H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 20
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Within cells, however, manyreactions are linked in pathways in which a product of onereaction serves as a reactant in another or is pumped out ofthe cell. In this more complex situation, when the rate of formation of a substance is equal to the rate of its consumption,the concentration of the substance remains constant, and thesystem of linked reactions for producing and consuming thatsubstance is said to be in a steady state (Figure 2-21). Oneconsequence of such linked reactions is that they prevent theaccumulation of excess intermediates, protecting cells fromthe harmful effects of intermediates that have the potential ofbeing toxic at high concentrations.2.3 • Chemical Equilibrium(a) Test tube equilibrium concentrationsBBBBBBBBBAAA(b) Intracellular steady-state concentrationsAABBBBBBCCCC▲ FIGURE 2-21 Comparison of reactions at equilibrium andsteady state.
(a) In the test tube, a biochemical reaction (A n B)eventually will reach equilibrium in which the rates of the forwardand reverse reactions are equal (as indicated by the reactionarrows of equal length). (b) In metabolic pathways within cells,the product B commonly would be consumed, in this example byconversion to C. A pathway of linked reactions is at steady statewhen the rate of formation of the intermediates (e.g., B) equalstheir rate of consumption. As indicated by the unequal length ofthe arrows, the individual reversible reactions constituting ametabolic pathway do not reach equilibrium. Moreover, theconcentrations of the intermediates at steady state differ fromwhat they would be at equilibrium.Dissociation Constants for Binding ReactionsReflect the Affinity of Interacting Molecules[P][D][PD]1 molecule of DNA and 10 molecules of the DNA-bindingprotein P.
In this case, given a Kd of 1010 M, 99 percent ofthe time this specific DNA sequence will have a molecule ofprotein bound to it, and 1 percent of the time it will not, eventhough the cell contains only 10 molecules of the protein!Clearly, P and D bind very tightly (have a high affinity), as reflected by the low value of the dissociation constant for theirbinding reaction.The large size of biological macromolecules, such as proteins, can result in the availability of multiple surfaces forcomplementary intermolecular interactions.
As a consequence, many macromolecules have the capacity to bindmultiple other molecules simultaneously. In some cases,these binding reactions are independent, with their own distinct Kd values that are constant. In other cases, binding ofa molecule at one site on a macromolecule can change thethree-dimensional shape of a distant site, thus altering thebinding interactions at that distant site. This is an importantmechanism by which one molecule can alter (regulate) theactivity of a second molecule (e.g., a protein) by changing itscapacity to interact with a third molecule. We examine thisregulatory mechanism in more detail in Chapter 3.Biological Fluids Have Characteristic pH ValuesThe concept of chemical equilibrium also applies to the binding of one molecule to another.
Many important cellularprocesses depend on such binding “reactions,” which involvethe making and breaking of various noncovalent interactionsrather than covalent bonds, as discussed above. A commonexample is the binding of a ligand (e.g., the hormone insulinor adrenaline) to its receptor on the surface of a cell, triggering a biological response.
Another example is the binding ofa protein to a specific sequence of base pairs in a moleculeof DNA, which frequently causes the expression of a nearbygene to increase or decrease (Chapter 11). If the equilibriumconstant for a binding reaction is known, the intracellularstability of the resulting complex can be predicted. To illustrate the general approach for determining the concentrationof noncovalently associated complexes, we will calculate theextent to which a protein is bound to DNA in a cell.Most commonly, binding reactions are described in termsof the dissociation constant Kd, which is the reciprocal of theequilibrium constant.
For the binding reaction P DPD, where PD is the specific complex of a protein (P) andDNA (D), the dissociation constant is given byKd 47(2-4)Typical reactions in which a protein binds to a specific DNAsequence have a Kd of 1010M, where M symbolizes molarity, or moles per liter (mol/L). To relate the magnitude of thisdissociation constant to the intracellular ratio of bound tounbound DNA, let’s consider the simple example of a bacterial cell having a volume of 1.5 1015 L and containingThe solvent inside cells and in all extracellular fluids is water.An important characteristic of any aqueous solution is theconcentration of positively charged hydrogen ions (H) andnegatively charged hydroxyl ions (OH).
