Saturated acyl-enzyme formation causes
Freeing of PAN-SH site 
In fatty acid synthesis

After priming reactions (first two reactions)


Acyl - malonyl - enzyme is formed


Central thiol (4-phosphopantetheine -SH group or Pan - SH group) of ACP is covalently linked with malonyl group
Peripheral thiol (cysteine -SH group) of 3-ketoacyl synthase is covalently linked to the acetyl group
During elongation steps


The acetyl group attached to peripheral thiol (cysteine -SH) is transferred to the malonyl group bound to central thiol (Pan -SH group). Simultaneously 1 CO2 is liberated from malonyl group. Thus:-


Central thiol (Pan -SH) contains 4 carbon atoms residue
Peripheral thiol (cysteine -SH group) becomes free


After elongation steps
Saturated acyl-enzyme is formed
4 carbon unit (butyryl) is transferred from central thiol (Pan -SH) to peripheral thiol (cysteine -SH group). Thus:-


Central thiol (Pan -SH) becomes free to accept new malonyl for next cycle
Peripheral thiol (cysteine -SH group) contains 4 carbon unit
Freeing of PAN-SH site
Freeing of cyst-SH site
Both
None