CFS 660: Proteins

What are the possible protein functionalities in biological systems?

enzymes (lipase, protease, amylase), transporters (beta-lactoglobin, myoglobin, hemoglobin), signaling (membrane proteins), structural (actin, myosin), and antibodies (immunoglobulins).

What are the possible protein functionalities in food systems?

nutritional (energy, essential amino acids), enzymes (catalysts, food quality, ingredient modification), Transporters (flavor binding), Structural (gels, emulsions, foams, water holding).

What are proteins?

Natural polymers of amino acids. That are coded into DNA. Peptide bonds link amino acids together. There are 4 different levels of structures. From a food chemistry perspective they contribute to texture, appearance, stability, and mouthfeel. 

Amino Acids

The monomer units of proteins. Must contain an amino group, side chain or R group. 

Electrostatic properties of amino acids

Side group may be neutral, positive, or negative. Charge depends on pH relative to a pK_a. The amount of electrostatic amino acids in a protein will determine the electric properties. Also dependent on the type of acid and alkaline in the environment

Positively charged amino acids

Lysine, arginine, histidine

Negatively charged amino acids

Aspartate and glutamate

pK_a1

the pH where the concentration of -COOH and -COO- are equal. 

pK_a2

the pH where the concentrations of -NH₃+ and -NH₂ are equal. 

pK_a3

if the amino acids have an ionizable side chains.

Amino acid hydrophobicity determines

protein structure, protein interactions, protein functionality. Side chains may be polar (hydrophilic) or non-polar (hydrophobic). 

Hydrophobic Effect

Tendency to minimize contact area between polar and non-polar groups. Reason for aggregation to minimize the area. 

Hydrophobic amino acids

Alanine, Glycine, Valine, Leucine, Isoleucine

Hydrophilic amino acids

Methionine, Proline, Tryptophan, Phenylalanine

Chemical reactivity may occur

with side chains. may occur during storage, processing or cooking. The R groups are targeted for modification.

what can chemical reactivity be used

can alter protein ingredient functionality and used to analytically measure proteins.

Examples of chemical reactions

disulfide bonds, maillard reactions, acetylation, deamination

Optical properties of amino acids

side groups interact with electromagnetic radiation. May effect functionality (antioxidants) and can be used to analytically measure proteins.

Amino acids with Aromatic R groups

Phenylalanine, Tyrosine, and Tryptophan

Peptide bonds

link amino acids together through a condensation reaction (loss of water). C terminus ends with a carboxylic acid, N terminus ends with an amino group. 

What is primary structure?

peptide bonds have limited rotation abilities (restricts possible protein backbone chain configurations. Cannot forma linear chain and will always rotate to the lower energy. From 0-100º. Will only be a linear chain if the rotation is at 0º or 100º. Peptide bonds cannot freely rotate with Proline. 

What does Primary structure depend on?

amino acid type, amino acid number, amino acid sequence, amino acid group bonding. Every protein has a unique primary structure that is coded in its genetic code and leads to a specific biological function.

Cystine bonds affect primary structure by

forming a disulfide bond. They can cross link and may have intramolecular or intermolecular interactions. Examples: egg, milk, soy, and wheat proteins

What are prosthetic groups?

Metals (metalloproteins), phosphates (phospoproteins), carbohydrates (glycoproteins), and lipids (lipoproteins). Some proteins are modified after synthesis by cellular enzymes, which add non-protein groups to polypeptide chain.

Alpha-Helix structure

a right handed turned helix. N-H to C=O hydrogen bonds between 4th succeeding amino acid in polypeptide chain, they will rotate one circle. Hydrogen bonds are parallel to axis. Typically amphiphilic (hydrophobic and hydrophilic groups). 3.6 residues per turns (one turn). Hydrogen bonds may stabilize the structure of the folded state. 

Beta-Sheet structure

C=O and N-H perpendicular to chain form inter-segment H-bonds. Parallel or antiparallel. Beta-strands typically 5-15 amino acids long, more stable than an alpha helix. High amounts of beta sheets make it more stable and hard to process. Anti-parallel beta sheets are more stable than parallel beta sheets.

Tertiary Structure

The 3D conformation adopted by a single protein chain, defined by position of all atoms in space.

What are the types of tertiary structures?

globular - many insoluble amino acids, protein tends to minimize surface/volume ratio. 

disordered - interacts well with water and takes up a random configuration.

fibrous - a strong secondary structure allows protein to retain a non-spherical shape.

Proteins fold to form these structures.

Quaternary Structures

three dimensional organization of multiple protein chains.