Enzymes: The Biological Catalysts

  Metabolic reactions always take place inside our bodies. For instance, digestion is an important process that takes place inside us. Often reactions such as digestion are accompanied by the use of catalysts. These biological catalysts are known as enzymes. Enzymes are responsible for speeding up a reaction without being consumed by the reaction itself. This is an important property of enzyme because they can be used over and over again.

  There thousands of different kinds of enzymes. Amylase, protease and lipase are just some of the common enzymes that one might have heard of. Every reaction needs an initial investment of energy for it to start. This energy is known as the activation energy.  The activation energy acts a barrier, which determines the total amount of energy required in order to start a reaction. Most of the time this activation energy is too high and as a result it would take a long time to finish the reaction. This is when enzymes become useful. An enzyme catalyzes a reaction by lowering the activation energy, enabling the reactant molecules to absorb enough energy to reach the transition state even at moderate temperatures. Since enzymes are specific, they determine which chemical process will be going on in a cell at any particular time.

How enzyme lowers the activation energy

 

  The reactant an enzyme acts on is referred to as the enzyme’s substrate. When a substrate and an enzyme bind they form the enzyme-substrate complex.  If you can recall, I mentioned that enzymes are specific. The specificity of enzymes results from the different kinds of amino sequences. The active site is a region in which the substrate binds to the enzyme. Induced fit allows chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction. Substrates are held at the the active site by weak bonds. These bonds can range from hydrogen bonds to ionic bonds. The active site lowers the activation energy either by providing a favorable environment or by participating directly in the reaction. The active site can also behave as a template for the substrates. The substrates are then converted to products before being released. The process described above is commonly known as the enzyme cycle.

   There are many factors that influence enzyme activity. The environment an enzyme works in is crucial for the enzyme’s activity.  The optimal temperature for a typical human enzyme is around 37 degrees Celsius. Enzymes at this temperature work well because substrates collide with active sites more frequently when the molecules move rapidly. Higher temperature does not however, guarantee increased enzyme activity. Higher temperatures have the ability to break hydrogen bonds, ionic bonds and other weak interactions that stabilize the shape of an enzyme.

How temperature influences enzyme activity

 

Similarly, the pH of the environment can influence enzyme activity. Some enzymes work best in acidic conditions while others work well in basic environment. For instance the enzyme pepsin is best suited to a pH level of around 2, whereas trypsin works diligently in a pH of 8.

How pH levels influence enzyme activity

    

 Many chemicals inhibit the action of specific enzymes. Competitive inhibitors mimic the substrate and compete for the active site. This way the productivity of enzymes is reduced since the inhibitors block substrates from entering the active sites. This kind of inhibition can be overcome if more substrates are produced so that when active sites are available, the substrates can quickly go attach to the enzymes’ active sites before the inhibitors. Conversely, noncompetitive inhibitors bind to the enzyme away from the active site and as a result change the shape of the enzyme.

  Cells regulate enzymatic reactions in order to foster necessary reactions and inhibit unnecessary ones. One way of regulating enzyme action is allosteric regulation. In allosteric regulation, a molecule binds to an an enzyme at a site other than the active site. This changes the enzyme activity. One example of allosteric regulation is the binding of an inhibitor. When an inhibitor binds, it causes the active site to change its original shape. This modification hinders substrates from binding. When the inhibitor gets released again, the enzyme goes back to its original form. In contrast allosteric activators increase the productivity of an enzyme for its substrate.

 

How allosteric inhibition and allosteric activation 

For this post, I used the AP version of my 8th Edition Biology Campbell book exclusively. It is a really useful book if you want to have a better interpretation of the content I discussed in this post.

Here are some of the links I used to help me with my post: 

http://bcs.whfreeman.com/thelifewire/content/chp06/0602002.html

http://www.worthington-biochem.com/introbiochem/default.html

 

What really happens to our food

   Have you ever wondered what really happens to that slice of pizza you just a couple of hours ago? Or maybe the science behind it? Well, yeah your slice of pizza ends up getting excreted out of your body after a while but what is it that transforms that slice of pizza to that putrid little (in some cases humongous) thing? Curious? Well buckle your seat belts as I take you on a journey in which you will learn what really happens to our food!
 
    Before I start I just want to clarify that many of you who are reading this might consider this content to be rudimentary and I do agree with you. Since this is my first EVER post I thought it would a good idea to start off with something so basic. Additionally this topic is also one of my all time favorites. I don't really know why. But it just is. And I happen to remember this effortlessly even though I learned this back in 8th grade. Don't worry I will post more extensive content later on. I will however, appreciate any suggestions on a topic that you find interesting. Something that would be beneficial for everyone. Thank You.

    Since we are going to talk about digestion I believe it will be useful for you to know what the alimentary canal is. The alimentary canal is a protracted tube running through the body. The interior portion of the alimentary canal is lined with layers of cells called the epithelium. New cells are produced simultaneously in order to replace the old cells, which might have been worn down by the movement of food. There are also cells present in the lining which produce a slimy liquid called mucus. Mucus helps in lubricating the lining and protects the lining from being attacked by digestive enzymes  present in the canal. The alimentary canal is made up longitudinal and circular muscles. A series of contraction along the alimentary canal helps to push the food in front. This process is called peristalsis.

