Related Experiment Videos
Evidence for an essential lysine in glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides
Abstract:
1. Pyridoxal 5'-phosphate inhibits glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides reversibly which Ki equals 0.04-0.06 mM. 2. This inhibition is competitive with respect to glucose 6-phosphate and non-competitive with respect to NADP+ or NAD+. Interaction between enzyme and excess pyridoxal 5'-phosphate follows pseudo-first-order kinetics and indicates that one molecule of inhibitor reacts with each active unit of enzyme. 3. Substrate and coenzyme protect the enzyme from inhibition by pyridoxal 5'-phosphate. Dissociation constants for NADP+ and glucose 6-phosphate were determined from their effects on the kinetics of enzyme--inhibitor interaction. 4. Reaction of the enzyme with pyridoxal 5'-phosphate produces a typical Schiff-base absorbance peak at 430 nm. Subsequent reduction with sodium borohydride leads to spectral changes characteristic for the formation of a secondary amine. 5. The irreversibly inactivated enzyme thus produced contains two moles of inhibitor per mole of enzyme (two subunits per mole). After protein hydrolysis, N-6-pyridoxyllysine can be identified by paper chromatography. 6. The enzyme is inhibited irreversibly by 1-fluoro-2,4-dinitrobenzene, even in the presence of excess 2-mercaptoethanol. At least one dinitrophenyl group is bound per active unit of enzyme; 4 to 5 moles of dinitrophenyl group are bound per mole of enzyme. NADP+ AND GLUCOSE 6-PHOSPHATE PROTECT AGAINST INHIBITION BY 1-FLUORO-2,4-DINITROBENZENE. The absorption spectrum of dinitrophenyl-enzyme corresponds to that for dinitrophenylated amino groups. 7. These studies indicate that there is an essential lysine at the active site of the enzyme. It is suggested that the function of this lysine is to bind glucose 6-phosphate. 8. It is proposed that a group of "active lysine" proteins may exist (in analogy with the "active serine" enzymes), which share a common structural feature at their substrate-binding site and to which pyridoxal 5'-phosphate binds specifically.
Insights
Pyridoxal 5'-phosphate reversibly inhibits glucose-6-phosphate dehydrogenase, acting competitively with glucose 6-phosphate. This reveals an essential lysine at the enzyme's active site, crucial for substrate binding.
Area of Science:
- Biochemistry
- Enzymology
Background:
- Glucose-6-phosphate dehydrogenase (G6PD) is a key enzyme in carbohydrate metabolism.
- Pyridoxal 5 ahydroximate (PLP) is a derivative of vitamin B6, acting as a cofactor in various enzymatic reactions.
Purpose of the Study:
- To investigate the inhibitory mechanism of pyridoxal 5 ahydroximate on glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides.
- To elucidate the role of active site residues in enzyme function and substrate binding.
Main Methods:
- Enzyme kinetics studies to determine inhibition constants (Ki) and binding interactions.
- Spectroscopic analysis (UV-Vis) to characterize enzyme-inhibitor complex formation.
- Chemical modification using 1-fluoro-2,4-dinitrobenzene and subsequent analysis (paper chromatography) to identify modified residues.
Main Results:
- Pyridoxal 5 ahydroximate reversibly inhibits G6PD competitively with glucose 6-phosphate and non-competitively with NADP+.
- The enzyme is irreversibly inhibited by 1-fluoro-2,4-dinitrobenzene, with modification occurring at an essential lysine residue.
- Spectroscopic data and chemical modification indicate the formation of a Schiff base and the presence of N-6-pyridoxyllysine upon inactivation.
Conclusions:
- An essential lysine residue is located at the active site of G6PD and is proposed to be involved in glucose 6-phosphate binding.
- The findings suggest the existence of 'active lysine' enzymes, analogous to 'active serine' enzymes, with specific structural features for cofactor binding.