| Michaelis-Menten constant | <chemistry> The true dissociation constant for the enzyme-substrate binary complex in a single-substrate rapid equilibrium enzyme-catalyzed reaction (usually symbolised by Ks), the concentration of the substrate at which half the true maximum velocity of an enzyme-catalyzed reaction is achieved (when velocities are measured under initial rate and steady state conditions). The ratio of rate constants (k2 + k3)/k1 in the single-substrate enzyme-catalyzed reaction: E + S &dblarr; ES &dblarr; E + products where E represents the free enzyme, S is the substrate, and ES is the central binary complex. The expression for the Michaelis constant will be more complex for multisubstrate reactions. An apparent Michaelis constant is a constant determined either under conditions that are not strictly steady state and initial rate or one that varies with the concentration of one or more cosubstrates. See: Michaelis-Menten equation. Synonym: Michaelis-Menten constant. (05 Mar 2000) |
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| Planck's constant | A constant, 6.6260755 × 10-34 J - s (joule-seconds) or 6.6260755 × 10-27 erg-seconds = 6.6260755 × 10-34 J Hz-1 (joule per hertz). (05 Mar 2000) |
| constant | A quantity that, under stated conditions, does not vary with changes in the environment. (05 Mar 2000) |
| constant coupling | Where several premature beats are seen, the interval between each of them and the preceding normal beat is constant. Synonym: constant coupling. Variable coupling, where several extrasystoles are seen, the interval between each of them and the preceding sinus beat varies. (05 Mar 2000) |
| constant field equation | An equation derived to predict membrane potentials in terms of the membrane's permeability to ions and their concentrations on either side. Synonym: constant field equation, Goldman-Hodgkin-Katz equation, GHK equation. (05 Mar 2000) |
| constant infusion pump | An electrically driven device for delivery from a reservoir of a constant, often very small, volume of solution over a prolonged period of time. (05 Mar 2000) |
| constant region | The stem and forking part of the Y-shaped antibody protein, consisting of amino acid chains, that is exactly the same in all antibody molecules within the same individual. (The ends of the Y-shaped molecule will vary widely between different antibodies). (09 Oct 1997) |
| haemoglobin Constant Spring | An abnormal haemoglobin having an extended polypeptide chain (31 additional amino acid residues) on the a chain (thus, the a chain is 172 amino acids long); approximately 20% of the individuals with Hb H disease also have this defect. (05 Mar 2000) |
| Hill constant | A measure of cooperativity in a binding process. A Hill coefficient of 1 indicates independent binding, a value of greater than 1 shows positive cooperativity binding of one ligand facilitates binding of subsequent ligands at other sites on the multimeric receptor complex. Worked out originally for the binding of oxygen to haemoglobin (Hill coefficient of 2.8). (18 Nov 1997) |
| sedimentation constant | The constant s in Svedberg's equation for estimating the molecular weight of a protein from the rate of movement in a centrifugal field:where M is the molecular weight, R the gas constant, T the absolute temperature, D the diffusion constant (in square centimeters per second), V the partial specific volume of the protein, ρ the density of the solvent. The constant s, with dimensions of time per unit of field force (s = drb/dt /ω2ro where rb is the position at time t, r0 is the position at time 0, and ω is the angular velocity) is usually between 1 × 10-13 and 200 × 10-13 second. The Svedberg unit (S) is arbitrarily set at 1 × 10-13 second and is very often used to describe the sedimentation rate of macromolecules; e.g., 4 S RNA. Synonym: sedimentation coefficient. (05 Mar 2000) |
| specificity constant | Ratio of the maximum velocity (Vmax) or kcat to the true Km value for a specific substrate in an enzyme-catalyzed reaction. (05 Mar 2000) |
| Newtonian constant of gravitation | A universal constant relating the gravitational force, f., attracting two masses, m1 and m2, toward each other when they are separated by a distance, r, in the equation: f = G(m1m2/r2); it has the value of 6.67259 × 10-8 dyne cm2 g-2 = 6.67259 × 10-11 m3 kg-1 s-2 in SI units. (05 Mar 2000) |
| decay constant | <physics, radiobiology> The fraction of the amount of a radionuclide that undergoes transition per unit time. Formally: Lamda=dP/dt Where dP is the probability of a given nucleus undergoing spontaneous nuclear transition in the time interval dt. (16 Dec 1997) |
| diffusion constant | For the translational diffusion of solutes, diffusion is described by Fick's First Law, that states that the amount of a substance crossing a given area is proportional to the spatial gradient of concentration and the diffusion constant (D), that is related to molecular size and shape. A useful derived relationship is that the mean square distance moved by molecules in time t is 6Dt. (18 Nov 1997) |
| disintegration constant | <physics, radiobiology> The fraction of the amount of a radionuclide that undergoes transition per unit time. Formally: Lamda=dP/dt Where dP is the probability of a given nucleus undergoing spontaneous nuclear transition in the time interval dt. (16 Dec 1997) |