Medical Physiology/Basic Biochemistry/Fats and Fatty Acids

Overview

Fats

Fats or lipids are the main way that the body stores excess nutrients. One Gram of fat will produce about 9 Kilocals, as opposed to 4 Kilocals for carbohydrate or protein.

Fats consist of 1-3 fatty acids attached to a glycerol molecule.

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Fatty acids consist of the general formular R-COOH, wher R is a carbohydrate chain.

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Fatty Acids can be metabolised in almost all cells of the body, the notable exception are brain cells, to produce ATP from ADP. They are broken down by a process called beta-oxidation to produce two carbon units which are then fed into the Citric Acid Cycle via Acetyl CoA.

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Other important lipid related compounds are Cholesterol, important in its own right and essential as a precursor for Steroids, and phospholipids which form the walls of all cell membranes.

Chemistry & Structure

Fatty Acids

Fatty acids consist of a carbon chain radicle and an acidic carboxyl group - R-COOH:

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A carbon chain with no double bonds is known as an saturated fatty acid; that with a single doublebond as a monounsaturated acid; and that with several saturated bonds as a polyunsaturated fatty acid.

Glycerides and Phospholipids

Fatty Acids are combined with Glycerol to make Glycerides. This is the form in which fat is stored in the body.

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A Phospholipid substitutes a phosphate bond for a fatty acid. Phospholipids are important as they makeup cell membranes. The addition of a phosphate bond means that the molecule has two poles, a hydrophilic pole (the phosphate pole) and a hydrophobic pole (the lipid pole). This is covered further when we study cell membranes and the details of the phospholipid bilayer.

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Cholesterol, Steroids and Eicanoids

Closely related to lipids are the steroids, whose structure contains four linked carbon rings. Cholesterol is the source of Steroids in the human body. Cholesterol is also an important component of cell membranes. Eicosanoids are modified twenty carbon fatty acids with a carbon ring, which makes the molecule appear to double back on itself. These molecules form important regulators of function. Prostaglandin is shown in the diagram below.

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Fats & Fatty Acid Metabolism

Ingested nutrients that are surplus to requirements are stored in the fat cells of the body as triglycerides. They are called upon as an energy source when glucose and glycogen stores are running low. Fat contains far fewer oxygen molecules than either sugars or proteins and is thus efficient for energy storage. One gram of fat will produce about 9 Kilocals (9 Cals - note the capital C) of energy compared with about 4 Cals for sugar and protein.

The first step in mobilising fat as an energy source is to split the triglyceride into Glycerol and the fatty acids. Glycerol can be metabolised to pyruvate, and the fatty acid will enter the mitochondria where it is processed via the Citric Acid Cycle.

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Fatty Acid Metabolism

beta-oxidation is a complex process which essentially chops 2-carbon chains of the fatty acid. This then combines with Acetyl CoA, which is metabolised in the Citric Acid Cycle.

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Most cells can utilise fatty acids directly, but note that they cannot be processed by brain cells which require glucose to function.

Summary of Fat Metabolism

The following flow chart illustrates the pathways of fat metabolism in the body. This will be explored further when we look at Digestion. Essentially, all surplus foods and nutrients are converted to fat, and stored in the fat stores of the body.

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Cholesterol Metabolism

Cholesterol is the source of all the Steroid drugs, as well as being important in its own right. Even in the absence of dietary intake (such as will occur with a vegan diet) it must be manufactured. Most synthesis takes place in the liver - about 25% - with the intestines, adrenals, and reproductive organs being other important sites of synthesis. The cholesterol molecule is synthesised from the two carbon molecules of Acetyl CoA in a somewhat complex process