Lysine amino groups

Fructose bisphosphate aldolase of animal muscle is a Class I aldolase, which forms a Schiff base or imme intermediate between the substrate (fructose-1,6-bisP or dihydroxyacetone-P) and a lysine amino group at the enzyme active site. The chemical evidence for this intermediate comes from studies with the aldolase and the reducing agent sodium borohydride, NaBH4. Incubation of fructose bisphosphate aldolase with dihydroxyacetone-P and NaBH4 inactivates the enzyme. Interestingly, no inactivation is observed if NaBH4 is added to the enzyme in the absence of substrate. 

Step 1 of Figure 29.14 Transimination The first step in transamination is trans-imination—the reaction of the PLP—enzyme imine with an a-amino acid to give a PLP—amino acid imine plus expelled enzyme as the leaving group. The reaction occurs by nucleophilic addition of the amino acid -NH2 group to the C=N bond of the PLP imine, much as an amine adds to the C=0 bond of a ketone or aldehyde in a nucleophilic addition reaction (Section 19.8). The pro-tonated diamine intermediate undergoes a proton transfer and expels the lysine amino group in the enzyme to complete the step. 

An interesting observation is that an enzyme may exhibit different pH activity profiles for various neutral substrates. The explanation of this is that the enzyme binds or transforms such various substrates differently. For example. Taka amylase has different pH optima for long chain amyloses and for low molecular mass substrates. Some specific chemical modifications of the side chains of the enzyme may also alter the pH activity profiles. Kobayashi, Miura and Ichisima (1992) modified the lysine amino groups using bifimctional reagent o-phtalaldehyde, and observed a pronounced shift in the pH-dependence of ohgomaltoside hydrolysis. 

For example, hexane 2,5-dione, a diketone metabolite of the solvent hexane, is reactive toward particular groups in proteins, the lysine amino groups, but is not especially chemically reactive in general terms (see chap. 7). 

Imidazole, lysine amino groups, and tyrosine hydroxyl groups react with diethylpyrocarbonate... 

It has been suggested that the modification of the protein s isoelectric point could result in an alteration of its pharmacokinetic profile. Avidin acylation was performed by lysine amino group derivatization with succinyl anhydride or other anhydrides, which allowed the isoelectric point to be shifted to more acidic values, depending on the level of modification. Indeed, the protein anionization induced a reduction of accumulation in the liver, but resulted only in a limited prolongation of residence time in the circulation [30, 31]. 

Thus modifying the lysine amino group in yeast proteins using maleic or citraconic anhydride altered the NA-protein interactions and facilitated separating proteins from NAs at pH 4.2. 

There are several enzymes that form a Schiff base between their substrates and either a lysine -amino group from the protein, the aldehyde electrophile of pyridoxal-5-phosphate or the keto group of a covalently bound pyruvyl moiety. The electron sink under all these conditions is either the imine or its protonated iminium form, that in many cases leads to the formation of an enamine by the variety of pathways outlined below. 

The KIEs for scission of the C—C bond to form the enamine (by measuring 45C02/44C02 ratios) of both the model amine and the acetoacetate decarboxylase catalyzed reactions were studied by O Leary and Baughn125. In the primary amine catalyzed reaction the KIE exhibited pH-dependence consistent with the kinetic data above. In the enzyme catalyzed reaction a pH independent k12/k13 KIE of 1.018 was reported, consistent with the idea that a nucleophilic attack by the lysine amino group and decarboxylation are both partially rate-limiting. 

Schmidt, D.E., and Westheim, F., 1971, Pk of lysine amino group at active site of acetoacetate decarboxylase. Biochemistry 10 1249-1253. 

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