Catabolism of Lipids

Suaib Luqman

Objectives

v To understand the catabolism of lipids
v What are the implications of lipid catabolism

Concept Map

3. Description

Definition:

The breakdown of intricate substances into simple ones in the living organisms with the discharge of energy. It is also known as destructive metabolism distinct from anabolism.The word catabolism is derived from the Greek word (kato and ballein) meaning to throw downward. It rivets each and every step of the metabolic procedure that slash down biomolecules. In catabolism, there are set of sequenced reaction in a pathway that breaks down complex and larger molecules into simple and smaller units with the energy release for its usage in anabolism. Lipid, Nucleic acid, Polysaccharide and Protein (Polymers) are broken down into Fatty acid, Nucleotide, Monosaccharide and Amino acid (Monomers) respectively.

During catabolism, polymers are broken down and the released monomers are utilized by the cells to either assemble a new polymer or mortify the monomers further to release energy and yield simple waste products. The simple waste products of the cell (Acetic acid, Ammonia, Carbon dioxide, Lactic acid, Urea) are produced by the oxidation process releasing chemical free energy (a little quantity is lost as heat) which is utilized to coerce the ATP synthesis. ATP transfers the energy to the cell for anabolism and catabolism provides the energy obligatory for the upkeeping and growth of the cells. Hence, anabolism is seen as a productive process and catabolism as a disparaging metabolism. Some of the best examples of the catabolic processes are breakdown of fat, breakdown of protein, citric acid cycle and glycolysis.

The catabolic processes are controlled by numerous signals including hormones (catabolic and anabolic) that stimulate metabolic processes. Adrenaline, Cortisol and Glucagon are the typical catabolic hormones whereas cytokines, hypocretin, melatonin and orexin are the hormones with least catabolic effects have already been revealed. In muscle tissue, some of these catabolic hormones articulate an anti-catabolic effect, for example, an anti-proteolytic effect was observed during adrenaline (epinephrine) administration which suppresses the metabolism. Other reports revealed that catecholamines (dopamine, epinephrine and nor-epinephrine) greatly diminish the pace of muscle catabolism.

In catabolism, the reactions are alike as the complex molecule being chewed up so that they could be utilized in an easier way. The process of digestion is a catabolic activity that begins with breakage of the bonds in larger and complex food molecules by water. The so formed simple and smaller molecules are subsequently transmitted to the cells to partake in a process that change biochemical energy to ATP. When food particles are broken down into simple and smaller nutrients, it releases the energy through an oxidation process. In addition to digestion, cellular respiration is also a catabolic method as it breaks down the simple and small molecules from digestion into even lesser ones and energy is created in form of ATP. Both digestion and cellular respiration are very essential processes as ATP is produced and used by the cells perform essential bodily functions including muscles movement and production of body heat.

The energy during the catabolic processes is released in the defined three phases.

1. The complex and large molecules (lipids, proteins and polysaccharides) are wrecked down resulting in the release of petite amounts of energy in the form of heat.

2. The simple and small molecules are oxidized to form ATP and heat via chemical energy ensuing the formation of any one of the three compounds namely acetate, oxaloacetate or α oxoglutarate which subsequently oxidized to form carbon dioxide.

3.The cyclic sequence reaction takes place through Krebs (TCA) cycle transfers the electrons and the hydrogen atoms from the transitional compounds to oxygen resulting in the formation of water. In cells, the events are imperative for ATP cohort and are referred to as oxidative phosphorylation and terminal respiration.

During the normal metabolic functions, the body faces a catabolic state that essentially characterizes the breakdown of foods and nutrients as a result of which they will have the aptitude to assemble again and append to the tissue or muscle growth process.

In human body, both catabolism and anabolism toil collectively in a bid to sustain a healthy energy level and sturdy, efficient muscle tissue. These metabolic processes ultimately help in the growth of muscle tissue and the energy release during the works help to retain appropriate muscle activity. The oxidation process facilitates the synthesis of ATP (essential chemical building blocks) and numerous ATP molecules give power to the cells so that it can transfer more energy to the anabolic process. In all cells, basically, catabolism performs the role of energy provider for appropriate growth and preservation.

