Structural and functional characterization of human NAD kinase

Area of Science:

• Biochemistry
• Molecular Biology
• Human Physiology

Background:

• Nicotinamide adenine dinucleotide phosphate (NADP) is vital for biosynthesis, energy metabolism, and signal transduction.
• NAD kinase catalyzes NADP synthesis, but its structure, function, and regulation in multicellular organisms remain largely uncharacterized.

Purpose of the Study:

• To identify and characterize the human NAD kinase enzyme.
• To investigate its structure, function, substrate specificity, and tissue expression.

Main Methods:

• Human NAD kinase cDNA was identified from database information and amplified from a fibroblast cDNA library.
• The cDNA was functionally overexpressed in Escherichia coli for protein production and characterization.
• Gene expression analysis was performed across various human tissues.

Main Results:

• A human NAD kinase cDNA encoding a 49 kDa protein was identified and functionally expressed.
• Human NAD kinase differs significantly in subunit molecular mass from prokaryotic counterparts.
• The enzyme forms a homotetramer, exhibits high substrate selectivity for NAD and ATP, and does not phosphorylate NAAD.
• The gene is expressed in most human tissues, with notable absence in skeletal muscle.

Conclusions:

• The study provides the first detailed characterization of human NAD kinase, revealing its distinct properties compared to prokaryotic enzymes.
• Findings suggest that the synthesis of the calcium-mobilizing molecule NAADP likely occurs via a different pathway.
• Human NAD kinase plays a specific role in cellular metabolism and signaling, with tissue-specific expression patterns.