Group-transfer reactions active carriers

Group-transfer reactions play a variety of roles. Reaction 5 is representative of such a reaction. A phosphoryl group is transferred from the activated phosphoryl-group carrier, ATP, to glucose. This reaction traps glucose in the cell so that further catabolism can take place. 

These carbanions can be formed (Figure 5.8) by proton abstraction from ketones resulting in aldol condensations, by proton abstraction from acetyl CoA, leading to Claisen ester condensation, and by decarboxylation of p-keto acids leading to a resonance-stabilised enolate, which can likewise add to an electrophilic centre. It should be noted that the reverse of decarboxylation also leads to formation of a carbon—carbon bond (this is again a group transfer reaction involving biotin as the carrier of the activated CO2 to be transferred). 

While peptide antibiotics are synthesized according to enzyme-controlled polymerization patterns, both proteins and nucleic acids are made by template mechanisms. Tire sequence of their monomer emits is determined by genetically encoded information. A key reaction in the formation of proteins is the transfer of activated aminoacyl groups to molecules of tRNA (Eq. 17-36). Tire tRNAs act as carriers or adapters as explained in detail in Chapter 29. Each aminoacyl-tRNA synthetase must recognize the correct tRNA and attach the correct amino acid to it. The tRNA then carries the activated amino acid to a ribosome, where it is placed, at the correct moment, in the active site. Peptidyltransferase, using a transacylation reaction, in an insertion mechanism transfers the C terminus of the growing peptide chain onto the amino group of... 

Pantothenic acid has a central role in energy-yielding metabolism as the functional moiety of coenzyme A (CoA), in the biosynthesis of fatty acids as the prosthetic group of acyl carrier protein, and through its role in CoA in the mitochondrial elongation of fatty acids the biosynthesis of steroids, porphyrins, and acetylcholine and other acyl transfer reactions, including postsynthetic acylation of proteins. Perhaps 4% of all known enzymes utilize CoA derivatives. CoA is also bound by disulfide links to protein cysteine residues in sporulating bacteria, where it may be involved with heat resistance of the spores, and in mitochondrial proteins, where it seems to be involved in the assembly of active cytochrome c oxidase and ATP synthetase complexes. 

We have seen that phosphoryl transfer can be used to drive otherwise endergonic reactions, alter the energy of conformation of a protein, or serve as a signal to alter the activity of a protein. The phosphoryl-group donor in all of these reactions is ATP. In other words, ATP is an activated carrier of phosphoryl groups because phosphoryl Pansfer from ATP is an exergonic process. The use of activated carriers is a recurring motif in biochemistry, and we will consider several such carriers here. 

Metabolism is characterized by common motifs. A small number of activated carriers, such as ATP, NADH, and acetyl CoA, are used in many metabolic pathways. NADPH, which carries two electrons at a high potential, provides reducing power in the biosynthesis of cell components from more-oxidized precursors. ATP and NADPH are continually generated and consumed. Most transfers of activated groups in metabolism are mediated by a recurring set of carriers. Moreover, key reaction types are used repeatedly in metabolic pathways. 

Malonate decarboxylase (EC 4.1.1.88) and citrate lyase (EC 4.1.3.6) are both large enzyme complexes that consist of multiple subunits, the smallest of which acts as an ACP. These two complexes catalyze the decarboxylation of malonate to acetate and CO2 and the Mg -dependent cleavage of citrate to acetate and oxaloacetate, respectively. Both have been shown to require a thiol-containing prosthetic group for activity. However, unlike the carrier proteins described in the previous section, the ACP subunits of these proteins are not phosphopantetheinylated by a reaction with CoA. Instead, they rely on a unique cofactor, 2 -(5"-triphosphoribosyl)-3 -dephospho-CoA (24, dePCoA-RibPPP), as source of a 2 -(5"-phosphoribosyl)-3 -dephospho-CoA prosthetic group, which is bound to a conserved serine residue of the ACP. ° ° A similar prosthetic group has been identified in citramalate lyase (EC 4.1.3.22). The proposed biosynthesis and subsequent transfer reactions of the cofactor 24 to the ACPs of these complexes are shown in Scheme 5. 

Carnitine serves as the carrier that transports activated long chain fatty acyl groups across the inner mitochondrial membrane (Fig. 23.4). Carnitine acyl transferases are able to reversibly transfer an activated fatty acyl group from CoA to the hydroxyl group of carnitine to form an acylcamitine ester. The reaction is reversible, so that the fatty acyl CoA derivative can be regenerated from the carnitine ester. 

Determination of ALT ALT (formerly glutamate pyruvate transaminase) catalyzes the equilibrium transfer reaction of the amino group from L-alanine to 2-oxoglutarate to form L-glutamate and pyruvate ALT requires pyridoxal phosphate as coenzyme, which acts as an amino carrier. It is found in the main organs, such as the liver, kidney, and heart. The ALT activity in serum is elevated in diseases of the liver. 

Fig. 9.1. Schematic representation of the ubiquitin-proteasome pathway. Ubiquitin moiecuies are activated by an El enzyme (shown green at 1 /3 scaie) in an ATP-dependent reaction, transferred to a cysteine residue (yeiiow) on an E2 or Ub carrier protein and subsequentiy attached to amino groups...

Some enzymes are nonfunctional until posttranslationally modified. Examples of these enzymes include the acyl- and carboxyltransferases. While lipoate and phosphopantetheine are necessary for acyl transfer chemistry, tethered biotin is used in carboxyl transfer chemistry. Biotin and lipoate tethering occur under a similar mechanism the natural small molecule is activated with ATP to form biotinyl-AMP or lipoyl-AMP (Scheme 20). A lysine from the target protein then attacks the activated acid and transfers the group to the protein. The phosphopantetheine moiety is transferred using its own enzyme, the phosphopantetheinyltrans-ferase (PPTase). The PPTase uses a nucleophilic hydroxy-containing amino acid, serine, to attach the phosphopantetheinyl (Ppant) arm found in coenzyme A to convert the apo (inactive) carrier protein to its holo (active) form. The reaction is Mg -dependent. 

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