CHEMISTRY OF MILK

Module 3. Milk proteins

Lesson 4
INTRODUCTION, DEFINITION ANS NOMENCLATURE OF MILK PROTEINS

4.1 Introduction

About two thirds of all cells consist of proteins and they have functional and structural roles in human body such as catalyst (enzymes), regulator (hormones), protection(Immunoglobulins), carrier proteins (haemoglobulin, lactoferrin, etc.) and as structuralproteins (collagens). Milk Protein is widely consumed human food proteins for people of all ages, particularly infants and children. Casein is unique due to their characteristic physical properties which are different from globular proteins. These proteins influence the behaviour and properties of other milk constituents. The protein content in a given milk products influences the structure of that product.

4.2 Definition

Proteins are defined as high molecular weight polymers of α-amino acids that are formed by living organisms. All these amino acids have L-configuration except glycine. The primary structure of proteins consists of a polypeptide chain of amino acids residues joined through peptide bonds.

Fig. 4.1 Primary structure of protein

(Source: http://www.web-books.com/MoBio/Free/Ch2B.htm)

Proteins differ from each other not only in their length of polypeptide chain but also in the proportion and sequence of the amino acids residues in them. Proteins of various species are said to be homologous if they are considered to have originated from a common ancestor. The most definitive criterion of homology is similarity of the amino acid sequence. The homologous proteins usually exhibit a common biological function but coincidence of function also can arise by convergence from different ancestral lines

4.3 Nomenclature of Milk Proteins

The Committee on Milk Protein Nomenclature,Classification, and Methodology of the Manufacturing Section of American Dairy Science Association (ADSA) for has made efforts to suggest suitable nomenclature to the milk proteins. Several research workers made attempts to suggest nomenclature basing on the resolution of this protein on the free boundary electrophoresis. The proteins which are having similar electrophoretic characteristics at pH 8.6 were actually heterogeneous mixtures of proteins.Waugh and Von Hipp et al. (1956) have shown that α-casein could be separated into calcium sensitive α _s-casein and calcium insensitive crude k-casein based upon its dissociation and differential solubility in the presence of calcium ions. A new terminology was suggested by Waugh et. al. (1956) for the action of rennin on α-casein fraction k casein for the primary reaction product of renninon k- casein and α_s-casein for the clots which are formed. But the role of other α-casein components and of βand or γ-casein in this transformation was not explained. The committee felt that in view of the advancements in the knowledge of the casein systems the committee preferred not recommend any nomenclature for the α-casein complex.

The American Dairy Science Association’s committee on milk protein nomenclature, classification and methodology has given a classification system for the known proteins in milk (Eigelet al., 1984; Farrell et al., 2004). According to this system of classification a Greek letter with or without a numerical precedes the class name when necessary to identify the family of proteins. The genetic variant of the protein is indicated by an upper case Arabic letter with or without a numerical super script immediately following the class name. Post-translational modifications are added in sequence: βcasein B 5P (f1 to 105) indicates that

a. Class of the protein is caseins

b. The family is β

c. The genetic variant is B

d. There are 5 post translational phosphorylations

e. The amino acid sequence from N-terminal amino acid through residue 1 through residue 105.