To the carboxyl end. The acetyl CoA that started the process is thus farthest from the carboxyl group of the finished fatty acid. Fatty acids are released from adipose by hydrolysis of their stored form, triacylglycerol. After release from adipocytes, unesterified fatty acids are transported in the blood bound to serum albumin to tissues such as liver, heart and muscle, where they are taken up and oxidized. The reaction normally occurs in the endoplasmic reticulum or the outer mitochondrial membrane.
Different enzymes are specific for fatty acids of different chain length.
Subsequent hydrolysis of the PP i by a pyrophosphatase draws the activation to completion. Fatty acyl carnitine is then trransported by an antiport in exchange for free carnitine to the inner surface of the inner mitochondrial membrane. This shuttle mechanism is required only for longer chain fatty acids. They cross the mitochondrial membranes, and are activated in the mitochondrion. The two processes are superficially the reverse of one another.