Enzymatic function in crystals of. delta. /sup 5/-3-ketosteroid isomerase: catalytic activity and binding of competitive inhibitors
Crystals of the steroid-metabolizing enzyme, ..delta../sup 5/-3-ketosteroid isomerase (EC 5.3.3.1) from Pseudomonas testosteroni, exhibit many enzymatic properties. Each enzyme subunit in the lattice binds a competitive inhibitor, progesterone, with the same stoichiometry (1:1) and affinity (K/sub D/ = 6 x 10/sup -6/ M) as the enzyme in solution. Another competitive inhibitor, 19-nortestosterone, competes with progesterone for the same binding sites in the crystal. The enzyme crystals catalyze the conversion of ..delta../sup 5/- to ..delta../sup 4/-ketosteroids, but because the enzyme is so efficient, and substrate diffusion into the crystal is so slow, substrate cannot penetrate deeply into the crystal before being converted to product. A general theoretical formulation is presented to account for the effects of substrate diffusion into enzyme crystals of different shapes and sizes. The dependence of apparent mean enzyme activity in steroid isomerase crystals as a function of crystal size is shown to be consistent with this theoretical formulation. These inhibitor binding and catalytic properties suggest that the enzyme is in an active conformation within these crystals.
- Research Organization:
- Univ. of Chicago, IL
- OSTI ID:
- 5132771
- Journal Information:
- J. Biol. Chem.; (United States), Vol. 259:14
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
59 BASIC BIOLOGICAL SCIENCES
CARBON 14
RADIOCHEMISTRY
ISOMERASES
ENZYME ACTIVITY
LABELLING
PROGESTERONE
TRITIUM
LABELLED COMPOUNDS
METABOLISM
STEROIDS
TRACER TECHNIQUES
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
CARBON ISOTOPES
CHEMISTRY
ENZYMES
EVEN-EVEN NUCLEI
HORMONES
HYDROGEN ISOTOPES
ISOTOPE APPLICATIONS
ISOTOPES
KETONES
LIGHT NUCLEI
NUCLEI
ODD-EVEN NUCLEI
ORGANIC COMPOUNDS
PREGNANES
RADIOISOTOPES
STEROID HORMONES
YEARS LIVING RADIOISOTOPES
400302* - Organic Chemistry- Isotope Effects- (-1987)
550500 - Metabolism