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Allolactose is a disaccharide similar to lactose. It consists of the D-galactose and D-glucose linked through a β1-6 Glycosidic bond instead of the β1-4 linkage of lactose. It may arise from the occasional transglycosylation of lactose by β-galactosidase.
It is an inducer of the lac operon in Escherichia coli and many other Gut flora. It binds to a subunit of the tetrameric lac repressor, which results in conformational changes and reduces the binding affinity of the lac repressor to the lac operator, thereby dissociating it from the lac operator. The absence of the repressor allows the transcription of the lac operon to proceed. A non-hydrolyzable analog of allolactose, isopropyl β-D-1-thiogalactopyranoside (IPTG), is normally used in molecular biology to induce the lac operon.
Mechanism of Allolactose Formation:
β-Galactosidase (lacZ) plays a dual role in the lac operon system. Not only does it break down lactose into glucose and galactose, but it also catalyzes the transformation of lactose into allolactose, the molecule that induces the lac operon. The enzyme facilitates this conversion via a glucose-binding site, which temporarily holds glucose after cleavage from lactose. Despite the enzyme’s relatively low affinity for glucose, the exact details of this glucose-binding site have remained difficult to pinpoint. Research using a modified version of β-galactosidase (G794A) has provided structural insights, confirming that the glucose in the trapped allolactose molecule binds to a specific site on the enzyme.
Incorporating Allolactose in Research:
Recent studies, such as the work by Toba, Watanabe, and Adachi (1982), have demonstrated the presence of non-lactose disaccharides, including allolactose (6-O-β-D-galactopyranosyl-D-glucose) and 6-O-β-D-galactopyranosyl-D-galactose, in commercially available yogurt. These disaccharides, alongside lactose and galactose, were identified through sophisticated gas-liquid chromatography (GLC) and mass spectrometry. The research highlighted that while lactose and galactose were found in higher concentrations (ranging from 2.11% to 3.13% and 1.11% to 1.52%, respectively), allolactose and 6-O-β-D-galactopyranosyl-D-galactose were present in much smaller quantities (0.03% to 0.09%). The ability to isolate these disaccharides from yogurt using methods like dialysis and chromatography has opened new insights into the sugar composition of yogurt, beyond the more commonly studied lactose and galactose.
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