Apart from providing energy to the cells for proper growth and function, catabolism occasionally acts as a pessimist that escorts to undesirable health effects. It occurs when the body has an enormously elevated rate of catabolism and the fat deposits of the body and muscle tissue becomes depleted. Generally, a majority of people who live healthy lifestyles (proper diet and exercise) do not stumble upon catabolic trouble. Nonetheless, hormone imbalance and high level of stress do elicit undesirable effects that spoil the normal balance amid catabolism and anabolism.

Lipid or Fat Catabolism

Lipids do have an assortment of uses including storage of energy in a compact form. Before being absorbed, fats or lipids ought to be solubilized and the process (saponification) engrosses the utilization of enzymes (e.gtriacylglycerol lipases), detergents (bile acids, free fatty acids) and partly hydrolyzed phospholipids. Since fats or lipids are accumulated in form of esters, the foremost storage form occurs as triacylglycerol (TAG). Lipases are the enzymes that liberate free fatty acids from accumulated fats or lipids. Hormone-sensitive lipases are restricted by insulin, cortisol and epinephrine.

In living organisms, lipids are the part of the membranes but cannot be elated transversely. They are broken down to fatty acid and glycerol by a group of extracellular enzymes called Lipases. Lipases not only attack the backbone of the larger lipid moiety but also assault the bond between the fatty acid and oxygen of the glycerol. Among lipases, phospholipases assail phospholipids and more or less four types of phospholipases occurs with dissimilar names relied on the type of bond they cleave. Phospholipases are not fussy about their substrate and will hit a glycerol ester link bearing fatty acid of any length attached to it. The outcome of this digestion is fatty acids of diverse chain lengths, glycerol and a hydrophilic head molecule. The hydrophilic head molecule could be one of numerous small organic molecules that are conduit by one or two reactions in Kreb’s cycle. 3 Phosphoglycerate (3PGA) is produced from glycerol (either by phospholipase C or phospholipase D) which ultimately results in the formation of pyruvate via glycolysis. The fatty acids are sullied by a method called β oxidation. In short, the activation of the fatty acid occurs by the toting up of CoA which necessitates energy in the form of ATP. The β carbon is afterward oxidized to a ketone (CH2 to C=O) by three reactions followed by the action of β ketothiolase that splits the fatty acid into Acetyl CoA with further addition of another CoA to the earlier oxidized β moiety of the fatty acid. Through consecutive rounds of oxidation, a fatty acid gets completely converted to Acetyl CoA. The fatty acids with odd numbers of carbon atoms, the ultimate reaction yield Acetyl CoA and CoA captivated to Propionyl CoA (a 3C fatty acid). In E. coli, Propionyl CoA gets converted to pyruvate and in diverse bacteria it is gripped differently.

During the catabolism of lipids or fats following vital points were observed.

Recycling of Constituents: In the catabolism of fatty acid, CoA acts as a major player and NAD and FAD (electron carriers) are used. At any time possible, the cell reuses an enzyme, coenzyme or a cofactor.
Channeling of metabolite: During catabolism, fatty acids are broken down to Acetyl CoA which is supplied to the TCA cycle. Most of the time cells also attempt to salvage the universal pathways. This is accomplished through the conversion of a common metabolite which is channeled into an already accessible pathway.

So, in catabolism of lipids also known as lipolysis, fats or lipids molecules are wrecked down into parts that can either be channeled to Kreb’s cycle or gets converted to pyruvate. TAG gets broken down to glycerol and fatty acid by hydrolysis. Glycerol enters Kreb’s cycle after its conversion to pyruvate in cytosol and fatty acid undergoes oxidation.

Summary

In this lecture we learnt about:

The catabolism of lipids or fats
The implications of lipids or fats in metabolism

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Weblinks

Books

Lehninger Principles of Biochemistry by David L. Nelson, ‎Albert L. Lehninger, ‎Michael M. Cox. 2008. https://books.google.co.in/books?isbn=071677108X
Lipids: Current perspectives by D John Betteridge. 1996. Issue 12259, Page 19 https://books.google.co.in/books?isbn=1853172316
Intestinal Lipid Metabolism by Charles M. Mansbach II, ‎Patrick Tso, ‎Arnis Kuksis. 2011. Page 358 https://books.google.co.in/books?isbn=146151195X Biochemistry of Lipids, Lipoproteins and Membranes by Neale Ridgway, ‎Roger McLeod. 2015. Page 565. https://books.google.co.in/books?isbn=0444634495