METABOLISM OF CARBOHYDRATES
Just the other day, after roaming around a certain mall for almost an hour, I decided to finally have my lunch in this fast food chain. Upon passing by various food stalls, it suddenly came to myrealization that for us, Filipinos, it is so difficult to imagine a mealwithout rice. Being our staple food, we have grown so accustomed to eating thisalmost every day. In return, rice, which can be cooked in a lot of ways, haveconsistently provided us with the nutrients and energy we need in our everydaylives. But how can this humble grain able to sustain a significant part of ourdaily food intake?
First, we must understand that rice is acarbohydrate and carbohydrates are essential in our diet. These are biologicalcompounds that contain carbon, hydrogen, and oxygen, and are mainly derived from plants and, sometimes, animals. Carbohydrates of biomedical importance can be classified as monosaccharides, disaccharides, oligosaccharides, or polysaccharides.
Monosaccharides are the simplest form of sugars and thus, can no longer be broken down into simpler compounds. An essential type of such is glucose, considered to be the most important monosaccharide. Other examples include galactose, mannose, fructose, ribose, and deoxyribose. Meanwhile, two monosaccharides joined together by glycosidic bonds is called a disaccharide. Two glucose molecules together compose a maltose or isomaltose; one galactose and one glucose, a lactose; and one glucose and one fructose, a sucrose (common table sugar). If there are three to ten monosaccharides condensed together, it is called an oligosaccharide, most of which cannot be digested in the body. Polysaccharides are condensed 10 or more monosaccharides and usually function in storing energy and maintaining structural integrity of cells. These include starch, cellulose, and glycogen.
Image 2. Carbohydrate groups.
Rice is a starchy food and, as such, stores a lot of energy. However, this can’t be easily absorbed by the body. It has to go through a series of steps for it to be utilized. Enzymes are used by the body to perform various metabolic processes. Upon consumption of starch, the enzyme salivary α-amylase, which is found in the mouth, acts upon the former to form an oligosaccharide or α-dextrin. Once it has reached the intestine, the α-dextrin will now be acted upon by the enzyme pancreatic α-amylase (produced by the exocrine pancreas) forming maltose, maltotriose (trisaccharide), or limit dextrin (oligosaccharide). The enzyme sucrose-isomaltase complex will convert maltose to glucose. The monosaccharide formed can now undergo absorption and metabolism.
Image 3. Digestion of Carbohydrates. (Lippincotts Illustrated Reviews Biochemistry 5E)
Going back to what I ate, it was a cup of white steamed rice, roasted chicken breast, and a bowl of mushroom soup. It also came with a small muffin and a glass of house-blend iced tea.
From what I mentioned earlier, the digestion of rice and other sources of carbohydrates will begin in the mouth. It will be continually digested until glucose is obtained. Afterwards, glycolysis (meaning breaking of sugar) will occur. Glycolysis is the main pathway for glucose to generate energy expressed as ATP. Due to the importance of this process, it is considered to be the first among other pathways of the metabolism of carbohydrates.
Glycolysis is composed of a series of metabolic processes that starts with glucose as its primary substrate. Generally, there are 10 various steps to come up with ATP, and each of which involves a specific enzyme catalyzing the changes. These steps are illustrated below.
Image 4. Steps in Glycolysis.
At the end of glycolysis, a total of 2 moles of ATP are produced per mole of glucose. The overall reaction of glycolysis is given as:
Glucose + 2 NAD+ + 2 ADP + 2 Pi à 2 Pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O
Aside from the steamed white rice, the iced tea and the muffin, being carbohydrates as well, will also enter glycolysis. However, their main path would not be mainly via glucose, rather via fructose and lactose, respectively. Iced tea has sucrose or table sugar as one of its components and thus is hydrolized to form glucose and fructose. The muffin, on the other hand, makes use of milk which has lactose. Enzymes in the intestines act on lactose to form glucose and galactose. How fructose and galactose enter the glycolytic pathway is illustrated in the following images, respectively.
Image 5. Steps in Fructose Metabolism.
Image 6. Steps in Galactose Metabolism.
What is stated above is the general pattern of how glucose is metabolized in the body. Given that it involves a number of processes, it is highly possible to develop any variations in these steps which can greatly affect the overall function of the body. For example, a defect in one of the enzymes in glycolysis, pyruvate kinase, may manifest in an individual a couple of symptoms: weakness, fatigue, shortness of breath, and dizziness. Their hemoglobin level can be predicted as less than normal with red blood cells showing a low level of lactate production. This is a clear manifestation of hemolytic anemia, the second leading enzyme deficiency-related cause of which is pyruvate kinase deficiency.
