Excerpt fromChapter 19:

Proteins

University of Louisiana at Lafayette

CHEM 125, LeonardAll images used with permission of publisher (Cengage Learning)

Zwitterion

2

H3N CH C O

O

R

+ −

•A zwitterion has two oppositely charged ions which exist in the same molecule.

•The zwitterion can only exist in solution which is at the isoelectric pH.

Zwitterion

3

H3N CH C O

O

R

+ −

•If the pH is too low, the solution is too acidic, the carboxylic salt end will gain H+.

+ H+

H3N CH C OH

O

R

+

positive charge

Zwitterion

4

H3N CH C O

O

R

+ −

•If the pH is too high, the solution is too basic.

•The amine salt end will lose H+.

− H+

negative charge

H2N CH C O

O

R

−

Structural Elements of Proteins

I. 1o Structural Elements

•The order and identity of each amino acid in the protein is it’s 1o structural element.

•Diseases like sickle-cell anemia is caused by a minor sequencing error in the hemoglobin protein.

Structural Elements of Proteins

II. 2o Structural Elements

•Alpha () helix – a coil held together by hydrogen bonds.

Small-scale folding patterns along the protein chain are the 2o structural elements:

Structural Elements of Proteins

•Alpha () helix – a coil held together by hydrogen bonds.

Structural Elements of Proteins

II. 2o Structural Elements

Small-scale folding patterns along the protein chain are the 2o structural elements:

•Alpha () helix – a coil held together by hydrogen bonds.

•Beta ()-pleated sheet – a folded sheet-like shape held together by hydrogen bonds.

Structural Elements of Proteins•Beta ()-pleated sheet – a folded sheet-like shape held together by hydrogen bonds.

Structural Elements of Proteins

II. 2o Structural Elements

Structural Elements of Proteins

III. 3o Structural Elements

Large-scale folding patterns which showhow 1 protein chain folds over itself:

A. Disulfide Linkage – 2 cysteine amino acids can link their thiol R groups.

B. Salt Bridge – attractions between the R groups of the acidic and basic amino acids.

C. Hydrogen Bonding – R groups with an H attached to an O, N, or F is attracted to other O, N, or F atoms.

D. Hydrophobic Interactions – nonpolar R groups attract other nonpolar R groups.

Structural Elements of Proteins

III. 3o Structural Elements

Structural Elements of Proteins

III. 3o Structural Elements

If the following two R groups were in close proximity along a protein chain, what 3o attractions would you see?

AA−C−NH2

OHO−CH2CH2−AA

a)

hydrogen bonding

Structural Elements of Proteins

III. 3o Structural Elements

If the following two R groups were in close proximity along a protein chain, what 3o attractions would you see?

b)

hydrophobic interaction

AA−CH2CH3

CH3CH2-CH−AA

CH3

Structural Elements of Proteins

IV. 4o Structural Elements

•Large-scale folding patterns which show how 2 or more protein chains fold over each other.

Hemoglobin: made up of4 protein chains.