Fragmentary enzyme

Fig. 8.4 Schematic representation of the wild-type and fragmentary alanine racemase. Only several of the relevant amino acid residues in the terminal portions are shown. The C-terminus of the N-terminal part of the fragmentary enzyme contains five extra amino acids residues. (Reproduced with permission from Toyama etal., J. Biol. Chem., 266, 13637 (1991)).

Although the overall conformation of the fragmentary enzyme appears to be indentical with that of the wild-type enzyme, it has only 40% of the activity of the native enzyme. The... 

The relative activity of the fragmentary enzyme to the wild-type enzyme was much higher than that of the clipped to the native DadB enzyme. This was explained by the assumption that the thermostable enzyme has more extensive hydrophobic interdomain interactions than the DadB enzyme with less thermostability.12 The importance of hydrophobic interdomain interactions for catalysis was pointed out by studies on lactate dehydrogenase.23,24 ... 

Other groups that can be attached covalently to enzymes include fatty acids, isoprenoid alcohols such as far-nesol, and carbohydrates. Although such modifications are widespread, our understanding of how cells use them to regulate enzymatic activities is still fragmentary. 

The flavoenzyme D-lactate dehydrogenase from yeast has been reported to contain zinc (134). An apoenzyme can be prepared and reactivated by Zn2+ or Co2+ (135). When yeast is grown in the presence of added Co2+, a Co(II) enzyme is synthesized. The biosynthetic Co(II) enzyme was found to have different catalytic properties compared to the enzyme reactivated from the apoenzyme (136). Only rather fragmentary data have been published on this subject, and the differences in cobalt binding obtained by the two methods of preparation are unknown. 

Direct effects of HS may be reflected in changes in photosynthesis and ATP formation in treated plants. In the case of photosynthesis, our information is still fragmentary and not very recent. Ferretti et al. (1991) showed that HS, applied to the culture medium, increased the activities of the enzymes involved in the photosynthetic sulfate reduction pathway, whereas Merlo et al. (1991) observed in maize leaves a decrease in starch content accompanied by an increase in soluble sugars. These positive biological effects appeared to be mediated by changes in the activity... 

A huge amount of data, often still fragmentary, on allosteric regulation of all different types of ribonucleotide reductases awaits unambiguous analyses as the ones described. It is certainly not overstating to say that, next to the chemistry of ribonucleotide reduction, the enzymes interaction with nucleotides and the physiologic consequences is one of the most peculiar pieces of biochemistry. 

The inducible enzymes are listed in Table II. The data are fragmentary but already indicate that several component enzymes are induced in four major systems the heme, microsomal oxygenase, gluconeogenic enzymes, and the direct glucose oxidation systems. The inducible systems show several similarities. (1) The inducible enzymes are not present in the neonate they make their appearance only at birth or a short time after. (2) They interact with one another. For example,... 

The data on the biosynthetic chain of chlorophyll are fragmentary, and there is no evidence that any of the reactions in the chain are reversible. At present the formulation of this biosynthetic chain rests mainly on the first assumption, namely, that biosynthesis proceeds in a stepwise maimer and that a number of known compounds may be assembled into a scheme that is compatible with our knowledge of organic and biological chemistry. Recent work with enzymes has provided added support for portions of this scheme. 

The alignment in Table I includes sequences from 12 different zymogens and enzymes, only two of which are known completely and a third is almost completed. Despite the fragmentary data, we are able to deduct some general structure-function relationships of the aspartate proteinases and of the gastric zymogens. 

---