Phosphoryl group, transfer

FIGURE 3.8 The activation energies for phosphoryl group-transfer reactions (200 to 400 kj/mol) are substantially larger than the free energy of hydrolysis of ATP ( — 30.5 kj/mol). 

A different mechanism operates in the wheat germ enzyme. 2,3-Bisphosphoglycerate is not a cofactor. Instead, the enzyme carries out intra-molecular phosphoryl group transfer (Figure 19.25). The C-3 phosphate is transferred to an active-site residue and then to the C-2 position of the original substrate molecule to form the product, 2-phosphoglycerate. 

Fig. 9. Transfer of mannitol bound to inside-out vesicles to the cytoplasmic volume. (A) Unphos-phorylated ll ". (B) Phosphorylated 11 . It is assumed that the phosphoryl group transfer from the enzyme to the sugar can only take place when the sugar is bound the cytoplasmic-facing binding site, E-Pcyt Mtl (see also Fig. 6).

The phosphoryl group transfer from the phosphoryl group donor to the sugar may proceed through multiple steps. For instance, in the case of 11 there is the transfer from P-HPr to domain A, then an internal transfer from domain A to domain B and finally to mannitol. 

Full details on the phosphorylation of water and alcohols by 4-nitrophenyl dihydrogen phosphate and the NfC H ) - and N(CH3) -salts of its mono- and dianion have been published 146>. Phosphoryl group transfer from the monoanion and dianion is thought to proceed via the monomeric POf ion. Addition of the sterically unhindered amine quinuclidine to an acetonitrile solution containing the phosphate monoanion and tert-butanol produces t-butyl phosphate at a faster rate than does the addition of the more hindered diisopropylethylamine. This nucleophilic catalysis of the phosphorylation reaction is also explained by the intermediacy of the POf ion. 

Phosphoryl group transfer reactions add or remove phosphoryl groups to or from cellular metabolites and macromolecules, and play a major role in biochemistry. Phosphoryl transfer is the most common enzymatic function coded by the yeast genome and, in addition to its importance in intermediary metabolism (see Chapter 5), the reaction is catalysed by a large number of central regulatory enzymes that are often part of signalling cascades, such as protein kinases, protein phosphatases, ATPases and GTPases. 

Figure 10.7 (a) In HAD enzymes Asp mediates phosphoryl group transfer to a variety of acceptors, including H20 (b) the catalytic scaffold around the essential Mg2+ion. (From Allen and Dunaway-Mariano, 2004. Copyright 2004, with permission from Elsevier.)... 

Allen, K.N. and Dunaway-Mariano, D. (2004) Phosphoryl group transfer evolution of a catalytic scaffold, TIBS, 29, 495-503. 

Hess, J.F. Bourret, R.B. Simon, M.L Histidine phosphorylation and phosphoryl group transfer in bacterial chemotaxis. Nature, 336, 139-143 (1988)... 

Although this reaction is fully reversible, the relatively high [ATP]/[ADP] ratio in cells normally drives the reaction to the right, with the net formation of NTPs and dNTPs. The enzyme actually catalyzes a two-step phosphoryl transfer, which is a classic case of a double-displacement (Ping-Pong) mechanism (Fig. 13-12 see also Fig. 6-13b). First, phosphoryl group transfer from ATP to an active-site His residue produces a phosphoenzyme... 

Phosphoryl group transfers from ATP result in an accumulation of ADP for example, when muscle is contracting vigorously, ADP accumulates and interferes with ATP-dependent contraction. During periods of intense demand for ATP, the cell lowers the ADP concentration, and at the same time acquires ATP, by the action of adenylate kinase ... 

The outcome of these coupled reactions, both reversible under cellular conditions, is that the energy released on oxidation of an aldehyde to a carboxylate group is conserved by the coupled formation of ATP from ADP and Pj. The formation of ATP by phosphoryl group transfer from a substrate such as 1,3-bisphosphoglycerate is referred to as a substrate-level phosphorylation, to distinguish this mechanism from respiration-linked phosphorylation. Substrate-level phosphorylations involve soluble enzymes and chemical intermediates (1,3-bisphosphoglycerate in this case). Respiration-linked phosphorylations, on the other hand, involve membrane -bound enzymes and transmembrane gradients of protons (Chapter 19). 

In the second glycolytic reaction that generates a compound with high phosphoryl group transfer potential, enolase promotes reversible removal of a molecule of water from 2-phosphoglycerate to yield phosphoenolpyruvate (PEP) ... 

---