Friday, 3 March 2017

Sweetness from light? Glucose is my molecule for March

My molecule for February was the metabolic, blood-sugar regulator Insulin, which made me realise that I haven't included glucose in my rogues' gallery of biologically important molecules. So I am putting that right for March with an overview of this molecule that has been central to the field of Biochemistry since it first began. The image on the left shows a cyclised molecule with 5 carbons and an oxygen atom at 12 o'clock. There are no double bonds, and importantly there are 5 hydroxyl groups at 2, 4, 6, 8 and 10 o'clock. However look closely, the OH at 10 o'clock is attached to a carbon atom that is not part of the hexameric ring. I think this is the easiest way to remember the molecular structure and chemical formula of glucose, which is C6-O6-H12 (for younger students, standard chemical structures exclude hydrogen atoms that are bonded to carbon atoms, to avoid clutter). This is sometimes called a Haworth projection. 

However, glucose is often depicted as a linear molecule (a Fischer projection, shown on the RHS) , with an aldehyde function at one end. The non-cyclic form of glucose is only present at low amounts (maybe 0.25%, in vivo: the remainder is cyclised). We are not done yet, since these are flat projections, and if there is one thing you will have realised by now, Biology is all about 3D shapes.  
In 3D terms, glucose molecules, when cyclised exist in a form often referred to as a "chair". This is shown on the left. The OH and H atoms that project from the ring carbons, do so in one of two ways: horizontally (equatorial) or perpendicular (axial). These two conformations exist in equilibrium, which can be of great importance when glucose encounters the active site of an enzyme. Can you think why? The other feature of glucose that again becomes important when it either acts as a substrate, or is involved in forming a linkage to a second sugar molecule: the orientation of the OH group at carbon 1. You can probably work out that the union of glucose and fructose (two monosaccharides) produces the disaccharide, sucrose, and it is the green OH at position 1, that forms a glycosidic bond to the OH at position 2 of a fructose molecule, which gives us α-D-glucopyranosyl β-D fructofuranoside, as shown on the right. The polymerisation of sugars, in particular glucose, produces carbohydrates such as starch and glycogen, both of which are found as storage products in living organisms, is a consequence of a dehydration reaction, which can occur at several OH groups. To recap then, the major form of metabolic glucose in living organisms, is cyclic and, since it is known to rotate plane polarised light in a right handed manner; we refer to it as a dextrose sugar (dexter is Latin for right handed). Which now brings me on to the question of why is glucose such a good source of energy?

Plants capture sunlight energy and infuse it with carbon dioxide to make glucose (among other things). As with any chemical reaction that occurs in vitro or in vivo, energy is involved. Just think of a typical petrol car. The fuel, gasoline is ignited in order to release the energy trapped in the chemical bonds within the fuel molecules to power the engine. In living organisms, sugars like glucose are placed at the start of an enzymatic conveyor belt and the trapped energy captured in the form of ATP , in order to provide us with the fuel we need to grow, move and think etc. The amount of energy contained in glucose is around 16kJ per gram (or if you prefer 3.75 kilocalories). If you consider the activity of a typical adult in the course of a day requires around 4 300 joules  (or 1000 calories), you can begin to appreciate the value of glucose as an energy source. I always think of bacterial media when I am trying to relate glucose quantities with living cells. Let's work it out. The molecular weight of glucose is about 180 and typically media contain 1% (w/v) glucose (10g/L). In my experience (depending on the strain and the growth conditions of course), a one litre culture of E.coli will yield around 5-10g wet weight of cells. Let's keep things simple, around 10g of glucose gives rise to 10g wet weight of bacterial cells (from a very small inoculum). Apart from a number of minerals, many lab strains of E.coli can grow on a very simple medium, provided with a carbon and nitrogen source. All of this is achieved by the catabolism of glucose which fuels, even in the case of a simple prokaryote, an incredibly sophisticated "molecular machine" that is capable of growth followed by cell division. It is clear I think that glucose is of paramount importance in most living organisms, justifying its choice as a molecule of the month, even if it is a late arrival!

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