Introduction to Metabolism

Introduction to Metabolism | Fuels for Energy | Energy Metabolism During Exercise

Overview of Energy Metabolism | Maximal Power and Capacity of the Three Energy Systems

Introduction to Metabolism

An exergonic reaction is one that gives up energy and is usually a spontaneous reaction. In the example A –> B below, the energy content of the product B is less than that of substrate A. Reaction C –> D is an example of an endergonic, nonspontaneous, uphill reaction. Here, the energy level of the product is greater than that of the substrate. This reaction will not occur unless there is an energy input. In the biological world, endergonic reactions such as C –> D are linked, or coupled to, and driven by exergonic reactions such as A –> B. In the example given, the overall process is A –> D. Note that the energy level of D, the final product, is less than that of A, the initial substrate.

Even though the reaction A® B is downhill and spontaneous, it is not likely to happen because there is an energy barrier, called energy of activation, that must first be overcome. In other words, some energy has to be put in to activate the system and "prime the pump." Enzymes are important because they have the effect of lowering the energy of activation and allowing the reaction to proceed spontaneously.

When a reaction has reached completion or equilibrium, there is no longer any net change from substrate to product. The equilibrium constant Keq is used to denote the concentrations of substrate to product at equilibrium. If the Keq is large, the reaction has potential for driving a biological system.

K_eq = [products] / [substrates]

While a chemical reaction would be able to proceed without assistance from an enzyme, it may take several hours to reach equilibrium. However, the reaction would reach equilibrium within a second or less in the presence of the specific enzyme. Enzymes accelerate reactions by factors of more than a million. They are highly specific but the rate of reaction varies with the substrate concentration. The maximal rate of reaction, V_max, is attained when the enzyme is saturated with substrate.

Control of these reactions can be exerted by the product formed several reactions down the pathway. As the concentration of the end-product increases, it exerts a negative feedback inhibition on an earlier enzyme in the pathway. Enzymes can also be inhibited by specific molecules, drugs, or poisons. A competitive inhibitor diminishes the rate of reaction by reducing the number of enzymes that can bind to a substrate.

Summary

A metabolic pathway is a series of reactions in which the product (P) of one reaction acts as a substrate (S) for the next reaction

A ––> B ––> C ––> D ––> E

1. every step must have an enzyme to catalyze the reaction
2. all of the components are referred to also as metabolites
3. metabolites within the pathway are referred to also as intermediates
4. concentrations of most intermediates remain fairly constant
5. flux is the number of molecules going through the pathway
6. most reactions in the pathway are at or near equilibrium
7. equilibrium reactions have a high enzyme activity
8. the P/S ratio controls the direction and rate of the reaction
9. non-equilibrium reactions act as control points in the pathway
10. non-equilibrium reactions have a low enzyme activity
11. regulation of non-equilibrium reactions are controlled by changing enzyme activity

Oxidation-Reduction Reactions

• oxidation - something loses electrons
• reduction - something gains electrons
• often times, dehydrogenation reactions are oxidative reactions in which electrons are lost as part of hydrogen atoms or hydride ions.
• two major coenzymes involved in energy metabolism are FAD and NAD
• FAD (oxidized form) can accept two hydrogen atoms to become NADH₂ (reduced form)

Notes on energy metabolism

• ATP is the universal "currency" for energy in biological systems
• ATP is a nucleotide consisting of an adenine, a ribose, and a triphosphate unit
• active form of ATP is usually a complex of ATP with Mg²⁺ or Mn²⁺
• a large amount of energy is released when ATP is hydrolyzed to ADP and Pi (inorganic phosphate)
• ATP turnover is very high-typical resting human consumes ~40 kg of ATP per 24 hr
• during high-intensity exercise, ATP turnover approaches 0.5 kg/min
• ATP can be formed extremely rapidly from carbohydrates (CHO) in the glycolytic pathway, however this pathway causes muscular fatigue very quickly
• CHO and fats are completely oxidized to CO₂, this process removes and transfers electrons through a metabolic pathway; the ultimate electron acceptor is O₂
• electrons are not transferred directly to O₂, rather they are transferred by electron carriers: nicotinamide adenine dinucleotide (NAD⁺) and flavin adenine dinucleotide (FAD)

Fuels for Energy

• only nutrients that contain energy are: carbohydrates (CHO), fats or triglycerides (TG), and proteins
• primary fuels (substrates) are carbohydrates and fats
• CHOs and fats only contain carbon (C), hydrogen (H), and oxygen (O); the end-products of CHO and fat metabolism are CO₂, H₂O, and ATP
• CHOs used by the body exist primarily as blood glucose and glycogen (a storage of glucose molecules within tissues) from muscle and liver
• fats - primary fat is triglyceride (TG); TG exists as 3 fatty acids (FAs) each connected to a glycerol molecule
• proteins - only supply 5-10% of energy; composed of subunits called amino acids which are mostly used as building materials for other metabolic functions

Energy Metabolism During Exercise

• energy in substrates is converted into "usable" energy by the body cells; muscles convert (transduce) chemical energy from foods into mechanical energy for movement
• adenosine triphosphate (ATP) - most commonly used high-energy molecule

adenine-ribose-P-P~P

• role of ATP in skeletal muscle contraction:

Ca²⁺

ATP + actin + myosin <=====> actomyosin + ADP + Pi + energy

relaxation <=====> contraction

• athletic events can be classified into three groups: power, speed, and endurance; there is an energy production system specific to each of these classifications: immediate, anaerobic, and aerobic, respectively

Overview of Energy Metabolism

• energy is released to muscles from the breakdown of ATP
• three energy systems generate ATP-immediate, anaerobic, and aerobic

Immediate energy system (ATP-PCr system)

• fastest supply of ATP
• uses no O₂

Anaerobic glycolysis system

• primary energy system when ATP requirements are high
• uses no O₂

Aerobic system

• primary system when ATP requirements are low
• uses O₂ in last step

All energy systems are used continuously! However, their relative contribution to the total ATP production varies depending upon the energy demands placed on the muscles. Examples:

a) very-short maximal exercise (< 5 s)-relies primarily on the immediate energy system for ATP

b) 10-120 s of maximal exercise-relies primarily on the anaerobic (glycolytic) system for ATP although, both the immediate and aerobic systems contribute to the total ATP production

c) > 180 s maximal exercise relies primarily on the aerobic system although, the anaerobic system still contributes to ATP production

Maximal Power and Capacity of the Three Energy Systems

System	Max Power (kcal•min^-1)	Max Capacity (kcal)
Immediate	36	11
Anaerobic glycolysis	16	15
Aerobic	10	480