Life in the Open Ocean. Joseph J. Torres

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Life in the Open Ocean - Joseph J. Torres


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volume increases during the course of the reaction, pressure will inhibit it.

      3 If volume decreases, pressure will enhance it.

      If reaction rates are expressed incorporating a pressure term, the equation looks like this:

      (2.2)equation

      where P is the pressure (atm), T is the absolute temperature (°K), R is the universal gas constant (82 cm3 atm °K−1 mol−1), KD is the rate constant at 1 atm, KP is the rate constant at P atm, and ∆V is the activation volume of the reaction; the change in system volume occurring during the rate‐limiting step of the reaction, in this case, the activation step.

      What is the source of changes in volume in enzymatic reactions? First, consider the total reaction system as the water in which the reactions take place, the enzyme itself, and its substrate or ligand. Those three elements of the system are water–protein–ligand (Hochachka and Somero 1984). Pressure will affect a reaction if there is a volume change in any element of the reaction system during the transition from reactants to products. Though intuitively one might expect that changes in the volume of the enzyme protein during the course of the reaction or production of a higher or lower volume product would be the main source of volume change, most volume changes are actually due to changes in the structure of water. The side‐chains of the enzyme protein’s amino acids are surrounded by a layer of highly organized water, and this water has a higher density (smaller volume) than the bulk water of the system. Enzymatic reactions by their nature alter the organization of water around the molecule.

      The alteration can take several forms (Hochachka and Somero 1984). During ligand binding, the highly organized water may be squeezed out into the bulk phase of the system as the two molecules come together, increasing the total volume of the reaction system. Similarly, when two subunits of an enzyme protein come together to form an aggregate, water can be squeezed out into the bulk phase, increasing system volume. Changes in conformation of the enzyme protein during the reaction can also result in volume change. In one case, exposure to the aqueous medium of hydrophobic residues normally packed within the molecule would increase system volume and would respond negatively to increased pressure. In the opposite case, a normally buried hydrophilic amino acid side‐chain exposed to the aqueous medium would allow water to become more densely organized around it, resulting in a decline in volume relative to the bulk water and exhibiting a positive response to increased pressure.

equation Schematic illustration of the effect of hydrostatic pressure on the apparent Michaelis constant (Km) NADH (upper) and pyruvate (lower) for M4-LDH's of four deep-living and three shallow-living species of marine teleost fishes.

      Source: Siebenaller and Somero (1979), figure 1 (p. 297) Reproduced with the permission of Springer.

Schematic illustration of effects of pressure on gill Na+, K+-ATPase activities in fishes from different habitats.

      Source: Gibbs (1997), figure 5 (p. 255). Reproduced with the permission of Academic Press.

      Source: Reprinted by permission from Springer Nature Customer Service Centre GmbH, Nature, Inefficient lactate dehydrogenases of deep‐sea fish, Somero and Siebenaller (1979), table 1 (p. 101).

Species (depth, body temp., common name) H (cal mol−1) S (cal mol−1 K−1) G (cal mol−1) Relative velocity
Pagothenia borchgrevinki (surface, −2 °C, ice fish) 10 467 −12.7 14 000 1.00
Sebastolobus alascanus (180–440 m, 4–12 °C, rock fish) 10 515 −12.6 14 009 0.98
Coryphaenoides acrolepis (1460–1840 m, 2–10 °C, rattail fish) 11 813 −8.7 14
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