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 heme groups. The cyclic decapeptide template contains four cysteine residues with different protecting groups which allow coupling of the unprotected helices carrying bromoacetyl units either at the N-terminus or the epsilon-amino group of a C-terminal lysine residue. The amphiphilic helices realize a water-soluble model of the cytochrome b core with tno parallel heme-binding helices alternating with two antiparallel helices shielding the two hydrophobic heme binding sites. The spectral properties resemble those of the natural protein. Characterization by mass spectrometry and circular dichroism support the anticipated structure. The free energy of folding shows a stabilizing effect by the two heme groups which have respective redox midpoint potentials of -106 and -170 mV. This modular protein combines the advantage of the structural organization of a TASP with the incorporation of functional groups.