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Biol219 Ch 6 Vocab
Biol219 Cell Biology Becker World of the Cell
| Term | Definition |
|---|---|
| metastable state | thermodynamically unstable, but they do not have enough energy to exceed the activation energy barrier. |
| catalyst | an agent that enhances the rate of a reaction by lowering the energy of activation |
| active site | where the substrates bind and the catalytic event occurs. |
| prosthetic groups | metal ions or small organic mole-function as electron acceptors because none of the amino acid side chains are good electron acceptors. |
| substrate specificity | the ability to discriminate between very similar molecules |
| group specificity | accept any of a whole group of substrates as long as they possess some common structural feature. |
| induced-fit model | substrate binding at the active site distorts both the enzyme and the substrate, thereby stabilizing the substrate molecules in their transition state and rendering certain substrate bonds more susceptible to catalytic attack. |
| substrate activation | to activate the substrate by subjecting it to the right chemical environment for catalysis. |
| 3 types of substrate activation | 1. Bond distortion. 2. Protontransfer. 3. Electron transfer. |
| Bond distortion | The change in enzyme conformation induced by initial substrate binding to the active site not only causes better complementarity and a tighter enzyme-substrate fit but also distorts one or more of its bonds, thereby weakening the bond and making it more s |
| Proton transfer | Theenzymemayalsoacceptor donate protons, thereby increasing the chemical reactivity of the substrate. This accounts for the importance of charged amino acids in active-site chemistry, which in turn explains why enzyme activity is so often pH dependent. |
| Electron transfer | s a further means of substrate activation, enzymes may also accept or donate electrons, thereby forming temporary covalent bonds between the enzyme and its substrate. |
| enzyme kinetics | quantitative aspects of enzyme catalysis and the rate of substrate conversion into products  |
| initial reaction velocity (v) | the rate of change in product concentration per unit time (e.g., mM/min). |
| maximum velocity (Vmax) | as [S] tends toward infinity, v approaches an upper limiting value. his value depends on the number of enzyme molecules and can therefore be increased only by adding more enzyme. |
| saturation | The inability of increasingly higher sub- strate concentrations to increase the reaction velocity beyond a finite upper value |
| Michaelis constant | Here, v is the initial reaction velocity, [S] is the initial sub- strate concentration, Vmax is the maximum velocity, and Km is the concentration of substrate that gives exactly half the maximum velocity. Vmax and Km |
| Michaelis–Menten equation | V=(max[S])/(v=K +[S]) |
| turnover number (kcat) | the rate at which substrate molecules are con- verted to product by a single enzyme molecule when the enzyme is operating at its maximum velocity. |
| Lineweaver–Burk equation. | Equation 6-12 is known as the Lineweaver–Burk equation. When it is plotted as 1/v versus 1/[S], as in Figure 6-10, the resulting double-reciprocal plot is linear in the general algebraic form y = mx + b, where m is the slope and b is the y-intercept. |
| allosteric effectors | Most of these substances have an inhibitory effect on enzyme activity, reducing the reaction rate with the desired substrate or sometimes blocking the reaction completely. |
| irreversible inhibitor | inds to the enzyme covalently, causing permanent loss of catalytic activity. Not surpris- ingly, irreversible inhibitors are usually toxic to cells. Ions of heavy metals are often irreversible inhibitors, as are nerve gas poisons and some insecticides. |
| reversible inhibitor | binds to an enzyme in a noncovalent, dissociable manner, such that the free and bound forms of the inhibitor exist in equilibrium with each other. |
| substrate-level regulation | Regulation that depends directly on the interactions of substrates and products with the enzyme |
| allosteric regulation and covalent modification. | These mechanisms allow cells to turn enzymes on or off or to fine-tune their reaction rates by modulating enzyme activities appropriately. |
| feedback (or end-product) inhibition | one of the most common mecha- nisms used by cells to ensure that the activities of reaction sequences are adjusted to cellular needs. |
| The five-step sequence whereby the amino acid isoleucine is synthesized from threonine | threonine deaminase, is regulated by the concentration of isoleucine within the cell. If isoleucine is being used by the cell the isoleucine concentration will be low and the cell will need more. High concentration will lead to inhibition of threonine de |
| allosteric regulation | all enzymes capable of allosteric regulation can exist in two different forms. In one of the two forms, the enzyme has a high affinity for its substrate(s), leading to high activity. In the other form, it has little or no affinity for its substrate, givi |
| allosteric enzymes | The two different forms of an allosteric enzyme are readily interconvertible and are, in fact, in equilibrium with each other. |
| allosteric effector. | Whether the active or inactive form of an allosteric enzyme is favored depends on the cellular concentration of the appropriate regulatory substance |
| phosphorylation | The addition of phosphate group |
| protein kinases | Enzymes that catalyze the phosphorylation of other enzymes (or of other proteins) |
| dephosphorylation | involves the removal of a phosphate group from a phosphorylated protein |
| protein phosphatases | catalyze dephosphorylation |
| proteolytic cleavage | covalent acti- vation of enzymes involves the one-time, irreversible removal of a portion of the polypeptide chain by an appropriate proteolytic (protein-degrading) enzyme |
| ribozymes | RNA catalysts |
| thermodynamics allows us to | assess the feasibility of a reaction, it says nothing about the likelihood that the reaction will actually occur at a reasonable rate in the cell. |
| For a given chemical reaction to occur in the cell | substrates must reach the transition state, which has a higher free energy than either the substrates or products. Reaching the transition state requires the input of activation energy. |
| ecause of this activation energy barrier, most biological compounds exist in an unreactive, metastable state? | To ensure that the activation energy requirement is met and the transition state is achieved, a catalyst is required, which is always an enzyme in biological systems. |
| Catalysts, whether inorganic or organic, act? | orming transient complexes with substrate molecules that lower the activation energy barrier and rapidly increase the rate of the particular reaction. |
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