Human Metabolism. Keith N. Frayn

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Название Human Metabolism
Автор произведения Keith N. Frayn
Жанр Медицина
Серия
Издательство Медицина
Год выпуска 0
isbn 9781119331469



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effects on the serum cholesterol concentration and propensity to heart disease will be discussed.

Figure with two panels labelled a and b showing the molecular shapes of different fatty acids. Panel a shows saturated fatty acid stearic acid (18:0), with a straight chain. Panel b shows mono-unsaturated fatty acid oleic acid (18:1 n-9), with a ‘bend’ in the chain at the double bond. Figure shows a cup of olive oil weighing 90 grams and a bowl of potatoes weighing 1.05 kilograms side by side to indicate a comparison of fat and carbohydrate as fuel sources. Raw potatoes are hydrated to almost exactly the same extent as glycogen in mammalian cells. Olive oil is similar to the fat stored in droplets in mature human adipocytes. The potatoes and olive oil here each provide 3.3 megajoules on oxidation. his emphasises the advantage of storing most of our energy in the body as triacylglycerol rather than as glycogen.

      1.2.2.3 Proteins

      Proteins are chains of amino acids linked through peptide bonds. Individual proteins are distinguished by the number and order of amino acids in the chain – the sequence, or primary structure. Within its normal environment, the chain of amino acids will assume a folded, three-dimensional shape, representing the secondary structure (local folding into α-helix and β-sheet) and tertiary structure (folding of the complete chain on itself). Two or more such folded peptide chains may then aggregate (quaternary structure) to form a complete enzyme or other functional protein.

      Protein is often considered as the structural material of the body, although it should not be thought of as the only structural material; it can only assume this function because of the complex arrangements of other cellular constituents, especially phospholipids forming cell membranes. Nevertheless, apart from water, protein is the largest single component in terms of mass of most tissues.1 Within the body, the majority of protein is present in the skeletal muscles, mainly because of their sheer weight (around 40% of the body weight) but also because each muscle cell is well packed with the proteins (actin and myosin) which constitute the contractile apparatus. But it is important to remember that most proteins act in an aqueous environment and are, therefore, associated with water. This is relevant if we consider the body’s protein reserves as a form of stored chemical energy. Since protein is associated with water, it suffers the same drawback as a form of energy storage as does glycogen; with every gram of protein are associated about 3g of water. It is not an energy-dense storage medium. Further, although protein undoubtedly represents a large source of energy that is drawn upon during starvation, it should be remembered that there is, in animals, no specific storage form of protein; all proteins have some function other than storage of energy. Thus, utilisation of protein as an energy source involves loss of the substance of the body. In evolutionary terms we might expect that this will be minimised (i.e. the use of the specific storage compounds glycogen and triacylglycerol will be favoured) and, as we shall see in later chapters, this is exactly the case.