Q.40 Which one of the following is coded by the ABO blood group locus in the human genome?
(A) Acyl transferase (B) Galactosyltransferase
(C) Transposase (D) β–Galactosidase
Answer: (B) Galactosyltransferase
The ABO blood group locus encodes a glycosyltransferase enzyme responsible for adding specific sugar residues to determine A, B, or AB blood types.
Option Analysis
Acyl transferase (A): This enzyme transfers acyl groups (like acetyl-CoA derivatives) in lipid or fatty acid metabolism. It plays no role in carbohydrate-based blood group antigen synthesis on red blood cells.
Galactosyltransferase (B): Correct. The B allele of the ABO gene encodes an α-1,3-galactosyltransferase that adds galactose from UDP-Gal to the H antigen precursor, forming the B antigen. The A allele encodes a related N-acetylgalactosaminyltransferase, but both are glycosyltransferases with galactosyltransferase activity in the B form.
Transposase (C): This enzyme catalyzes DNA transposition in mobile genetic elements like transposons. It has no involvement in blood group glycosylation or human ABO genetics.
β-Galactosidase (D): A hydrolytic enzyme that cleaves terminal galactose residues from glycoconjugates. It degrades antigens rather than synthesizes them, opposite to ABO transferase function.
The ABO blood group locus in the human genome plays a pivotal role in determining blood types through glycosyltransferase activity, particularly galactosyltransferase for B antigens. Located on chromosome 9q34, this gene modifies H antigen precursors on red blood cells, crucial for transfusion compatibility and organ transplants.
Molecular Mechanism
A and B alleles encode distinct transferases: A adds N-acetylgalactosamine, while B uses galactosyltransferase to add galactose, differing by just four amino acids. The O allele produces an inactive enzyme, leaving H antigen unmodified.
Exam Relevance
In competitive exams like CSIR NET or GATE Biotechnology, questions test ABO blood group locus function, emphasizing galactosyltransferase over unrelated enzymes like acyl transferase or transposase.
Clinical Importance
ABO mismatches cause hemolytic reactions; understanding the encoded galactosyltransferase aids in precise genotyping for safe transfusions.
