Esterase activity of hemoglobin. Differences between HB A and HB S

Area of Science:

• Biochemistry
• Enzymology
• Protein Chemistry

Background:

• Proteins and peptides can catalyze the hydrolysis of p-nitrophenyl acetate (p-NPA).
• Histidine residues are generally believed to mediate this catalytic effect.
• Oxyhemoglobin A (oxy Hb A) also demonstrates esterase activity.

Purpose of the Study:

• To investigate the esterase activity of oxyhemoglobin A and compare it with other hemoglobin variants.
• To explore the role of histidine residues in the catalytic hydrolysis of p-NPA by oxyhemoglobin.
• To use p-NPA hydrolysis as a probe for conformational differences in hemoglobin.

Main Methods:

• Kinetic studies of p-NPA hydrolysis in the presence of various oxyhemoglobin forms (oxy Hb A, oxy Hb S, oxy Hb Deer Lodge, oxy Hb F).
• Investigation of the effect of 2,3-diphosphoglyceric acid on the reaction rate.
• Analysis of pH dependence and studies with free amino acids.

Main Results:

• Oxy Hb A exhibits intermediate esterase activity compared to bovine albumin and carbonic anhydrase.
• Oxy Hb S showed approximately 2 times slower hydrolysis of p-NPA than oxy Hb A.
• Oxy Hb Deer Lodge (His beta 2 to Arg) hydrolysis rate approached that of oxy Hb S, while oxy Hb F was similar to oxy Hb A.
• Evidence supports the involvement of histidine residues, particularly His beta 2, in the catalytic activity.
• The reaction's pH dependence and effects of 2,3-diphosphoglyceric acid further implicate histidine.

Conclusions:

• Oxyhemoglobin possesses inherent esterase activity, with variations observed between different hemoglobin types.
• Subtle conformational differences exist between oxy Hb S and oxy Hb A in solution, likely involving His beta 2 and its microenvironment.
• Catalytic hydrolysis of p-NPA serves as a sensitive method for probing conformational states of macromolecules like hemoglobin.