FOOD CHEMISTRY

Lesson 6. ENZYME CATALYZED REACTIONS INVOLVING HYDROLYSIS AND PROTEOLYSIS

Module 2. Food proteins

Lesson 6

ENZYME CATALYZED REACTIONS INVOLVING HYDROLYSIS AND PROTEOLYSIS

6.1 Introduction

Processes involving proteolysis play an important role in the production of many foods. Proteolysis can occur as a result of proteolytic enzymes present in the food itself or those from microbial sources. This large group of enzymes is divided into two large subgroups

1. Peptidases (exopeptidases) - These enzymes cleave amino acids or dipeptide in a stepwise manner from the terminal end of protein.

2. Proteinases (endopeptidases)- These enzymes hydrolyze the linkages within the peptide chain and do not attack terminal peptide bonds.

6.2 Types of proteolytic enzymes

Proteolytic enzymes can be divided into four groups: the acid proteases, the serine proteases, the sulfhydryl proteases, and the metal containing proteases.

6.2.1 Acid proteases

Those that have pH optimum at low pH. e.g. pepsin, rennin (chymosin). In the dairy industry, in cheese manufacture, thef ormation of casein curd is achieved with chymosin or rennin. Rennin is present in the fourth stomach of the suckling calf. Rennin can also be produced by genetically engineered microorganism. The coagulation of milk by rennin occurs in two stages. In the first, enzymatic stage, the enzyme acts on κ-casein (hydrolysis of peptide bond between Phe₁₀₅-Met₁₀₆) resulting in the formation of insoluble para-κ-casein and a soluble glycomacropeptide. The second stage involves the clotting of the modified casein micelles by calcium ions. Rennin is essentially free of other undesirable proteinases and is, therefore, especially suitable for cheesemaking.

6.2.2 Serine proteases

They have the presence of a serine and a histidine residue in their active sites. e.g. chymotrypsin, trypsin, plasmin, thrombin. Serine proteases are produced by a great number of bacteria and fungi. Chymotrypsin and trypsin are pancreatic enzymes that carry out their function in the intestinal tract. Trypsin cleaves linkages of amino acid residues with a basic side chain (lysyl or arginyl bonds).

6.2.3 Sulfhydryl proteases

Require sulfhydryl group (–SH) for activity. They are mostly of plant origin e.g. papain, ficin, bromelain. The active sites of these plant enzymes contain a cysteine and a histidine group that are essential for enzyme activity. These enzymes catalyze the hydrolysis of peptide, ester and amide bonds. Haze is a result of the combination of polypeptide and tannin molecules in beer giving rise to easily observed particles. Proteolytic enzymes (papain, ficin, bromelain) prevent this type of haze by reducing the polypeptide size.

6.2.4 Metal containing proteases

These enzymes are exopeptidases. They require a metal for activity and are inhibited by metal chelating compounds e.g. aminopeptidases, carboxypeptidases A and B, dipeptidases. Most of these enzymes contain zinc. Carboxypeptidases remove amino acids from the end of peptide chains that carry a free α-carboxyl group. Aminopeptidases remove amino acids from the free α-amino end of the peptide chain.

6.3 Application of proteolytic enzymes in foods

Enzymes are used for protein hydrolysis to:

1. To provide a wide variety of proteins known as enzymatically modified proteins e.g. egg protein, whey protein

2. Improving functional properties of proteins

3. For solubilization of denatured proteins

4. For maintenance of protein solubility in acid media

5. Increasing digestibility

6. Decomposition of those proteins that possess undesirable properties