Metabolism 101: Oxidation & Reduction

Oxidation! Anti-oxidants! DNA damage! Cancer! Woaa woaa woaa! Let’s step back for a second and actually understand what oxidation is. Also, let’s not forget oxidation’s partner in crime, reduction and how the two are vital for metabolism.

As I’m sure you know, atoms are made up of electrons. These electrons are not static are a capable of moving from one atom to another. That’s exactly what oxidation and reduction are all about. The atom or molecule that loses an electron is a reducing agent and becomes oxidized. Alternatively, the atom that gains an electron is an oxidizing agent and gets reduced. This is the basic principle behind oxidation-reduction (redox) reactions. Just as simple as that 🙂

Another trick to understand this is to think of “reduction” as the atom having a lower charge. Think about it, a reduced atom would have gained an extra electron and hence would have a negative (or lower) charge.

Now lets put this in the context of nasty oxidants (free radicals) like ozone. Well the thing with ozone is that it’s a poor unhappy molecule that is dire need of an electron to become happy. So what does it do? It will rip away an electron from another molecule. So selfish! The issue is that the molecule it stole the electron from will become unhappy and “damaged”. Quite an issue if the molecule we’re talking about is our DNA.

Well… anti-oxidants to the rescue!!! These good guys are basically reducing agents that will give off their extra electron to oxidants to neutralize them. Hence, eat your berries and no one will get hurt 😉

You might be wondering how does all of this relate to metabolism? When you burn macronutrients, you are essentially oxidizing them. Thus, we need oxidizing agents that will pull the electrons off of the macronutrients. Specifically, these agents are coenzymes called FAD, NAD+, and NADP+. Each one of these acts as an electron carrier and is capable of holding on to 2 electrons. These electrons are crucial as they will take part in a process called oxidative phosphorylation which generates most ATP in macronutrient breakdown.

Alternatively, coenzymes are important in the synthesis of fatty acids where they serve the complete opposite role. They will donate the electrons that they carry not to generate energy or ATP but to put it into storage as fat.

Furthermore, NAD+ or NADH+ H+ balance is crucial for determining whether your body will undergo aerobic or anaerobic respiration. This will all make sense in future posts.

Now that I’ve covered what energy is and oxidation & reduction, I still want to go over what enzymes are. After that, it’s all juicy metabolism information 😉

Metabolism 101: Energy & Reactions

Metabolism! Fascinating topic! Do you know what the word actually means? Technically metabolism is a series of interconnected pathways that are responsible for energy production or synthesis of molecules using energy. Woaa! That’s a mouthful! Metabolism is a very vast and complicated field. Most biochemistry students have to take a course specifically on metabolism which only covers the basics. Hence, in the next series of posts I have decided to go over the most important aspects of metabolism which will help you understand how it works and its relation to nutrition.

I’ve decided to begin by addressing an important question. What is energy within a biological system and why do we need to consume calories?

Energy is an interesting concept because it can neither be created nor destroyed.  However, it can be transformed into a different form of energy or be transferred between a system and its surroundings. In a living organism energy is obtained by breaking down macronutrients and trapping their energy as adenosine triphosphate (ATP). The ATP can then be used to make vital bodily reactions occur.

The thing is that there’re two types of reactions those that require energy (endothermic) and those that give off energy (exothermic). These reactions can be coupled in that the energy from an exothermic reaction will power the endothermic reaction. 

An exothermic reaction is when  the reaction’s reactants are more energetic than the products. Thus, there is energy given off during the reaction which can then be stored or used to make another reaction happen. A typical exothermic reaction would be the metabolism of glucose. The glucose molecule is highly energetic and our body has learned to break it down through a series of reactions that turn glucose into carbon dioxide and water. Do note that there is an initial energy investment required but using enzymes this step can be skipped.

Glucose-Exothermic Rx

An endothermic reaction is the complete opposite of an exothermic reaction. The initial reactants have little energy but after the reaction the products have a high level of energy. Therefore, energy must be absorbed or used up in order for the reaction to occur. In the case of ATP, energy from glucose breakdown is used to attach a phosphate group to ADP to form the highly energetic ATP molecule.

ATP-Endothermic Rx

The energy trapped in the form of ATP can then be used to make other reactions occur. In order to move our muscles, ATP molecules are broken down into ADP and phosphate. This reaction involves an exothermic reaction of ATP being broken down into ADP and phosphate using an enzyme called actomyosin ATPase. The potential energy from ATP is then converted from into kinetic energy and thus displacement.

So back to the question. If energy cannot be made or destroyed, why do we need to consume calories? Well when we use up ATP, the energy gets transformed into something we can no longer use or is lost to the surroundings in the form of heat. Technically, this energy is not “destroyed” but is no longer available to us and must be replenished.

Now that I’ve covered the role of energy in our bodies, I will follow up on posts on oxidation & reduction and enzymes. Although these ideas might sound dry they’re fundamental to understanding metabolism 😉

Macronutrients 101: Metabolism Overview

Now that we have taken a look at the three key macronutrients- carbohydrates, lipids, and proteins, it is important to know when our body utilizes these fuels. In subsequent posts I will go into more detail with regards to different metabolic pathways but for now I thought that giving you a brief overview would be useful.

