THE KINETICS OF ENZYME REACTIONS: SCHüTZ'S LAW

1. A review of the applicability of Schütz''s Law to enzymic reactions is given. 2. The theoretical deductions of the Law, (a) on the basis of the law of mass action, (b) on the basis of the adsorption theory, are given and the significance of the assumptions made in these deductions pointed out. 3. It is shown that the true critical increment for an enzymic reaction is equal to twice the critical increment calculated from the Schütz constant k_s, if the heat of decomposition of the enzyme-products complex be neglected. 4. Experiments are described on the tryptic hydrolysis of casein at 30°C. and 404C. The foregoing considerations are applied to the experimental results obtained.     

A New Way to Degrade Heme: THE MYCOBACTERIUM TUBERCULOSIS ENZYME MhuD CATALYZES HEME DEGRADATION WITHOUT GENERATING CO*?

MhuD is an oxygen-dependent heme-degrading enzyme from Mycobacterium tuberculosis with high sequence similarity (∼45%) to Staphylococcus aureus IsdG and IsdI. Spectroscopic and mutagenesis studies indicate that the catalytically active 1:1 heme-MhuD complex has an active site structure similar to those of IsdG and IsdI, including the nonplanarity (ruffling) of the heme group bound to the enzyme. Distinct from the canonical heme degradation, we have found that the MhuD catalysis does not generate CO. Product analyses by electrospray ionization-MS and NMR show that MhuD cleaves heme at the α-meso position but retains the meso-carbon atom at the cleavage site, which is removed by canonical heme oxygenases. The novel tetrapyrrole product of MhuD, termed “mycobilin,” has an aldehyde group at the cleavage site and a carbonyl group at either the β-meso or the δ-meso position. Consequently, MhuD catalysis does not involve verdoheme, the key intermediate of ring cleavage by canonical heme oxygenase enzymes. Ruffled heme is apparently responsible for the heme degradation mechanism unique to MhuD. In addition, MhuD heme degradation without CO liberation is biologically significant as one of the signals of M. tuberculosis transition to dormancy is mediated by the production of host CO.     

THE ENDOGENOUS μ-OPIOID AGONISTS,ENDOMORPHIN 1 AND 2, HAVE VASODILATOR ACTIVITY IN THE HINDQUARTERS VASCULAR BED OF THE RAT

Endomorphin 1 and endomorphin 2 are newly-discovered endogenous ligands for the μ-opioid receptor. In the present study, responses to intra-arterial injections of endomorphin 1 and 2 were investigated in the hindquarters vascular bed of the rat. Under constant-flow conditions, endomorphin 1 and 2 induced dose-dependent decreases in hindquarters perfusion pressure when injected in doses of 3–100 nmol into the hindquarters perfusion circuit. Vasodilator responses to endomorphin 1 and 2 and met-enkephalin were attenuated by the opioid receptor antagonist naloxone (2 mg/kg i.v.) at a time when vasodilator responses to isoproterenol were not altered. In terms of relative vasodilator activity, endomorphin 1 and 2 were similar to ATP, 100-fold less potent than isoproterneol, and 10,000-fold less potent than acetylcholine. These data demonstrate that endomorphin 1 and 2 have significant naloxone-sensitive vasodilator activity in the hindquarters vascular bed of the rat. © 1997 Elsevier Science Inc.     

THE COPEPODA OF THE WU-L1 LAKE, WU-SIH, KIANGSU PROVINCE. Ⅱ. CYCLOPOIDA

This is the second part of a series of papers dealing with the Copepoda-fauna of theWu-Li Lake, Wu-Sih, Kiangsu Province, China. It comprises 21 species of Cyclopoida,of which 5 species (those marked with an asterisk in the list) are for the first time re-corded in China and two species of Tropocyclops are found to be new to science. All     

Human Cytochrome P450 21A2, the Major Steroid 21-Hydroxylase: STRUCTURE OF THE ENZYME·PROGESTERONE SUBSTRATE COMPLEX AND RATE-LIMITING C–H BOND CLEAVAGE*

Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase, and deficiency of this enzyme is involved in ∼95% of cases of human congenital adrenal hyperplasia, a disorder of adrenal steroidogenesis. A structure of the bovine enzyme that we published previously (Zhao, B., Lei, L., Kagawa, N., Sundaramoorthy, M., Banerjee, S., Nagy, L. D., Guengerich, F. P., and Waterman, M. R. (2012) Three-dimensional structure of steroid 21-hydroxylase (cytochrome P450 21A2) with two substrates reveals locations of disease-associated variants. J. Biol. Chem. 287, 10613–10622), containing two molecules of the substrate 17α-hydroxyprogesterone, has been used as a template for understanding genetic deficiencies. We have now obtained a crystal structure of human P450 21A2 in complex with progesterone, a substrate in adrenal 21-hydroxylation. Substrate binding and release were fast for human P450 21A2 with both substrates, and pre-steady-state kinetics showed a partial burst but only with progesterone as substrate and not 17α-hydroxyprogesterone. High intermolecular non-competitive kinetic deuterium isotope effects on both k_cat and k_cat/K_m, from 5 to 11, were observed with both substrates, indicative of rate-limiting C–H bond cleavage and suggesting that the juxtaposition of the C21 carbon in the active site is critical for efficient oxidation. The estimated rate of binding of the substrate progesterone (k_on 2.4 × 10⁷ m⁻¹ s⁻¹) is only ∼2-fold greater than the catalytic efficiency (k_cat/K_m = 1.3 × 10⁷ m⁻¹ s⁻¹) with this substrate, suggesting that the rate of substrate binding may also be partially rate-limiting. The structure of the human P450 21A2-substrate complex provides direct insight into mechanistic effects of genetic variants.     

A SURVEY OF THE HOLARCTIC LINYPHIIDAE （ARANEAE）, OF THE ERIGONINE GENUS ZORNELLA JACKSON, 1932

The Holarctic erigonid genus Zornella has been surveyed. Four spedes are recognized in this genus, two Palaearctic： Z. aultrigera （L. Koch, 11879） ＆ Z. orientalis sp. nov., and two Nearcfic： Z. arrnata （Banks, 1906） ＆ Z. ayptodon Chamherlin, 1920. It is demouslrated that neither nearcfic species is agnspedtic with Z. atltrigera as was thought by Holm and other authors. Z. sp. nov. is describcd from northeastern Siberia. All of the species are illustrated and their distributions mapped.