External aldimine

The ct-amino group of the substrate SAM replaces that of active site lysine as the Schiff base partner of the cofactor (external aldimine. Scheme 2(b)) and the C-a proton of SAM is next abstracted by the e-NH2 function of active site lysine to form a quinoid intermediate (Scheme 2(c)). 

ACC external aldimine undergoes transaldimination to release ACC and complete the catalytic sequence (Scheme 2(e)), according to McCarthy et... 

Covalent adduct of ACS with the external aldimine of L-vinylglycine with PLP... 

LeMagueres, R Im, H. Dvorak, A. Strych, U. Benedik, M. Krause, K. L. Crystal Structure at 1.45 A Resolution of Alanine Racemase from a Pathogenic Bacterium, Pseudomonas aeruginosa, Contains Both Internal and External Aldimine Forms. Biochemistry 2003, 42, 14752-14761. 

D). The coenzyme is shown as the internal aldimine with Lys 258 (see Fig. 14-6,14-10). The positive and negative contours on the two sides of the coenzyme ring indicate that the coenzyme tilts over to form the external aldimine when substrates react.413 (D) Superimposed structure of the active site of the enzyme in its free form as in (A) (bold lines) and the refined structure of the a-methylaspartate complex, (dashed lines).411 This illustrates the tilting of the coenzyme ring, which is also shown in Eq. 14-39 and Fig. 14-10. Courtesy of Arthur Amone and Sangkee Rhee. 

Beta-chloroalanine and serine O-sulfate can undergo (3 elimination (as in Eq. 14-29) in active sites of glutamate decarboxylase or aspartate aminotransferase. The enzymes then form free aminoacrylate, a reactive molecule that can undergo an aldol-type condensation with the external aldimine to give the following product.1... 

Before discussing the reactions of Schiff bases of PLP we should consider one fact that was not known in 1952. PLP is bound into an enzyme s active site as a Schiff base with a specific lysine side chain before a substate binds. This is often called the internal aldimine. When the substrate binds it reacts with the internal Schiff base by a two-step process called transimination (Eq. 14-26) to form the substrate Schiff base, which is also called the external aldimine. 

Notice that each step in the overall sequence changes the electronic or steric characteristics of the complex in a way that facilitates the next step.246 This is an important principle that is applicable throughout enzymology For an enzyme to be an efficient catalyst each step must lead to a change that sets the stage for the next. These consecutive steps often require proton transfers, and each such transfer will influence the subsequent step in the sequence. Some steps also require alterations in the conformation of substrate, coenzyme, and enzyme. One of these is the transimination sequence (Eqs. 14-26,14-39). On the basis of the observed loss of circular dichroism in the external aldimine, Ivanov and Karpeisky suggested that a... 

Fig. 5.3 Spatial orientation of substrate binding site of AspAT. A substrate, Asp, which forms an external aldimine complex with PLP interacts with Arg-292 and Arg-386 through ionic interactions.

The -amino group of lysine-87 forms external aldimine E in Fig. 7.6 and is released after L-serine binds to form ES I in Fig. 7.6 Lysine-87 is ideally situated to facilitate catalysis by removing the a-proton of L-serine.7 We find that a mutant form of the a2Pi complex in which the p subunit lysine-87 is replaced by threonine (K87T) is inactive but binds L-serine and L-tryptophan slowly and extremely tightly.92-97 These results provide evidence that... 

The value of reduced LD in the quinonoid band of the enzyme complexes with L-alanine and oxindolyl-L-alanine proved to be close to that in the absorption band of the external aldimine formed with L-threonine (Table 9.2). It does not follow from this coincidence that the external aldimine and quinonoid have a similar orientation in the active site. The two intermediates, in fact, differ in their orientation this is because the directions of the transition dipole moment in the pyridine ring of the aldimine and quinonoid are very different.72 ... 

The a-proton of the external aldimine is then abstracted by a basic residue Bi and the quinonoid intermediate is formed (Fig. 9.13, III). The rate constants of formation of the quinonoid intermediate from L-tryptophan and S-benzyl-L-cysteine are estimated be 940 and 47.6 sec-1, respectively this demonstrates a drastic effect of the leaving group on labilization of the a-proton.78 ... 

The a -amino group of the amino acid substrate displaces the e-amino group of the active-site lysine residue. In other words, an internal aldimine becomes an external aldimine. The amino acid-PLP Schiff base that is formed remains tightly bound to the enzyme by multiple noncovalent interactions. 

The Schiff base between the amino acid substrate and PLP, the external aldimine, loses a proton from the a-carbon atom of the amino acid to form a quinonoid intermediate (Figure 23.10). 

Figure 23.10. Transamination Mechanism. The external aldimine loses a proton to form a quinonoid intermediate. Reprotonation of this intermediate at the aldehyde carbon atom yields a ketimine. This intermediate is hydrolyzed to generate the a-ketoacid product and pyridoxamine phosphate.

An essential step in the transamination reaction is the protonation of the quinonoid intermediate to form the external aldimine. The chirality of the amino acid formed is determined by the direction from which this proton is added to the quinonoid form. (Figure 24.10). This protonation step determines the 1 configuration of the amino acids produced. The interaction between the conserved arginine residue and the a-carboxylate group helps orient the substrate so that, when the lysine residue transfers a proton to the face of the quinonoid intermediate, it generates an aldimine with an 1 configuration at the C center. 

Figure 24.9. Amino Acid Biosynthesis by Transamination. Within a transaminase, the internal aldimine is converted into pyridoxamine phosphate (PMP) by reaction with glutamate. PMP then reacts with an a-ketoacid to generate a ketimine. This intermediate is converted into a quinonoid intermediate, which in turn yields an external aldimine. The aldimine is cleaved to release the newly formed amino acid to complete the cycle.

An external aldimine forms with SAM, which is deprotonated to form the quinonoid intermediate. The deprotonated carbon atom attacks the carbon atom adjacent to the sulfur atom to form the cyclopropane ring and release methyl thio-adenosine, the other product. 

---