Chapter 2 An Introduction to Metabolism
The word metabolism refers to a set of life-sustaining chemical reactions that occur in organisms. The three main purposes of having a metabolism are:
- Converting food into energy to run cellular processes.
- Converting food into building blocks for proteins, lipids, nucleic acids, and some carbohydrates.
- Eliminating metabolic waste products.
Our metabolism is made up of hundreds of enzymatic reactions organized into discrete pathways. These pathways proceed in a stepwise fashion, transforming substrates into end products through many specific chemical intermediates.
2.1 Metabolic Fuels and Dietary Components
In order to survive, humans must meet two rudimentary requirements:
- We must be able to synthesize everything our cells need that are not supplied by our diet.
- We need to protect our internal environment from toxins and changing conditions in our external environment.
To meet these requirements, there are four basic types of pathways to metabolize our dietary components:
- Fuel oxidation
- Fuel storage and metabolism
- Biosynthetic pathways
- Detoxification and waste disposal
Biosynthetic pathways allow us to synthesize nutrients that are needed by our bodies, but are not supplied in our diet.
Detoxification and waste disposal pathways are metabolic pathways devoted to removing toxins that can be present in our diets or in the air we breathe, introduced into our bodies as drugs, or generated internally from the metabolism of dietary components.
Xenobiotics are dietary components that have no value to the body (and hence must be disposed of).
2.2 Carbohydrates: Monosaccharides
Monosaccharides generally exist as rings in solution: the carbonyl group (from aldehydes or ketones) has reacted with a hydroxyl (-OH) group in the same molecule to form a five or six member ring:
The original carbonyl group has now become an anmeric carbon. A hydroxyl group that points down (below the ring) is called the \(\alpha\) position (and vice versa for the \(\beta\) position). In the actual, three-dimensional structure, the ring is not planar: the monosaccharides adopt a chair conformation to maximize the distance between the hydroxyl groups:
knitr::include_graphics("_book/_main_files/figure-html/")In solution, the hydroxyl groups on the anomeric carbon spontaneously changes from the \(\alpha\) to the \(\beta\) position through a process called mutarotation (catalyzed by enzymes called mutarotases). When the ring opens, the straight-chain aldehyde or ketone is formed.
Note that when the anomeric carbon forms a bond with another molecule, the sugar can no longer mutarotate.
2.3 More About Carbohydrates
A disaccharide contains two monosaccharides that have been joined by an O-glycosidic bond. Lactose - the sugar present in milk - is formed via a galactose and glucose unit that have been linked through a \(\beta\)(1-4) bond formed between the \(\beta\)-OH group of the anomeric carbon of galactose and the hydroxyl group on the 4th carbon of the glucose.
Oligosaccharides contain from 3 to roughly 12 monosaccharides linked together. They are often found attached through N- or O-glycosidic bonds to proteins to form glycoproteins. Polysaccharides contain tens to thousands of monosaccharides joined by glycosidic bonds to form linear chains or branched structures.
Starch is an example of a branched polymer: it is linked through \(\alpha\)(1-4) and \(\alpha\)(1-6) bonds.
2.3.1 Three key polymers from glucose monosaccharides:
Study and remember the following graphic:
Figure 2.1: Key Polymers from Glucose Monosaccharides (taken from BioNinja)
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2.4 Saturated and Unsaturated Fatty Acids
There are important advantages in storing energy in the form of fats:
- Carbon in fatty acids are almost completely reduced. Hence, oxidation of fatty acids will yield more energy in the form of ATP in the form of carbon.
- Fatty acids are not as hydrated as monosaccharides; hence, they can be more densely packed than monosaccharides!