LACTOSE SYNTHASE.pptx

Lactose synthase is an enzyme that is responsible for the synthesis of lactose, which is a sugar found in milk. The lactose synthase enzyme is composed of two subunits, alpha-lactalbumin and beta-1,4-galactosyltransferase.

Alpha-lactalbumin is a regulatory protein that activates the beta-1,4-galactosyltransferase subunit, which then transfers a galactose molecule from UDP-galactose to glucose, forming lactose. This process occurs in the mammary glands of lactating females, where lactose is a primary component of milk.

Lactose synthase is important for the production of lactose in milk, which provides a source of nutrition for new born mammals. It also has applications in the food industry, where it is used to synthesize lactose for various products such as infant formula, ice cream, and cheese.
This is a summarised account on this enzyme in a very simple language.

1. LACTOSE SYNTHASE ENZYME TECHNOLOGY (BTY405) CA – 2 PRESENTATION SHARANYA CHAUDHURI 12108335

2. INTRODUCTION Lactose synthase (E.C – 2.4.1.22) It is an oligomeric enzyme which generates lactose from UDP galactose . It is isolated from milk. It is a member of the transferase family of enzymes. This enzyme is important for the synthesis of carbohydrate components of glycoproteins . Reaction it catalyses for lactose synthesis – UDP- galactose + glucose ⇌ UDP + lactose

3. Fig. – Reaction catalysed by Lactose Synthase

4. Lactose synthase is a typical example of an enzyme in which a non functional subunit modifies the behaviour of the functional subunit. The 2 subunits of Lactose synthase are – a. N- acetyllactoseamine synthase (functional subunit) b. Alpha- lactalbumin (non – functional subunit) The lactose synthase (LS) enzyme is a 1:1 complex of a catalytic component, beta1,4-galactosyltransferse (beta4Gal-T1) and a regulatory component, alpha- lactalbumin (LA), a mammary gland-specific protein. Fig. - Crystal structure of a lactose synthase complex

5. N-ACETYLLACTOSEAMINE SYNTHASE Better known as β-1,4- galactosyltransferase - 1. It contains the catalytic component. In the absence of α- lactalbumin , β-l,4-galactosyltransferase catalyses the reaction : It falls under the category of type 2 membrane proteins commonly found in Golgi. It consists of two flexible loops: small loop and large loop. The small loop consists of a Trp residue (Trp314) with surrounding glycine residues, meanwhile the large loop makes up amino acid residues 345 to 365. The Trp residue in the small loop moves allowing for the sugar nucleotide to be locked into the binding site . This causes a conformational change in the large loop which then creates sites for oligosaccharide and metal ion binding, and protein-protein interactions for alpha- lactalbumin .

6. 2 . ALPHA-LACTALBUMIN Consists of the regulatory component of Lactose synthase. Alpha- lactalbumin is a Ca 2+  binding protein specific to mammary glands . Alpha- lactalbumin , which is expressed in response to  prolactin , increases the affinity of N- acetyllactosamine synthase for its substrate, causing increased production of  lactose  during lactation. Without alpha- lactalbumin , the affinity of   N - acetylglucosamine to glucose is 1000-fold lower. Alpha - l actalbumin acts as the regulatory protein of lactose synthase (EC 2.4.1.22), a role in which it modulates the affinity of the catalytic component UDP- galactose β- N - acetylglucosaminide β1,4- galactosyltransferase-1 ( β4- galactosyltransferase-1; EC 2.4.1.38), for acceptor substrates through a reversible protein–protein interaction.

7. MECHANISM N- acetyllactoseamine is produced and stored in the mammary gland during pregnancy, when levels of α- lactalbumin are low . After the birth of the baby, reduced synthesis of the hormone  progesterone  in the mother leads to increased synthesis of the luteotrophic hormone ( prolactin ), stimulating the production of α- lactalbumin in the mammary gland . This combines with the stored β-1,4-galcosyltransferase to form lactose synthase, an enzyme which facilitates production of the lactose component of the milk required for the new-born baby. Lactose synthase catalyses the reaction:  UDP- galactose  + glucose ⇌ UDP + lactose. Thus it can be seen that the presence of the α- lactalbumin sub-unit changes the specificity of the enzyme, causing it to transfer  galactose  to glucose rather than to  N-   acetylgiucosamine . The overall process involves conformational changes in the region of the active site of the enzyme.

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