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Galactose is an epimer of glucose, differing only at the C4 position in their open chain forms.
Ref: NCERT Class 12 Chemistry > Chapter 14: Biomolecules > Topic: Carbohydrates - Disaccharides and Glycosidic Linkage
Lactose is composed of galactose and glucose, linked by a β-1,4-glycosidic bond.
Ref: NCERT Class 12 Chemistry > Chapter 14: Biomolecules > Topic: Carbohydrates - Disaccharides and Glycosidic Linkage
Galactose sensing and induction depend on Gal3 protein, paralog of Gal1 galactokinase lacking kinase activity but preserving ligand-binding pocket. In presence of galactose and ATP, Gal3 undergoes conformational change promoting high-affinity interaction with Gal80 repressor both cytoplasmic and nuclear fractions, sequestering Gal80 away from Gal4-bound promoters. Gal3-Gal80 sequestration frees Gal4 activation domain recruiting coactivators initiating transcription of GAL1, GAL10, GAL7 genes required for galactose catabolism via Leloir pathway. Gal1 itself contributes to maintenance after init
Ref: NCBI Bookshelf, Yeast Gene Regulation: Galactose Inactivation of Gal80 by Gal3
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