Metabolism of Lipids

Metabolism of lipids, also known as fat metabolism, is the process of synthesis, processing, and degradation of lipids. Lipids are biomolecules that are insoluble in water and can only be dissolved in organic solvents. Lipids, which are digested as foods, are assimilated into the body by this process. Metabolism of lipids occurs in union with carbohydrate metabolism, as glucose can be converted to fats. Metabolism of lipids plays a key role in maintaining lipids in our bodies. Disruptions in this process may cause various illnesses. Let us see in detail about lipid metabolism, classification of lipids, fat metabolism, stages in metabolism, fatty acid metabolism, and lipid metabolism Slideshare.

Classification of lipids:

Lipids are categorised into the following types:

1. Cholesterol and its derivatives
2. High Fatty Acid

• Saturated Fatty Acid
• Unsaturated Fatty Acid

1. Triacylglycerols
2. Phospholipids

• Sphingomyelins
• Phosphoglycerides
• Glycosphingolipids

• Cerebrosides
• Gangliosides
• Sulfatides

Stages in the metabolism of lipids:

Fat metabolism occurs in two important sites: the pancreas and the liver. Metabolism of lipids occurs in the following steps. They are

• Lipid digestion 

It is the first step involved in fat metabolism. The lipid which is ingested through food is broken down into simpler compounds for efficiency. The lipase enzyme plays a major role in lipid digestion. It decomposes complex triglycerides into simple monoglycerides. The lipids are broken down in the small intestine and remain intact from the mouth to the stomach.

• Lipid absorption 

This is the second step that facilitates the metabolism of lipids. The fats are absorbed by both the stomach and the intestine. Simple fatty acids are absorbed by the stomach, and some higher-order fatty acids are absorbed by the intestine. The monoglycerides diffuse through the membrane and rejoin as triglycerides in the cytosol. 

• Lipid transportation 

Lipoproteins are the transport proteins for lipids. Lipid molecules like monoglycerides, triglycerides, cholesterol, and membrane lipids are hydrophobic and require lipoproteins for transportation. Lipoproteins are both hydrophobic and hydrophilic in structure (i.e., amphipathic) and allow the transportation of fatty acids through the blood. The types of lipoproteins mostly vary based on density.  

• Lipid catabolism

Catabolism of lipids occurs in two stages. They are:

1. Triglycerides break down into fatty acids.
2. Acetyl-coenzyme A is produced from fatty acids. 

The steps involved in the catabolism of lipids are as follows:

1. Glycolysis: Glucose to Pyruvate
2. Beta-oxidation: Triglycerides are converted into three fatty acid molecules and a glycerol molecule. 
3. Activation step: Acyl-CoA synthetase to  fatty acyl-CoA
4. Oxidation step: Acyl-CoA to trans-∆²-Enoyl-CoA
5. Hydration step: trans-∆²-Enoyl-CoA to L-3-Hydroxyacyl-CoA
6. Second oxidation step: L-3-Hydroxyacyl-CoA to 3-Ketoacyl-CoA
7. Cleavage: 3-Ketoacyl-CoA to Acyl-CoA and Acetyl-CoA.

• Lipid biosynthesis

It is the production process of lipids. Lipids stored in the tissues and organs are released by the following biosynthesis process:

1. Membrane lipid biosynthesis
2. Triglyceride biosynthesis 
3. Fatty acid biosynthesis 
4. Cholesterol biosynthesis 

Fatty acid metabolism:

Fatty acid metabolism plays a major role in the production of energy, growth, and development. It is the process of the breakdown of fatty acids facilitated by a series of pathways. It occurs in two different processes. They are:

• Catabolic process – energy is generated through this process.
• Anabolic process – an energy-requiring process that creates new biomolecules. 

β-oxidation is one of the important steps that happen in fatty acid metabolism. The final product of this metabolism is Acyl-CoA.

Enzymes involved in fat metabolism:

The list of enzymes that play a crucial role in the metabolism of lipids are as follows:

• Lipases
• Phosphodiesterases
• Sphingosine kinases
• Phospholipase A2
• Cholesterol acyltransferase (ACAT)
• Fatty acid synthase
• carnitine palmitoyl transferase (CPT1)
• Phospholipase C
• Autotaxin
• Lipin
• Alk-Smase
• Adiponutrin
• Arylsulfatase A

About 40 enzymes are involved in processing the pathways of lipid metabolism. The absence of any one of these enzymes from the cycle will result in serious fat metabolism-related disorders. 

The Importance of Fat Metabolism:

Metabolism of lipids plays a major role in providing energy to the body, insulates the body, enhances organ functions, helps in energy conservation, cell membrane construction, synthesis of fats, protects organs of the body, regulates hormone secretion, provides energy for organ function, transports nutrients, maintains homeostasis, and performs many other vital functions. 

Lipid Metabolism Disorders:

Malfunctions in the metabolism of lipids can lead to some undesirable changes in the body that result in the following diseases:

• Cholesterol 
• Cardiovascular disease
• Diabetes
• Gaucher’s disease
• Tay-Sachs disease
• Niemann-Pick disease. 
• Atherosclerosis 
• Hypercholesterolemia
• Stroke
• Abetalipoproteinemia
• Liver disease
• Hypoglycemia 

Conclusion:

Lipid metabolism plays a key role in regulating body functions. Lipid is one of the important constituents of the human body which has to be degraded by a series of metabolism pathways to be utilised by our body. Recent research on lipid metabolism is working in such a way to target cancer cells using this pathway. Researchers believe that fat metabolism can be a key for cancer therapy, tissue engineering, and metabolomics studies. Likewise, many effective pieces of research are being conducted on the metabolism of lipids to obtain highly beneficial outcomes.