. AMINO ACID STEREOCHEMISTRY
In 19 of the 20 amino acids, the -carbon is chiral.
With few exceptions, the amino acids in living systems are in the L form.
Glycine is the achiral amino acid.

5. AMINO ACID ZWITTERIONS
Amino acids exist as zwitterions, a dipolar ion that results from an internal acid-base reaction.
Note that the net charge of the zwitterion is zero.

6. ZWITTERIONS, cont.
The isoelectric point is the pH at which an amino acid has a net charge of zero.
At pH values above the isoelectric point, the amino acid has a net negative value.
At pH values below the isoelectric point, the amino acid has a net positive value.
Amino acid solutions can act as buffers because they react with both H3O+ and OH-.
“R” Group
Isoelectric Point
Neutral
About pH 6
Acidic
Less than pH 6
Basic
More than pH 6

7. AMINO ACID REACTIONS
Oxidation of cysteine, the only –SH containing amino acid, to form a disulfide (-S-S-) bridge:

8. REACTIONS, cont.
Peptide bond formation – amino acids can make polymers by forming amide (peptide) linkages

9. REACTIONS, cont.
Peptides – an amino acid polymer of short chain length
Polypeptide – intermediate chain length polymer, less than 50 amino acids
Protein – polymer with more than 50 amino acids
Amino acid residue – an amino acid that is part of a polymer
By convention, peptides are written with the N-terminal residue on the left and the C-terminal residue on the right

10. IMPORTANT PEPTIDES
Vasopressin and oxytocin – hormones released by pituitary gland
Both have disulfide bridges
Vasopressin (antidiuretic hormone) decreases urine formation
Oxytocin causes uterine contractions.

11. IMPORTANT PEPTIDES, cont.
Adrenocorticotropic hormone – released by pituitary gland
Has no disulfide bridges
Regulates the production of steroids by the adrenal gland

12. PROTEIN SIZE

13. PROTEIN FUNCTIONS

14. PROTEIN FUNCTIONS

15. PROTEIN SHAPES
Fibrous proteins – long rod-shaped or stringlike molecules that intertwine to form fibers (collagen, elastin, keratin)
Globular proteins – spherical-shaped proteins that form stable suspensions in water, or is water soluble (hemoglobin, enzymes)

16. PROTEIN COMPOSITION Simple proteins – contain only amino acid residues
Conjugated proteins – contain amino acid residues and other organic or inorganic components (prosthetic groups)

17. PRIMARY PROTEIN STRUCTURE
The linear sequence of amino acids in a protein chain
Determines secondary and tertiary structures

18. SECONDARY PROTEIN STRUCTURE
Determined by hydrogen bonding between amide groups of amino acid residues in the chain.
Two basic types of secondary structure: -helix – keratin,
myosin,
epidermin,
Fibrin
-pleated sheets – found extensively only
in silk protein
Random coil molecular structure

19. -HELIX

20. -PLEATED SHEET

21. TERTIARY PROTEIN STRUCTURE
The specific 3-D shape of a protein resulting from interactions between “R” groups of amino acid residues
“R” group interactions include: Disulfide bridges – form between cysteine residues Salt bridges – ionic bonds form between acidic and basic residues Hydrogen bonds – form between polar residues Hydrophobic interactions – form between nonpolar residues

22. TERTIARY STRUCTURE, cont.

23. TERTIARY STRUCTURE, cont.
In an aqueous environment, the interaction of hydrophilic and hydrophobic side chains with water also determines shape.

24. QUATERNARY PROTEIN STRUCTURE
All proteins have primary, secondary, and tertiary structure, but not all proteins have quaternary structure.
Quaternary structure is the arrangement of subunits that form a larger protein.
Subunits are polypeptides that have primary, secondary, and tertiary structure.
Conjugated proteins with quaternary structure contain subunits as well as prosthetic groups, which may be organic or inorganic components.
Heme is an example of a prosthetic group:

25. QUATERNARY PROTEIN STRUCTURE
Hemoglobin
4 chains (subunits): two identical alpha chains and two identical beta chains
4 heme groups
Hydrophobic forces hold subunits together

26. PROTEIN HYDROLYSIS
Heat and acid or base can completely hydrolyze proteins.
This is an important process in protein digestion.

27. PROTEIN DENATURATION
The process by which a protein loses its characteristic quaternary, tertiary, and secondary structure
Leads to a loss of biological activity (function)

28. PROTEIN DENATURATION