DESIGN, SYNTHESIS, AND PROPERTIES OF A NOVEL CYTOCHROME-B MODEL

The modular strategy of a template-assembled synthetic protein (TASP) was used for the de novo synthesis of a 122-residue, antiparallel four -helix bundle protein which accommodates two bis-histidine ligated hem e groups. The cyclic decapeptide template contains four cysteine resid ues with different protecting groups which allow coupling of the unpro tected helices carrying bromoacetyl units either at the N-terminus or the epsilon-amino group of a C-terminal lysine residue. The amphiphili c helices realize a water-soluble model of the cytochrome b core with tno parallel heme-binding helices alternating with two antiparallel he lices shielding the two hydrophobic heme binding sites. The spectral p roperties resemble those of the natural protein. Characterization by m ass spectrometry and circular dichroism support the anticipated struct ure. The free energy of folding shows a stabilizing effect by the two heme groups which have respective redox midpoint potentials of -106 an d -170 mV. This modular protein combines the advantage of the structur al organization of a TASP with the incorporation of functional groups.