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Biomedical subjects

B M Britt

Publications and source records attributed to B M Britt.

6 recordsLinked to original sources
PubMed2001

Binding thermodynamics of the transition state analogue coformycin and of the ground state analogue 1-deazaadenosine to bovine adenosine deaminase.

Binding of the transition state analogue coformycin and the ground state analogue 1-deaazadenosine to bovine adenosine deaminase have been thermodynamically characterized. The heat capacity changes for coformycin and 1-deazaadenosine binding are -4.7 +/- 0.8 kJ/mole-K and -1.2 +/- 0.1 kJ/mole-K, respectively. Since the predominant source of heat capacity change in enzyme interactions are changes in the extent of exposure of nonpolar amino acid side chains to the aqueous environment and the hydrophobic effect is the predominant factor in native structure stabilization, we propose that the binding of either class of ligand is associated with a stabilizing enzyme conformational change with coformycin producing the far greater effect. Analysis of the T dependence of the second order rate constant for formation of the enzyme/coformycin complex further reveals that the conformational change is not rate limiting. We propose that the enzyme may facilitate catalysis via the formation of a stabilizing conformation at the reaction transition state.

Adenosine Deaminase↗
PubMed1998

Evidence for a low temperature transition state binding preference in bovine adenosine deaminase.

Arrhenius plots of the interactions of bovine adenosine deaminase (ADA) and of coformycin-inhibited ADA with adenosine are non-linear and reveal that coformycin significantly increases the activation energy for reaction only at temperatures well below the normal operating temperature of the enzyme (38.3 degrees C). This apparent enhanced affinity of the enzyme for the transition state analog at low temperature is confirmed from determinations of coformycin binding at 38.3 degrees C (KI = 5.3 x 10(-11) M) and at 21 degrees C (KI = 1.1 x 10(-11) M). It is suggested that these data are inconsistent with a model for general enzyme catalysis that requires an initial transition state complementary active site. Instead, it is suggested that an initial active site transition state complementarity is undesirable and the tendency of the enzyme to exist in this conformer at low temperatures is responsible for its inefficient interaction with adenosine substrate.

Adenosine↗
PubMed1998

Sequence similarities of glyceraldehyde-3-phosphate dehydrogenases, phosphoglycerate kinases, and pyruvate kinases are species optimal temperature-dependent.

Data are presented that suggest enzyme sequence similarities among species are not solely a function of their evolutionary relationship. It is demonstrated that sequence similarities of glyceraldehyde-3-phosphate dehydrogenases, phosphoglycerate kinases, and pyruvate kinases from yeast, bacteria, mammals and a bird possess a significant species optimal thriving temperature dependence that crosses through conventional phylogenetic divisions. It is therefore suggested that species which are distantly related evolutionarily may possess some degree of enzyme sequence similarity if they happen to thrive at near the same optimal temperature; conversely, organisms which are closely related evolutionarily but function at radically different temperatures will possess a sequence dissimilarity that may mask the close relatedness.

Animals↗
PubMed1997

For enzymes, bigger is better.

Previously published data are re-examined in order to address two fundamental questions concerning enzyme catalysis: Why are enzymes so big? How is the substrate binding energy realized in the transition state? Relationships are shown that demonstrate (1) an increased enzyme:substrate mass ratio is associated with greater stabilization of the transition state and with increased substrate binding energy, and (2) tighter substrate binding is associated with greater transition state stabilization. It is argued that the conventional view of enzyme catalysis cannot account for these trends while the Shifting Specificity Model can. It is postulated that enzymes have evolved to be massive so that the interaction of the substrate with the active site alters the global conformation of the enzyme in a meaningful way; that is, the interaction alters the active site from an initial substrate-specific geometry to a transition state-specific geometry. It is also postulated that strong enzyme-substrate interactions better facilitate this active site transformation, thus, providing a mechanism for the realization of the substrate binding energy in the transition state of the chemical transformation.

Catalysis↗
PubMed1993

A shifting specificity model for enzyme catalysis.

A new model for general enzyme catalysis challenges the idea that transition state complementarity of enzyme active sites to the reactions they catalyze is the sole source of their catalytic efficacy. The "shifting specificity" model rejects the widely held view that strong interactions of the enzyme with substrate inhibit catalytic efficiency and previously published data are presented which show that a strong interaction of substrate with the enzyme active site actually facilitates its conversion to product. Furthermore, this new model defines a role for the entire enzyme molecule unlike most theories of enzyme catalysis which are concerned only with the interaction of the active site with substrate. This shifting specificity model for general enzyme catalysis may be expressed succinctly as: (i) enzymes have evolved to bind substrates; (ii) enzyme/substrate complexes have evolved to bind transition states; (iii) a stronger interaction of substrate with the enzyme facilitates a more rapid conversion to product. This last effect results from a more efficient modulation of the global enzyme conformation by tight-binding substrates. It is suggested that the addition of atomic mass to the enzyme, which interacts with the enzyme in the same manner in which the enzyme domains interact with one another, must alter the low-frequency, global vibrations of the enzyme to produce a different overall conformation. Nature has selected for that conformational change which shifts the active site complementarity from substrate-specific to transition state-specific. Thus, this model suggests a means for an efficient realization of the substrate-binding energy in the transition state of the reaction. General aspects of this model are discussed in light of the current view of enzyme catalysis.

Catalysis↗