Protein Structure

FDSC400

Protein Functions

• Biological? • Food?

Protein Structure

20 Amino Acids

Primary

Secondary

Tertiary

Quaternary

Denatured

( )

Coded in DNA

Self assembly to a single (native) structure. Depends on primary structure and solution conditions

Common in foods. Many non-native forms depending on protein structure, solution conditions (& history) and ingredient interactions

Amino Acids

• The monomer unit of proteins

C CH

NH2

R

HO

O

•R is the side chain.•One of 20 different chemical compounds•Some R-groups are acid (other alkali)•Some R-groups are water soluble (others are not)

Chiral carbon (L-series)

Amino Acids

Polar• Uncharged. Ser, Thr,

Asn, Gln, Cys

• Positive (basic). Arg, Lys, His

• Negative (acidic). Asp, Glu,

Non-Polar• Aliphatic. Ala, Ile,

Leu, Met, Pro, Val

• Aromatic. Phe, Trp, Tyr

Example Amino Acids

Alanine

Glutamic acid

Phenylalanine

Peptide Bonds

C CH

NH2

R

HO

O

C CH

NH2

R

HO

O

C CH

NH

R

HO

OC C

HNH2

R

O

Water

Amino acids

Peptide Bonds

O

C N

H

O

C N

H

:

-

+

NH

NH

O

O

R

NH+

NH

+

O

O

R

Disulfide Bonds

• Two cysteine molecules under oxidizing conditions

• Intermolecular or intramolecular cross-link

CH2

SH CH2

SH

CH2

SSCH2

[O]

-Helix

• N-H to C=O hydrogen bonds in 4th succeeding A.A.

• Hydrogen bonds parallel to axis

• Typically amphiphilic

Amphiphilic 2° Structures

Hydrophilic

Hydrophobic

-Sheet

• C=O and N-H perpendicular to chain form inter-segment H-bonds

• Parallel or antiparallel -strands typically 5-15 A.A.• More stable than -helix

-sheet

Hydrophobic Hydrophobic amino acidsamino acids

Peptide chainPeptide chain

Protein FoldingProtein Folding

Tertiary Structure

Types of Tertiary Structure

Globular Disordered Fibrous

Many insoluble amino acids, protein tends to minimize surface/volume ratio

Interacts well with water and takes up a random configuration

Strong secondary structure allows protein to retain a non-spherical shape

Quaternary Structure

Folded protein unable to contain some hydrophobic residues

Dimerized protein shields the hydrophobic amino acids from water