Glucose

Hi, in today's post we'll be discussing part of the next topic in our Veterinary Biochemistry unit, that is carbohydrates, which consists of six individual parts. Today we'll talk about the first part which is an introduction to carbohydrates and glucose. I'll define some key words and explain the glycosidic bond. I'll also discuss homoeostatic glucose concentrations and glucose transporters.

Carbohydrates

First of all, carbohydrates are molecules that consist of carbon, hydrogen, oxygen and sometimes nitrogen. They also contain hydroxide groups and aldehyde and ketone groups. They are also called saccharides or sugars. Carbohydrates are important because they are the precursors for many important substances such as: lipids, non-essential amino acids, RNA and DNA. They are also important in the release of energy from ATP. There are several types of carbohydrates, including:

• Monosaccharides: these are the simplest carbohydrates and cannot be hydrolysed into simpler forms. eg fructose.
•  Disaccharides: these are two monosaccharides linked by a glycosidic bond. eg sucrose
• Polysaccharides: these are many monosaccharides linked by glycosidic bonds. eg. cellulose

The Glycosidic Bond

Glycosidic bonds are formed by the -OH group of an anomeric carbon and a second carbon. The linkages are either α or β depending on the anomeric isomer. The diagram below shows what an anomeric isomer looks like.

An Anomeric Isomer

The only difference between the two molecules is whether or not the -OH group lies above or below the plane of the molecule. If the bond is below the plane, it is a α glycosidic bond. It will be a β glycosidic bond if the bond lies above the plane of the molecule. Enzymes are highly specific for each type of glycosidic bond.

Homeostatic Glucose Concentrations

The concentration of glucose in blood plasma is tightly controlled. Normal plasma glucose levels in monogastric animals (those with one stomach chamber, eg humans and pigs) are between 4 and 5 mM. In ruminants (an animal with a rumen eg. cattle and sheep) normal plasma glucose levels lie between 3 and 4 mM. The difference between monogastrics and ruminants is due to the fact that in ruminants, glucose is fermented and the animal absorbs volatile fatty acids where as in monogastrics glucose is absorbed. Hypoglycaemia occurs when the blood glucose levels fall below half of the normal blood glucose values. Hyperglycaemia occurs when the blood glucose levels are higher than 10mM for extended periods of time.

Glucose Transporters (GLUT)

GLUT proteins allow the facilitated passive diffusion of glucose into cells. ie they help cells absorb glucose. All cells contain GLUT proteins but they may vary in their kinetic properties. There are 4 key GLUT proteins:

• GLUT1: found in erythrocytes, the brain, placenta and kidney. They have a Km of 1mM and are not responsive to insulin
• GLUT2: found in the liver, beta cells of the pancreas and the kidney. They have a Km of 10-20 mM and aren't insulin responsive
• GLUT3: found in the brain and many other tissues, they have a Km of less than 1mM and are also not responsive to insulin
• GLUT4: found in the muscles, heart and adipose tissue. They have a Km of 5mM and are responsive to Insulin.

Glucose Dependence of Tissues

Erythrocytes (red blood cells) require GLUT1 transporters to receive their glucose. They have no mitochondria and so they rely on glucose for their energy, which is generated via glycolysis. They are unable to use fats or amino acids as an energy source because they require a mitochondria. 

The brain and nervous tissue use GLUT1 and GLUT3 transporters to receive their energy. They are entirely glucose dependent and require aerobic metabolism. 

Type II muscle (white muscle) use GLUT4 transporters to receive their glucose. This type of muscle receives a limited oxygen supply and have few mitochondria. During anaerobic metabolism, they get glucose from glycogen stores. 

The liver uses GLUT2 transporters to receive its glucose. The liver is central to glucose homeostasis and act as a glucose sink. The metabolism that occurs in the liver is not glucose dependent.

And that's it :) If you have any questions please feel free to ask :)