Bioenergetics The tiny hummingbirds can store enough fuel to fly a distance of

distance of 500 miles without resting. This achievement is possible

by living cells. Metabolism also includes coordination, regulation and energy

Most organism use the same general pathway for extraction and utilization of energy.
All living organisms are divided into two major classes:
Autotrophs – can use atmospheric carbon dioxide as a sole source of carbon for the synthesis of macromolecules. Autotrophs use the sun energy for biosynthetic purposes. Heterotrophs – obtain energy by ingesting complex carbon-containing compounds.
Heterotrophs are divided into aerobs and anaerobs.

internal concentrations of inorganic ions, metabolites and enzymes
2. Organisms extract

acid biosynthesis)
A sequence of reactions that has a specific purpose

Metabolic pathway may be:

energy
Anabolic reactions - synthesize molecules for cell maintenance, growth

Metabolic pathways can be grouped into two paths – catabolism and anabolism

Catabolism is characterized by oxidation reactions and by release of free energy which is transformed to ATP. Anabolism is characterized by reduction reactions and by utilization of energy accumulated in ATP molecules.

Catabolism and anabolism are tightly linked together by their coordinated energy requirements: catabolic processes release the energy from food and collect it in the ATP; anabolic processes use the free energy stored in ATP to perform work.

multi-step pathways provide energy in smaller stepwise amounts
Each enzyme

Metabolism Proceeds by Discrete Steps

Single-step vs multi-step pathways

A multistep enzyme pathway releases energy in smaller amounts that can be used by the cell

changing conditions
Most pathways are irreversible
Flux - flow of material

Metabolic Pathways Are Regulated

level

synthesis by inhibiting an early step (usually the first “committed”

Feedback inhibition

Metabolite early in the pathway activates an enzyme further down the pathway

Feed-forward activation

covalent modification
Protein kinases phosphorylate enzymes (+ ATP)
Protein phosphatases remove phosphoryl

Covalent modification for enzyme regulation

cascade
The initial signal may be amplified by the “cascade” nature

lipids to respective building blocks.
Stage II. Amino acids, fatty

a final common pathway.
Different proteins, fats and carbohydrates enter the

Anabolism can also be divided into stages, however the anabolic pathways are characterized by divergence.
Monosaccharide synthesis begin with CO2, oxaloacetate, pyruvate or lactate. Amino acids are synthesized from acetyl CoA, pyruvate or keto acids of Krebs cycle. Fatty acids are constructed from acetyl CoA.
On the next stage monosaccharides, amino acids and fatty acids are used for the synthesis of polysaccharides, proteins and fats.

a cell
- simultaneous operation of opposing metabolic paths
- high local

Compartmentation of Metabolic Processes in Cell

cell.

Oxidation-reduction reactions are those in which electrons are transferred from one molecule or atom to another

Enzymes: oxidoreductases

Coenzymes: NAD+, NADP+, FAD+, FMN+

Example:

one molecule to another (intermolecular) or group transfer within a

Enzymes: transferases

Examples:

Phosphorylation Acylation Glycosylation

into two molecules
- esterases

Enzymes: hydrolases

Example:

a double bond in a nonhydrolytic (without water), nonoxidative elimination

Example:

Enzymes: lyases

1. Intramolecular hydrogen atom shifts changing the location of

Enzymes: isomerases

Example:

6. Bond formation reactions using energy from ATP
Enzymes: ligases (synthetases)

intermediates. Use sensitive isotopic tracers (3H, 14C etc)
Verify pathway steps

Experimental Methods for Studying Metabolism

of the mitochondria contains pyruvate dehydrogenase complex

pathway
Synthesis of glycogen
Degradation of glycogen
Glycolysis
Gluconeogenesis
Lactate
Ethanol
Acetyl Co A

in glucose is released in glycolysis.
Glycolysis is preliminary phase, preparing

Pyruvate formed in the aerobic conditions undergoes conversion to acetyl CoA by pyruvate dehydrogenase complex.

Pyruvate dehydrogenase complex is a bridge between glycolysis and aerobic metabolism – citric acid cycle.

Pyruvate dehydrogenase complex and enzymes of cytric acid cycle are located in the matrix of mitochondria.

OXIDATIVE DECARBOXYLATION OF PYRUVATE

transports pyruvate from the intermembrane space into the matrix in

Entry of Pyruvate into the Mitochondrion

Pyruvate freely diffuses through the outer membrane of mitochon-dria through the channels formed by transmembrane proteins porins.

3 enzymes, 5 coenzymes and other proteins.
Conversion of Pyruvate

Pyruvate dehydrogenase complex is giant, with molecular mass ranging from 4 to 10 million daltons.

Electron micrograph of the pyruvate dehydrogenase complex from E. coli.

TPP (thiamine pyrophosphate), lipoamide, HS-CoA, FAD+, NAD+.

TPP is a prosthetic

The building block of TPP is vitamin B1 (thiamin); NAD – vitamin B5 (nicotinamide); FAD – vitamin B2 (riboflavin), HS-CoA – vitamin B3 (pantothenic acid), lipoamide – lipoic acid

classic example of multienzyme complex
The oxidative decarboxylation of pyruvate catalized

wheel – the citric acid cycle – to generate energy

Acids
Amino Acids
The citric acid cycle is the final common pathway

Most fuel molecules enter the cycle as acetyl coenzyme A.

the reactions of the citric acid cycle take place inside

Hans Adolf Krebs. Biochemist; born in Germany. Worked in Britain. His discovery in 1937 of the ‘Krebs cycle’ of chemical reactions was critical to the understanding of cell metabolism and earned him the 1953 Nobel Prize for Physiology or Medicine.

condenses with a two-carbon acetyl unit to yield a six-carbon

The function of the citric acid cycle is the harvesting of high-energy electrons from acetyl CoA.

reaction with C-C bond formation
Addition of C2 unit (acetyl) to

of cis-aconitate
Stereospecific addition of H2O to cis-aconitate to form isocitrate

reaction)
One of four oxidation-reduction reactions of the cycle
Hydride ion from

identical mechanisms
E1 - a-ketoglutarate dehydrogenase (with TPP)

is conserved as GTP or ATP in higher animals (or

clusters
Embedded in the inner mitochondrial membrane
Electrons are transferred from

6. The Succinate Dehydrogenase Complex

of fumarate to form L-malate
Only the L isomer of malate

enter the cycle in the form of acetyl CoA.

9 ATP (2.5 ATP per NADH, and 1.5 ATP per FADH2) are produced during oxidative phosphorylation.
1 ATP is directly formed in the citric acid cycle.
1 acetyl CoA generates approximately 10 molecules of ATP.

catabolic and anabolic).

Functions of the Citric Acid Cycle

The cycle is involved in the aerobic catabolism of carbohydrates, lipids and amino acids.

Intermediates of the cycle are starting points for many anabolic reactions.

Yields energy in the form of GTP (ATP).

Yields reducing power in the form of NADH2 and FADH2.

modulators
(2) Covalent modification of cycle enzymes
(3) Supply of acetyl CoA

Three enzymes have regulatory properties
citrate synthase (is allosterically inhibited by NADH, ATP, succinyl CoA, citrate – feedback inhibition)
isocitrate dehydrogenase (allosteric effectors: (+) ADP; (-) NADH, ATP. Bacterial ICDH can be covalently modified by kinase/phosphatase)
-ketoglutarate dehydrogenase complex (inhibition by ATP, succinyl CoA and NADH

cycle provides intermediates for biosyntheses