Metabolic Pathways

Metabolic Pathway Definition

Let's start by looking at the definition of a metabolic pathway.

Metabolic pathways generally consist of a sequence of reactions activated by enzymes where the product of the previous reaction becomes the starting point or reactant for the following reaction.

• Enzymes are proteins that speed up or catalyze chemical reactions in the body.

• Proteins are organic compounds that perform essential functions such as transporting materials, controlling physiological processes such as growth, speeding up chemical reactions, storing things, etc. They consist of smaller molecules called amino acids that can be linked together to form polypeptides.

• Organic compounds are compounds that contain mainly carbon and can sustain life. Organic compounds are also commonly made of hydrogen, oxygen, or nitrogen.

Major metabolic pathways mostly consist of the synthesis of organic compounds that contribute to reproduction, cell growth, repair, energy uptake, etc.

Types of metabolic pathways

There are three types of metabolic pathways that you need to be familiar with: anabolic, catabolic and amphibolic pathways.

One of the most important metabolic pathways can involve the breakdown or buildup of organic compounds called carbohydrates to synthesize energy for our bodies.

• Carbohydrates are organic compounds consisting of hydrogen, carbon, and oxygen that store energy. They are the body's primary source of energy.

Figure 1: Types of metabolic pathways. Daniela Lin, Vaia Originals.

Metabolic Pathways Chart

There are a ton of metabolic pathways, some of which are shown in the chart below (figure 2). So, let's do an overview of some of the most important metabolic pathways in humans:

Essential definitions to understand the processes in the table above and for the next few sections are:

• Glycogen is a polysaccharide used by animals, fungi, and bacteria to store energy. Glycogen is made up of many bonded glucose molecules. In contrast, glucose, a monosaccharide, is the simplest form of sugar from which carbohydrates can be constructed.

• Polysaccharides are carbohydrates with multiple amino acids bonded together.

• NADH or nicotinamide adenine dinucleotide is a coenzyme that acts as an energy carrier as it transfers electrons from one reaction to the next. NADPH does the same thing as NADH; it's just involved in photosynthesis instead.

• \(\text {FADH}_2\) or flavin adenine dinucleotide is a coenzyme that acts as an energy carrier, just like NADH. We use flavin adenine dinucleotide sometimes instead of NADH because one step of the Citric Acid Cycle doesn't have enough energy to reduce NAD+.

• A coenzyme or cofactor is a compound that's not a protein that helps an enzyme function.

• Oxidation occurs when a reactant loses electrons during a chemical reaction. In comparison, reduction occurs when a reactant gains electrons during a chemical reaction.

• ATP, or adenosine triphosphate, is an organic compound that provides energy to our cells.

• Fatty Acids are the building blocks of lipids, the organic compounds that store energy and make up the cell membrane. Examples of lipids are oils and waxes.

• DNA stands for deoxyribonucleic acid. It is a double-stranded molecule that carries around the genetic information of living organisms.

• RNA stands for ribonucleic acid. It is a single-stranded molecule made from DNA that synthesizes proteins.

If the chart above didn't show how complex metabolism is, figure 2 certainly does! To the eye, it just looks like a mess of lines, but these lines represent the connections of all the interconnected processes' reactants, intermediates, and products. Our body really does a lot to keep us alive!

Figure 2: Metabolic pathways in the body illustrated. Daniela Lin, Vaia Originals.

Are you overwhelmed by the big picture yet? Well, don't worry! Next, we'll look at some of the most important examples of metabolic pathways to better understand metabolism.

Examples of Metabolic Pathways

Let's take a look at two of the most vital processes that allow living organisms to gain energy and break it down for usage: photosynthesis and cellular respiration.

Photosynthesis is an Anabolic Pathway

Photosynthesis is the process plants use to make energy.

Plants use these sugars for their own needs, but we can consume the plants to gain their energy. The overall reaction for photosynthesis is:

$$ 6CO_2+ 6H_2O + \text{solar energy} \longrightarrow C_6H_{12}O_6 + 6O_2 $$

The steps to photosynthesis are:

1. Light-dependent reactions: Solar energy is converted to chemical energy in the form of ATP and NADPH.

• Protein complexes and chlorophyll molecules in the photosystems together produce chemical energy.

• Since chemical energy in the form of ATP and NADPH are being formed, this process is anabolic.

2. Light-independent reactions or The Calvin cycle: Uses chemical energy from the light-dependent reactions to form glucose.

• Since glucose is formed from ATP and NADPH, this process is also anabolic.

Cellular Respiration is a Catabolic Pathway

Up next, we have cellular respiration.

The overall reaction for cellular respiration is:

\(C_6H_{12}O_6 + 6O_2 \longrightarrow 6CO_2+ 6H_2O + \text {chemical energy}\)

The steps to cellular respiration are as follows:

1. Glycolysis: Glycolysis is the process of breaking down glucose, making it a catabolic process.

• Glycolysis begins with glucose and ends up broken down into pyruvate.

2. Pyruvate Oxidation

• The conversion or oxidation of pyruvate from glycolysis to acetyl-COA, an essential cofactor.

• This process is catabolic since it involves oxidizing pyruvate into acetyl-COA.

3. Citric Acid Cycle (TCA or Kreb's Cycle):

• Starts with the product from pyruvate oxidation and reduces it to NADH (nicotinamide adenine dinucleotide).

• This process is amphibolic or both anabolic and catabolic.

• The catabolic part occurs when acetyl-COA is oxidized into carbon dioxide.

• The anabolic part occurs when NADH and \(\text {FADH}_2\) are synthesized.

4. Oxidative Phosphorylation:

• Oxidative Phosphorylation involves the breakdown of electron carriers NADH and \(\text {FADH}_2\) to make ATP.

• The breakdown of the electron carriers makes it a catabolic process.

Take notice that the overall reaction of cellular respiration and photosynthesis are almost the opposite. This is because they work in tandem, as plants use the energy from the sun to convert carbon dioxide that other organisms release through cellular respiration into glucose, which we break down for energy.

Simultaneously, plants also release oxygen that we use to breathe and perform cellular respiration with. The broken-down glucose allows us to utilize chemical energy in the form of ATP, which can provide energy for many cellular processes. This "cycle of life" is shown in Figure 3, which is crucial for survival.

Figure 3: Metabolic processes compared. Daniela Lin, Vaia Originals.

Keep in mind that there are other metabolic processes that are not aerobic or do not involve oxygen. These processes are called anaerobic processes, such as fermentation. Anaerobic metabolism can break down carbohydrates for energy in the absence of oxygen.

Scientists think that anaerobic processes like glycolysis evolved many years ago when there was no oxygen in the atmosphere. Despite the complexity of metabolism, living organisms still share some pathways indicating our shared evolutionary history.