Principles of Protein Structure - G. Schultz 1982

Protein Evolution
Protein Differentiation
α-Lactalbumin and Lysozyme

Lactalbumin [517, 528] and Lysozyme [518, 529—531] provide a classic example of two Proteins with similar sequences but different Functions and varying rates of mutation fixation. The hypothesis of structural similarity between the two proteins was first proposed in 1958 and confirmed about a decade later [523, 533] by comparing their Amino acid sequences. Several properties of both proteins important for comparison are listed in Table 9.3. The three-dimensional Structure of bovine lactalbumin was determined by building a model [534] based on The structure of hen egg-white lysozyme, followed by energy Minimization [501, 535]. This Procedure assumes identical folding of both chains, which is quite justifiable given the high sequence similarity between the two proteins (Table 9.3). This example also demonstrates how data from one protein can be utilized for the structural analysis of distantly related homologous proteins.

Class="center">Table 9.3 Properties of α-lactalbumin [528] and lysozyme [518, 531]

α-Lactalbumin

Lysozyme

Estimated divergence time from a common ancestor [145]

3∙108 years

Fate following divergence

Participates in the evolution of a new biological function

Retains the ancestral function

Mutation fixation rate [1451

22 PAM/108 years

10 PAM/108 years

Amino acid differences between the two human proteins

82 ≡ 62% ≡ 126 PAM

Function

Serves to initiate and terminate lactose synthesis by interacting with a membrane-bound protein (Golgi apparatus)

Hydrolyzes Introduction/37.html">Bacterial Cell wall Polysaccharides

Occurrence

Found in milk (15% of total milk protein)

Found in many Tissues and secretions of all major- taxonomic groups; it remains unclear [241] whether all Enzymes exhibiting lysozyme activity are homologous


Last update: 06/08/2026

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