A diagram of peristalsis

     Now that we can interpret what peristalsis is, we can move on to understand how digestion really takes place. Digestion is predominantly a chemical process in which large insoluble molecules are broken to small soluble molecules. It is a process by which solid food is dissolved to make a solution. Enzymes-biological catalysts- helps in dissolving the food.

    

    Many might not realize this but digestion actually starts the moment you take in food into your mouth. Munch! Munch! Chewing your food (the only good part) is how everything starts. The chewed food is mixed with saliva. Salivary amylase, the enzyme present in saliva helps in breaking down starch to maltose. For the food to enter the esophagus-the tube that transports food to the stomach-it has to pass the windpipe. Gosh! The windpipe? How come we don't choke when we eat? When we swallow, our tongue presses upward and back against the mouth. This as a result forces the bolus to the back of our mouth. A soft palate closes the nasal cavity. The top of the windpipe is circumscribed by the larynx cartilage. This cartilage is pulled upwards so that the glottis:the opening of the windpipe lies under the tongue. A flap of cartilage called the epiglottis prevents food from going down the windpipe instead of the esophagus.

A diagram explaining how food travels down the esophagus to the stomach 

       Say hello to the stomach! Stomach is used to store food which then ends up getting turned to liquid in order to be released to remaining portion of the alimentary canals. The glands in the stomach produce gastric juice containing protease, an enzyme responsible for breaking down protein into soluble compounds known as peptides. The stomach lining also manufactures HCl, a strong acid that helps pepsin to work at its best level. Wait a minute. There's acid in my stomach?! How come I am not...dead?! Remember the mucus lining I mentioned earlier? Damage to the stomach wall by acid is prevented by the mucus. The acid also kills any bacteria that might have present in your food. Bacteria in your food? Ugh! Where on earth do you get your pizza from?! Just kidding.

    After spending some time in the stomach, the liquid products of digestion enters the small intestine through the duodenum: the first part of the small intestine. Pancreatic juice from the pancreas and bile from the liver are poured into the duodenum  to act on the food present over there. In case you were wondering, the pancreas is a digestive gland sited below the stomach. It is responsible for making numerous enzymes which in turn can act on all classes of food. Pancreatic amylase attacks starch and converts it to maltose. Lipase breaks down fats to fatty acids and glycerol. The pancreatic juice also contains sodium hydrogen-carbonate, an alkali which partly neutralizes the acid which was present from before. In contrast bile is a green fluid made by the liver. Bile is stored in the gall bladder which is later delivered to the duodenum through the bile duct. The green pigment is a result of the breakdown of hemoglobin in the liver.  Bile salts present in bile helps in emulsifying fats.

A diagram of the stomach, liver, pancreas and their respective parts

    You might be wondering "Do the enzymes get digested too?" The simple answer is no. The complicated answer? Well, the proteases released earlier are in inactive state. pepsin is produced as pepsinogen and doesn't become active until it encounters HCl in the stomach. Similarly, trypsin, one of the proteases secreted from pancreas is released as trypsyinogen and is later activated by the enzyme enterokinase. In this manner the body ensures that the enzymes do not get digested. Additionally the secretion of liquids is controlled by hormones released by the body.
 
   The small intestine contains the ileum. Almost all of the absorption of the digested food takes place here in the ileum. It is efficient in absorbing nutrients for numerous reasons. Some of which include a large absorbing surface, an internal surface filled with villi and the epithelium lining which is known for its thinness. Each villus also has a dense network of capillaries. Amino acids and glucose travel into the epithelial cells and then through the walls of the capillaries in the villus and finally into the bloodstream. The capillaries join to form the vein, which in turn forms the hepatic portal vein. This vein carries all the blood from the intestine to the liver. When the products are released from the liver, they enter into the bloodstream. Sometimes fatty acids and glycerol are combined to form fats again in the epithelium. These fats then travel to the lacteal and eventually flows into the lymphatic system, which makes a network throughout the body and gradually empties the stored fat to the bloodstream to be absorbed by cells. The absorption of the food molecules aren't just done through diffusion but it is also done using active transport. Amino acids, sugars and ions of mineral salts are absorbed through the process of active transport.

A diagram of villus

     The content travelling to the large intestine generally consist of water, cellulose and vegetable fiber, mucus and innumerable dead cells. Despite the lack of any presence of enzymes, with the help of bacteria in colon, the large intestine digests some of the fiber to be used as fatty acids. Some of the bile salts are also absorbed and returned to the liver. In addition the colon also absorbs much of the water from the undigested residues. This step is important because if the colon doesn't absorb the water then the body would have to suffer from dehydration. After spending some hours inside your body the unwanted materials in your pizza is excreted from body in the form of feces. The journey is finally over!





Here are the links of some cool sites which I used for my post: 
http://www.biologyguide.net/
http://www.mrothery.co.uk/