A biosynthetic reaction of exactly this type synthesizes the amino acid glutamine (Figure 2–35B). We will see shortly that similar (but more complex) mechanisms are also used to produce nearly all of the large molecules of the cell.
Figure 2–35 an example of an energetically unfavorable biosynthetic reaction driven by aTP hydrolysis. (a) schematic illustration of the formation of a–B in the condensation reaction described in the text. (B) The biosynthesis of the common amino acid glutamine from glutamic acid and ammonia. Glutamic acid is first converted to a high-energy phosphorylated intermediate (corresponding to the compound B–o–po3 described in the text), which then reacts with ammonia (corresponding to a–h) to form glutamine. in this example, both steps occur on the surface of the same enzyme, glutamine synthetase. The high-energy bonds are shaded red; here, as elsewhere throughout the book, the symbol pi = hpo42–, and a yellow “circled p” = po3 .
naDh and naDph are important electron Carriers
Other important activated carrier molecules participate in oxidation–reduction reactions and are commonly part of coupled reactions in cells. These activated carriers are specialized to carry electrons held at a high energy level (sometimes called “high-energy” electrons) and hydrogen atoms. The most important of these electron carriers are NAD+ (nicotinamide adenine dinucleotide) and the closely related molecule NADP+ (nicotinamide adenine dinucleotide phosphate). Each picks up a “packet of energy” corresponding to two electrons plus a proton (H+), and they are thereby converted to NADH (reduced nicotinamide adenine dinucleotide) and NADPH (reduced nicotinamide adenine dinucleotide phosphate), respectively (Figure 2–36). These molecules can therefore be regarded as carriers of hydride ions (the H+ plus two electrons, or H–).
Like ATP, NADPH is an activated carrier that participates in many important biosynthetic reactions that would otherwise be energetically unfavorable. The NADPH is produced according to the general scheme shown in Figure 2–36A. During a special set of energy-yielding catabolic reactions, two hydrogen atoms are removed from a substrate molecule. Both electrons but just one proton (that is, a hydride ion, H–) are added to the nicotinamide ring of NADP+ to form NADPH; the second proton (H+) is released into solution. This is a typical oxidation–reduction reaction, in which the substrate is oxidized and NADP+ is reduced.
NADPH readily gives up the hydride ion it carries in a subsequent oxidation–reduction reaction, because the nicotinamide ring can achieve a more stable arrangement of electrons without it. In this subsequent reaction, which
Figure 2–36 NaDPH, an important carrier of electrons.
HC (a) naDph is produced in reactions of the general type shown on the left, in which two hydrogen atoms are removed from a substrate.
The oxidized form of the carrier molecule, naDp+, receives one hydrogen atom plus an electron (a hydride ion); the proton (h+) from the other h atom is released into solution. Because naDph holds its hydride ion in a high-energy linkage, the hydride ion can easily be transferred to other molecules, as shown on the right. (B) and (C) The structures of naDp+ and naDph. The part of the naDp+ molecule known as the nicotinamide ring accepts the hydride ion, h–, forming naDph. The molecules naD+ and naDh are identical in structure to naDp+ and naDph, respectively, except that they lack the indicated phosphate group.