Because these ionsare the dissociation products of H2O, they are constituentsof all living systems, and they are liberated by many reactionsthat take place between organic molecules within cells.When a water molecule dissociates, one of its polarHOO bonds breaks. The resulting hydrogen ion, often referred to as a proton, has a short lifetime as a free particleand quickly combines with a water molecule to form a hydronium ion (H3O). For convenience, however, we refer tothe concentration of hydrogen ions in a solution, [H], eventhough this really represents the concentration of hydroniumions, [H3O]. Dissociation of H2O generates one OH ionalong with each H.
The dissociation of water is a reversiblereaction,H2 OH OHAt 25 ºC, [H][OH] 1014 M2, so that in pure water,[H] [OH] 107 M.The concentration of hydrogen ions in a solution is expressed conventionally as its pH, defined as the negative logof the hydrogen ion concentration. The pH of pure water at25 ºC is 7:pH log[H ] log117 log[H ]107It is important to keep in mind that a 1 unit difference inpH represents a tenfold difference in the concentration of48CHAPTER 2 • Chemical Foundationsprotons. On the pH scale, 7.0 is considered neutral: pH values below 7.0 indicate acidic solutions (higher [H]), andvalues above 7.0 indicate basic (alkaline) solutions.
For instance, gastric juice, which is rich in hydrochloric acid (HCl),has a pH of about 1. Its [H] is roughly a millionfold greaterthan that of cytoplasm with a pH of about 7.Although the cytosol of cells normally has a pH of about7.2, the pH is much lower (about 4.5) in the interior of lysosomes, one type of organelle in eukaryotic cells. The manydegradative enzymes within lysosomes function optimally inan acidic environment, whereas their action is inhibited inthe near neutral environment of the cytoplasm. This illustrates that maintenance of a specific pH is imperative forproper functioning of some cellular structures. On the otherhand, dramatic shifts in cellular pH may play an importantrole in controlling cellular activity.
For example, the pH ofthe cytoplasm of an unfertilized sea urchin egg is 6.6. Within1 minute of fertilization, however, the pH rises to 7.2; that is,the H concentration decreases to about one-fourth its original value, a change that is necessary for subsequent growthand division of the egg.The equilibrium constant for this reaction, denoted Ka (subscript a for “acid”), is defined as Ka [H][A]/[HA]. Taking the logarithm of both sides and rearranging the resultyields a very useful relation between the equilibrium constantand pH:pH pKa log[A][HA](2-5)where pKa equals –log Ka.From this expression, commonly known as the HendersonHasselbalch equation, it can be seen that the pKa of any acidis equal to the pH at which half the molecules are dissociatedand half are neutral (undissociated).
This is because when pKa pH, then log ([A]/[HA]) 0, and therefore [A] [HA].The Henderson-Hasselbalch equation allows us to calculate thedegree of dissociation of an acid if both the pH of the solutionand the pKa of the acid are known. Experimentally, by measuring the [A] and [HA] as a function of the solution’s pH, onecan calculate the pKa of the acid and thus the equilibriumconstant Ka for the dissociation reaction.Hydrogen Ions Are Releasedby Acids and Taken Up by BasesBuffers Maintain the pH of Intracellularand Extracellular FluidsIn general, an acid is any molecule, ion, or chemical groupthat tends to release a hydrogen ion (H), such as hydrochloric acid (HCl) and the carboxyl group (OCOOH),which tends to dissociate to form the negatively charged carboxylate ion (OCOO).
Likewise, a base is any molecule,ion, or chemical group that readily combines with a H,such as the hydroxyl ion (OH), ammonia (NH3), whichforms an ammonium ion (NH4), and the amino group(ONH2).When acid is added to an aqueous solution, the [H] increases (the pH goes down). Conversely, when a base isadded to a solution, the [H] decreases (the pH goes up). Because [H][OH] 1014M2, any increase in [H] is coupled with a decrease in [OH], and vice versa.Many biological molecules contain both acidic and basicgroups.
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