First off, how does our body store energy? There is a molecule called adenosine triphosphate (ATP) which is the energy currency within a cell. Literally, you can think of it as money. In the process of breaking dowm macronutrients into energy, a phosphate group gets attached to adenosine diphosphate (ADP) to convert it into ATP. Kind of like you depositing money in the bank from doing your job. Furthermore, when energy is required ATP will let go of one phosphate group to become ADP and cause a reaction to occur. This is similar to you giving  money to someone for doing a service for you. Remember that all macronutrients can get converted into ATP, the trick is how fast this process will occur and oxygen availability. Additionally, fat metabolism requires adequate oxygen supply while carbohydrates could be burned without oxygen.

ATP

When at rest or undergoing very light physical activity, our body’s energy demands are low. Therefore, there’s plenty of time to generate ATP from macronutrients. The most efficient way to do so would be by burning fat as it yields more ATP per gram and oxygen is readily available. At the same time, the body can conserve its glucose stores in case it needs a quick burst of energy for a “fight or flight” response.

Once the body’s energy demand go up like during jogging or aerobic exercise, burning fat alone is simply not fast enough. There is still enough oxygen to continue fat metabolism but now glucose must also be metabolized to meet the increased ATP requirements. Hence, our bodies will begin to break down our body’s glycogen stores into glucose. Unfortunately, our glycogen stores are limited and can sustain aerobic activity for a limited amount of time. This effect can be observed with marathon runners as they “hit the wall” if they run of glycogen.

With heavy exercise or sprinting, our oxygen levels drop. Without oxygen, we cannot metabolize fat and must solely rely on burning glucose. Even so, our bodies will burn this glucose in an inefficient anaerobic pathways that results in lactose production. As a result we feel cramps in our muscles and can only sustain this intense exercise for a short duration.

It is important to note that the body doesn’t really use protein as fuel. Doing so is inefficient, slow and yields toxic by-products like urea that must be removed from the body. The only time our bodies will burn protein is during times of extreme starvation. 

Do these rules apply to all organs? Well technically what I’ve just described is particular to muscles. Our nervous system can only burn glucose or ketone bodies (by product of fat produced during starvation). Alternatively, our adipose tissue will preferentially burn fat. All in all, muscles, brain and adipose tissue will make up most of our required daily calories.

So what’s the deal with the macronutrient ratio? Although the research is still controversial, I would assume that it would depend on your activity level. If you lead a sedentary lifestyle, then stay clear of carbs as they will get stored as fat. However, if you lead a healthy active lifestyle then don’t shy away from carbs 😉 Just make sure they’re the good unrefined kind!

Macronutrients 101: Protein Structure

Now that I’ve discussed what proteins do, let’s ask the million dollar question! Where in the food chain do proteins come from?

If you think they come from animals than you’re absolutely wrong. Those animals actually got their proteins from plants. Yes plants

Proteins unlike other macronutrients contain nitrogen atoms. The issue is that nitrogen is naturally found as a dimer with an extremely stable triple bond. Neither plants nor animals are able to break this bond. However, nitrogen-fixing soil bacteria possess the specific enzymes needed to break it down. These commensal bacteria reside at the roots of legume plants where they exchange the accessible nitrogen for glucose. The plants than utilize the ammonia to form amino acids and in turn proteins.

So what are these amino acids? Well, amino acids are monomeric units that make up proteins. There’re 20 different amino acids found in human proteins and they all have the same generic structure. There is a central (alpha) carbon atom that has an amine group (base), a carboxyl group (acid), and a variable side chain group.

The amine and carboxyl groups of two separate amino acids can undergo an acid-base reaction to form a peptide bond. Through this simple reaction, amino acids can form long chains called polypeptides. Now if you think about carbohydrates, large dietary polysaccharides like starch are only made up of glucose. However, having 20 different amino acids allows polypeptides to be constructed in multitude of diverse patterns.

The polypeptide sequence is dictated by our genes. One gene is a DNA sequence that will code for an amino acid sequence of a polypeptide. Within the polypeptide, amino acids will interact with each other to give it some 3D “secondary” structure. The two most common structures are alpha-helices or beta-sheets. These form through interactions within the invariable regions of the amino acid.

The variable groups of amino acids will then interact with each other to give the protein additional “tertiary” structural features. For example, two cysteine amino acids can form a disulfide bridge between its two sulfur atoms. Hence, this feature will create a fold within the polypeptide. All of these structural features make the proteins unique and determine their physiological function.

Lastly, a protein can be made up of multiple polypeptide chains and can include prosthetic (non-amino acid) groups. A common example of such protein is hemoglobin in red blood cells. This protein is made up of four polypeptide chains and heme “iron” containing prosthetic groups. Below is a ribbon diagram of what hemoglobin looks like based on x-ray crystallography predictions.

Beautiful ain’t it? These gorgeous proteins can be produced by our bodies, all from tiny little amino acids. Proteins are the essence of our bodies and control most of our physiological functions. In fact, I’ve taken an undergraduate course that was purely on proteins. There’s so much to learn about them! One thing is for certain is that we really don’t need much dietary protein, especially animal protein. 😉