Proteins, aromatic compounds bound

Induced Circular Dichroism of Aromatic Compounds Bound to Proteins... 

Evaporation and drying leads to more extensive reactions than in UHT milk that include degradation of proteins and thiamine, Maillard reaction between proteins (preferably in bound lysine) and lactose, degradation of 0x0- and hydroxycarboxyhc acids and oxidation of fatty acids. PhosphoKpids of fat particles are particularly susceptible to oxidation. It is therefore mainly alkane-2-ones, lactones and the Maillard reaction products that are responsible for the odour of condensed and powdered mflk. In addition to these compounds, typical aromatic substances are benzalde-hyde, acetophenone, 2-aminoacetophenone, furfiiryl alcohol and benzothi azole. 

An alternative method for removing Zn from the active site of zinc proteins is to use aromatic nitroso compounds such as 3-nitrosoben-zamide and 6-nitroso-l,2-benzopyrone (382). These agents can oxidize Zn-bound cysteine S and can inhibit HIV-1 infection in human lymphocytes. They also eject zinc from isolated HIV-1 nucleocapsid zinc-fingers and from intact HIV-1 virions. 

In addition to the specificity-determining P, aromatic side chain, the amide groups of this substrate can form specific hydrogen bonds to the protein (Fig. 12-10). These hydrogen bonds presumably help the enzyme to recognize the compound, which is bound with Km of 0.03 M and is hydrolyzed (with liberation of NH3)288 289 with /c at -0.17 s"1. 

Fig. 20. Schematic view of the antihyperlipoproteinemic compound bezafibrate bound to deoxyhemoglobin A in the central cavity of the tetramer. The protein-ligand interactions include an amino-aromatic interaction involving asparagine-108 and an aromatic-aromatic interaction involving tryptophan-37, . Reproduced with permission from Perutz et al. (1986).

In view of these results, two observations may be made (a) as a corollary to studies of protein ultraviolet spectra in any particular nonaqueous solvent, the spectral properties of relevant simple compounds in that solvent must be investigated and (5) any changes in protein spectra produced as a result of modification of the native protein conformation in a particular nonaqueous solvent must be superimposed on changes resulting simply from the replacement of the aqueous environment by the nonaqueous one of generally different polarizability and refractive index. In the extreme case, for example, it may make little or no difference spectrally whether the aromatic chromophores remain internally bound within the protein molecule, or whether they become exposed to the solvent, and hence no useful information about protein conformations can be expected. More studies have to be made to clarify to what extent spectral changes can be useful in the investigation of proteins in nonaqueous solvents. 

---