Enzyme up close
doi:10.1038/nindia.2008.301 Published online 17 October 2008
By studying the mutated version of an enzyme that helps maintain the acid-base balance of blood, researchers have been able to pin-point its exact identity. The study results will provide insights into how the mutated enzyme works and leads to physiological anomalies.
Known as human carbonic anhydrase II (HCA II), the enzyme aids in the interconversion between carbon dioxide and bicarbonate to maintain the acid-base balance in blood and other tissues and helps transport carbon dioxide out of tissues. HCA II is a zinc metalloenzyme. There is a water molecule very close to zinc. This zinc bound water molecule is generally referred to as the 'zinc water'. During catalysis, a proton (hydrogen ion) is ferried from 'zinc water' to histidine 64 (an amino acid) within the enzyme.
The researchers were interested to probe the transfer of proton in different mutants of HCA II and compare them to the ones encountered in the wild type (original) enzyme. For the study, the researchers resorted to computer-based simulation studies.
The researchers identified several proton transfer pathways. One path involves His-64 and there are several alternative paths (in mutants) devoid of His-64. The proton moves via the hydrogen-bonded networks of water molecules and amino acid residues on the enzyme.
Studies have shown that HCA II deficiency syndrome (due to mutation in gene that codes for the enzyme) ushers in renal tubular acidosis, osteoporosis, and in some cases mental retardation. Such mutation lowers the catalytic turnover of the enzyme by a factor of three.
"Our theory may be useful in understanding exactly how the proton transfer pathways are affected by the mutation," says lead researcher Srabani Taraphder from the department of chemistry, Indian Institute of Technology, Kharagpur. The team is in the process of studying this problem.
- Roy, A. et al. A Theoretical Study on the Detection of Proton Transfer Pathways in Some Mutants of Human Carbonic Anhydrase II. J. Phys. Chem. B. doi: 10.1021/jp0757309 (2008)