Biological Energy, the Mitochondria, Disease & CoQ10
Mitochondria make energy that power life
Humans convert energy stored in proteins, fats, and complex carbohydrates into biologically useful energy in three steps. Step 1 is the digestion and breakdown of macromolecules into fundamental components– sugars, fats, and amino acids. In Step 2, these core components are oxidized to produce carbon dioxide and reduced nicotinamide adenine dinucleotide (NADH). In Step 3, the energy stored in NADH is harnessed through a succession of steps resulting in the generation of adenosine tri-phosphate (ATP) and consumption of oxygen– respiration. ATP is often referred to as the universal energy currency of biology in that it is used to power the vast majority of biological reactions. This step is also termed oxidative phosphorylation, connoting the generation of ATP from ADP + Pi, and the consumption of oxygen. Oxidative phosphorylation takes place within the mitochondria.
Central to life forms is how they derive energy to reproduce, mature and survive. Humans extract energy from biochemicals contained in food. This process at the physiological level is called digestion. Once reduced to their basic components, sugars, fats and amino acids are transported into a specialized organelle– the mitochondria.
The mitochondria perform many functions of which its central one is energy production. While at its most basic level mitochondria operate as combustion engines, converting sugars, fats and amino acids into carbon dioxide, water and energy, a more accurate description is that of a hybrid automobile. They do contain a combustion engine, but also contain an electrical generator and a molecular motor. Mitochondria operate by three basic principles:
The combustion engine converts sugars, fats and amino acids and oxygen to carbon dioxide and water and yield energy.
The generator harnesses the energy yielded in combustion as electron flow, to charge a battery.
The battery drives a molecular motor to make biological energy in the form of adenosine triphosphate (ATP).
The majority of human energy is derived from the mitochondria, and virtually every cell in our body is dependent upon the proper function of mitochondria to meet its energy needs.
Human cells contain a variety of specialized structures that support different functions. These include the housing of genetic information in the nucleus (red); the generation, folding, transport, and processing of proteins by the endoplasmic reticulum (orange) and Golgi apparatus (pink); and the generation of energy by the mitochondria (green). Mitochondria range in size and shape. An individual cell can contain as few as a dozen to tens of thousands of mitochondria, depending on the tissue type and its energy demands. Each mitochondrion contains its own genetic instruction set, or mtDNA genome. The mtDNA genome is 16,569 bases in length, and resembles that of a bacterial genome– harkening to the endosymbiotic theory of mitochondrial origin as a (prokaryotic) bacterium. The genome codes for 37 genes, 24 of which are needed for mtDNA translation and 13 encode for subunits of the respiratory chain. The mitochondrion is a double membraned organelle, containing an inner and outer membrane, an inner membrane space, and a matrix. Contained within the inner membrane is the electron transport system– so named for its role in the transfer of electrons across a succession of proteins complexes. The electron transport system is comprised of groups of proteins and cofactors termed complexes that are responsible for proton pumping and electron transfer. The transient accumulation of protons (H+) within the inner membrane space serves as a chemical battery that powers complex V– ATP synthase. ATP synthase is a composite of multiple proteins that functions as a nano-machine powered by the proton gradient (battery) and drives the reaction of ADP + Pi -> ATP. Electrons destined for complex V are deposited into oxygen to form water. This terminal reaction: 4H+ 4e- + 2O2 -> 2H2O gives rise to the term respiration– connoting the pivotal role oxygen consumption plays in the generating of energy. The coupling of ATP formation and oxygen consumption is also referred to as oxidative phosphorylation.