cycles

1.urea cycle

2.citric acid cycle                                                                                                                                          

3.ketogenesis                                                                                                                                        

       Ketogenesis is the process by which ketone bodies are produced as a result of fatty acid breakdown.                                                                                                                          

 4.electron transport chain

5.biosynthesis of cholesterol                                                                                                                 

       Slightly less than half of the cholesterol in the body derives from biosynthesis de novo. Biosynthesis in the liver accounts for approximately 10%, and in the intestines approximately 15%, of the amount produced each day. Cholesterol synthesis occurs in the cytoplasm and microsomes (ER) from the two-carbon acetate group of acetyl-CoA.

The acetyl-CoA utilized for cholesterol biosynthesis is derived from an oxidation reaction (e.g., fatty acids or pyruvate) in the mitochondria and is transported to the cytoplasm by the same process as that described for fatty acid synthesis (see the Figure below). Acetyl-CoA can also be synthesized from cytosolic acetate derived from cytoplasmic oxidation of ethanolwhich is initiated by cytoplasmic alcohol dehydrogenase (ADH3). All the reduction reactions of cholesterol biosynthesis use NADPH as a cofactor. The isoprenoid intermediates of cholesterol biosynthesis can be diverted to other synthesis reactions, such as those for dolichol (used in the synthesis of N-linked glycoproteins, coenzyme Q (of the oxidative phosphorylation pathway) or the side chain of heme-a. Additionally, these intermediates are used in the lipid modification of some proteins  

                                                                                                

The process of cholesterol synthesis has five major steps:

1. Acetyl-CoAs are converted to 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA)

2. HMG-CoA is converted to mevalonate

3. Mevalonate is converted to the isoprene based molecule, isopentenyl pyrophosphate (IPP), with the concomitant loss of CO₂

4. IPP is converted to squalene

5. Squalene is converted to cholesterol.