B

Hydrophobic interactions

van der Waals interactions

Any two atoms in close proximity the alignment of the hydrogen-bonded atoms, but they are always significantly weaker than covalent bonds. In aqueous solvent at 25 °C, the available thermal energy can be of the same order of magnitude as the strength of these weak interactions, and the interaction between solute and solvent (water) molecules is nearly as favorable as solute-solute interactions. Consequently, hydrogen bonds and ionic, hydrophobic, and van der Waals interactions are continually formed and broken.

Although these four types of interactions are individually weak relative to covalent bonds, the cumulative effect of many such interactions can be very significant. For example, the noncovalent binding of an enzyme to its substrate may involve several hydrogen bonds and one or more ionic interactions, as well as hydrophobic and van der Waals interactions. The formation of each of these weak bonds contributes to a net decrease in the free energy of the system. We can calculate the stability of a noncovalent interaction, such as that of a small molecule hydrogen-bonded to its macromolecular partner, from the binding energy. Stability, as measured by the equilibrium constant (see below) of the binding reaction, varies exponentially with binding energy. The dissociation of two biomolecules (such as an enzyme and its bound substrate) associated noncovalently through multiple weak interactions requires all these interactions to be disrupted at the same time. Because the interactions fluctuate randomly, such simultaneous disruptions are very unlikely. The molecular stability bestowed by 5 or 20 weak interactions is therefore much greater than would be expected intuitively from a simple summation of small binding energies.

Macromolecules such as proteins, DNA, and RNA contain so many sites of potential hydrogen bonding or ionic, van der Waals, or hydrophobic interactions that the cumulative effect of the many small binding forces can be enormous. For macromolecules, the most stable (that is, the native) structure is usually that in which weak-bonding possibilities are maximized. The folding of a single polypeptide or polynucleotide chain into its three-dimensional shape is determined by this principle. The binding of an antigen to a specific antibody depends on the cumulative effects of many weak interactions. As noted earlier, the energy released when an enzyme binds noncovalently to its substrate is the main source of the enzyme's catalytic power. The binding of a hormone or a neurotransmitter to its cellular receptor protein is the result of weak interactions. One consequence of the large size of enzymes and receptors is that their extensive surfaces provide many opportunities for weak interactions. At the molecular level, the complementarity between interacting biomolecules reflects the complementarity and weak interactions between polar, charged, and hydrophobic groups on the surfaces of the molecules.

When the structure of a protein such as hemoglobin (Fig. 2-9) is determined by x-ray crystallography (see

FIGURE 2-9 Water binding in hemoglobin. The crystal structure of hemoglobin, shown (a) with bound water molecules (red spheres) and (b) without the water molecules. These water molecules are so firmly bound to the protein that they affect the x-ray diffraction pattern as though they were fixed parts of the crystal. The gray structures with red and orange atoms are the four hemes of hemoglobin, discussed in detail in Chapter 5.

FIGURE 2-9 Water binding in hemoglobin. The crystal structure of hemoglobin, shown (a) with bound water molecules (red spheres) and (b) without the water molecules. These water molecules are so firmly bound to the protein that they affect the x-ray diffraction pattern as though they were fixed parts of the crystal. The gray structures with red and orange atoms are the four hemes of hemoglobin, discussed in detail in Chapter 5.

Box 4-4, p. XX), water molecules are often found to be bound so tightly as to be part of the crystal structure; the same is true for water in crystals of RNA or DNA. These bound water molecules, which can also be detected in aqueous solutions by nuclear magnetic resonance, have distinctly different properties from those of the "bulk" water of the solvent. They are, for example, not osmotically active (see below). For many proteins, tightly bound water molecules are essential to their function. In a reaction central to the process of photosynthesis, for example, light drives protons across a biological membrane as electrons flow through a series of electron-carrying proteins (see Fig. 19-XX). One of these proteins, cytochrome f, has a chain of five bound water molecules (Fig. 2-10) that may provide a path for protons to move through the membrane by a process known as "proton hopping" (described below). Another such light-driven proton pump, bacteriorhodopsin, almost certainly uses a chain of precisely oriented bound water molecules in the transmembrane movement of protons (see Fig. 19-XX).

FIGURE 2-10 Water chain in cytochrome /.Water is bound in a proton channel of the membrane protein cytochrome f which is part of the energy-trapping machinery of photosynthesis in chloroplasts (see Fig. 19-XX). Five water molecules are hydrogen-bonded to each other and to functional groups of the protein, which include the side chains of valine, proline, arginine, alanine, two asparagine, and two gluta-mine residues. The protein has a bound heme (see Fig. 5-1), its iron ion facilitating electron flow during photosynthesis. Electron flow is coupled to the movement of protons across the membrane, which probably involves "electron hopping" (see Fig. 2-14) through this chain of bound water molecules.

FIGURE 2-10 Water chain in cytochrome /.Water is bound in a proton channel of the membrane protein cytochrome f which is part of the energy-trapping machinery of photosynthesis in chloroplasts (see Fig. 19-XX). Five water molecules are hydrogen-bonded to each other and to functional groups of the protein, which include the side chains of valine, proline, arginine, alanine, two asparagine, and two gluta-mine residues. The protein has a bound heme (see Fig. 5-1), its iron ion facilitating electron flow during photosynthesis. Electron flow is coupled to the movement of protons across the membrane, which probably involves "electron hopping" (see Fig. 2-14) through this chain of bound water molecules.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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