Introduction to Metabolism
Metabolism is the entire network of chemical reactions carried out by living cells. Metabolites are small molecules intermediary in the degradation or biosynthesis of biopolymers. Intermediary metabolism is the term used to refer to reactions involving there intermediaries. Catabolic reactions degrade molecules to liberate smaller molecules and energy. Anabolic reactions synthesize biomolecules.
Catabolism supplies the energy for biosynthetic pathways. This energy is in one of two forms: adenosine triphosphate (ATP) or the reduced coenzymes NADH or NADPH.
Metabolism can be divided into the metabolism of the four major biomolecules: carbohydrates, lipids, amino acids and nucleotides. Each of these molecules involve distinct sequences of metabolic reactions called pathways. Pathways may be linear, may branch, or may be circular or spiral.
Defining the beginning or end of a pathway is difficult. Metabolism usually leads to products that undergo further change. The start or end point of a pathway may be assigned somewhat arbitrarily, according to tradition or ease of study. It is possible to link reactions and pathways to describe extended metabolic routes. Metabolism has multiple steps, and intermediates are merely precursors to the final product. The interdependence of metabolic pathways means that we must not only understand individual pathways, but their interrelationships as well.
oops! still working...
Take Quiz: [Q1] [Q2] [Q3]
Pathway Regulation
Metabolism occurs in multiple steps for several reasons. First, energy inputs and outputs need to be controlled in order to conserve energy in some form, instead of loosing it as heat. Second, enzymes have limited reaction specificity, each is able to catalyze a single step in the pathway. Finally, intermediates shared by different pathways allow the exchange of energy and chemical compounds between pathways.
Metabolic pathways proceed in one direction. The flow of materials through the pathway depends on the supply of substrate and removal of products, (which occur at multiple points along the pathway, as well as the activities of the enzymes catalyzing the individual reactions.
Regulation of metabolic pathways take place through regulatory enzymes. Enzymes themselves can be controlled be regulating the enzyme location (regulated transport), enzyme amount (rate of synthesis or degradation) or enzyme activity via covalent modifications or effector molecules. Types of enzyme regulation in metabolic pathways include feedback inhibition and feed-forward activation. In feedback inhibition, an excess of product inhibits the regulatory enzyme at an earlier step. In feed-forward activation, an early metabolite activates an enzyme further down the pathway.Take Quiz: [Q1] [Q2] [Q3]
Energy Transfer
When a reacting system is not at equilibrium, the tendency to move towards equilibrium represents a driving force of magnitude equal to the free-energy change (delta G) of the reaction. The greater the amount of energy released, the further the reaction proceeds towards product formation before reaching equilibrium.
In more generic terms, delta G is related to the quilibrium of a reaction:
delta G = delta Gº + RT ln ([products] / [reactants])
where delta Gº is the standard free energy change for the reaction at 25ºC (298 K), 1 atm and 1.0 M. R is the gas constant (1.987 cal / mol K or 8.315 J / mole) and T is the absolute temperature.
Delta G is a measure of how far a reaction is frm equilibrium and represents the amount of work that can be done by a reaction. You get more useful work from reactions far from equilibrium.
What drives a system to react in certain direction is the ratio of products to reactants [P] / [R]. When delta G = 0, the reaction is at equilibrium and the concentrations of products and reactants will not change.
In an exergonic reaction, delta G < 0 and the reaction happes in the direction of the products, releasing energy until equilibrium is reached. Initial reactant concentrations are higher than equilibrium concentrations.
In an endergonic reaction, delta G > 0 and the reaction happens in the direction of the reactants, absorbing energy until equilibrium is reached. Initial product concentrations are higher than equilibrium concentrations.
The delta G of any reaction proceeding spontaneously in its normal direction (Reactants to Products) towards equilibrium is always negative, becomes less negative as the reaction proceeds, and is zero at the point of equilibrium, indication that no more work can be done.
Take Quiz: [Q1] [Q2] [Q3]
Delta G of Biological Systems
Topic not yet available
Take Quiz: [Q1] [Q2]
Advance Topics:
Energy Molecules
Topic not yet available
Take Quiz: [Q1] [Q2] [Q3]
Carbohydrate Metabolism
Topic not yet available
Take Quiz: [Q1] [Q2] [Q3]
Continue
to "Enzymes" or take a test: [T1]
[T2] [T3].
Need more practice? Answer the following review questions:
Questions not yet available