ATP energy metabolism in a two-step pathway with gene-expression regulation. The lower part represents a simplified ATP/ADP energy metabolism process, in which ADP acquires energy from the supply process (s) (i.e. phosphorylation) and produce ATP; also ATP can release energy in the demand process (d) (i.e. hydrolysis) and be converted to ADP. The supply is catalyzed by enzyme E (or enzymes stemming from an operon). The upper part describes the synthesis of enzyme E in process a and its degradation in process b.