Which enzyme is responsible for conferring H activity on the red cell membrane?

The enzyme responsible for conferring H activity on the red cell membrane is L-fucosyl transferase. This enzyme plays a crucial role in the biosynthesis of the H antigen, which is the precursor for the A and B antigens in the ABO blood group system.

L-fucosyl transferase catalyzes the transfer of fucose, a type of sugar, to the terminal position of a galactose residue on a type 1 precursor chain, creating the H antigen that is present on red blood cells. The presence of this antigen is critical, as it serves as the base for the further attachment of the specific sugar modifications that define the A and B blood types.

In contrast, other enzymes listed, like galactosyl transferase and N-acetylgalactosaminyl transferase, are involved in the modification of the H antigen to form A or B antigens but do not play a role in the initial formation of the H antigen itself. N-acetylglucosaminyl transferase also does not directly contribute to the conferring of H activity. Therefore, L-fucosyl transferase is specifically highlighted for its function in establishing the foundational H antigen on the red cell membrane.