Introduction to Mobile Energy Carriers
Energy is moved around and transferred within the cell in a variety of ways. One critical mechanism that nature has developed is the use of recyclable molecular energy carriers. While there are several major recyclable energy carriers, they all share some common functional features:
Properties of Key Cellular Molecular Energy Carriers
- We think of the energy carriers as existing in "pools" of available carriers. One could, by analogy, consider these mobile energy carriers analogous to the delivery vehicles of parcel carriers—the company has a certain "pool" of available vehicles at any one time to pickup and make deliveries.
- Each individual carrier in the pool can exist in one of multiple distinct states: it is either carrying a "load" of energy, a fractional load, or is "empty". The molecule can interconvert between "loaded" and empty and thus can be recycled. Again by analogy, the delivery vehicles can be either carrying packages or be empty and switch between these states.
- The balance or ratio in the pool between "loaded" and "unloaded" carriers is important for cellular function, is regulated by the cell, and can often tell us something about the state of a cell. Likewise, a parcel carrier service keeps close tabs on how full or empty their delivery vehicles are—if they are too full, there may be insufficient "empty" trucks to pick up new packages; if they are too empty, business must not be going well or it is shut down. There is an appropriate balance for different situations.
In this course, we will examine two major types of molecular recyclable energy carriers: (1) the adenine nucleotides, specifically: nicotinamide adenine dinucleotide (NAD+), a close relative, nicotinamide adenine dinucleotide phosphate (NADP+), and flavin adenine dinucleotide (FAD2+) and (2) nucleotide mono-, di-, and triphosphates, with particular attention paid to adenosine triphosphate (ATP). Each of these two types of molecules is involved in energy transfer that involves different classes of chemical reactions. Adenine nucleotides are primarily associated with redox chemistry, while the nucleotide triphosphates are associated with transfers of energy that are linked to the hydrolysis or condensation of inorganic phosphates.