Biochem Exam Bioenergetics

Bioenergetics

Bioenergetics is the study of energy changes

accompanying biochemical reactions in living

organisms

• Malnutrition is associated with energy imbalance

in humans

– Undernutrition is low energy consumption or

availability (Protein Energy Malnutrition)

– Overnutrition leads to storage of excess energy

reserves in the body (obesity)

– Starvation (no energy consumption over time) leads to

death when energy reserves are completely depleted

Energy Generation

How do we measure the amount of

energy available to a system for doing

work?

– Gibbs change in free energy (G)

– Two types of chemical reactions involve

changes in energy

• Exergonic reaction – a reaction that proceeds

spontaneously with the loss of free energy (-triangleG)

• Endergonic reaction – a reaction that proceeds

only if sufficient free energy is available to be

gained (+triangleG)

– These are coupled reactions (occur together)

Coupled Reactions

Reactions that gain energy by chemical linkage

to oxidative reactions that lose energy are

called “coupled reactions”

– The conversion of A → B occurs spontaneously

with the release of free energy (-triangleG; exergonic)

– It is coupled to second reaction in which this

released free energy is gained for the conversion of

C  D (+triangleG; endergonic)

• Some free energy released from exergonic reactions will

always remain as surplus and will be lost as heat

Pathways involving Energy

Metabolism – all chemical processes,

including reactions that occur within a living

organism to maintain life

– Catabolism – process where substrates are broken

down or sometimes oxidized with the release of

free energy

– Anabolism – process where reactions are used to

synthesize or build molecules with the gaining of

free energy

• In humans, high energy intermediates are used to store

the free energy between these two processes since they

usually don’t occur together (not coupled) but at

different times

High Energy Intermediates

ATP – adenosine triphosphate (2 high energy

phosphate bonds store the free energy)

– Most common form of chemical energy in humans

– Classified as a nucleotide

– Stored in very small amounts in most cells

– Produced during catabolism

• Creatine phosphate (phosphocreatine)

– Stored in higher amounts in skeletal muscle and

brain

– Helps maintain ATP levels in actively working

tissues/cells for a short period of time

sources of free energy/uses of free energy

Catabolic pathways/processes where ATP

or creatine phosphate are produced

– Oxidative phosphorylation (respiratory chain)

– Citric acid cycle

– Glycolysis

– Phosphagenesis

Anabolic pathways that require or need

ATP as a substrate

– Gluconeogenesis

– Glycogenesis

– Lipogenesis

– Protein synthesis

– RNA and DNA synthesis

ATP Driven Reactions

Allows thermodynamically unfavorable

reactions (endergonic) to occur

• Example; phosphorylation reactions

(1) ATP → ADP + Pi (exergonic: release of

energy)

(-triangleG = -30.5 kJ/mol)

(2) Glucose + Pi → Glucose 6-phosphate + H2O

(+triangleG = +13.8 kJ/mol) (endergonic: needs

energy to proceed)

Energy Generation

Coupled Together:

Glucose + ATP → Glucose 6-phosphate +

ADP

13.8 + (-30.5)

triangleG = -16.7 kJ/mol (heat production)

Thus, the phosphorylation of glucose occurs

irreversibly due to the differing energy values

Biological Oxidations

Humans are dependent on a supply of

oxygen to produce sufficient amounts of

ATP to survive

– Respiratory Chain

• Formation of ATP by which hydrogen atoms react

with O2 to form water

• Oxidation: gain of O2 or loss of H atoms or loss of

electrons

• Reduction: gain of H atoms or gain of electrons,

or loss of O2

– Oxidation does not always require O2

Oxidoreductases

Oxidases – catalyze the removal of

hydrogen atoms from one substrate using

O2 (the other substrate) as a hydrogen

acceptor

– Forms H2O or H2O2

• AH2 + O2 → A + H2O2

– Usually uses Fe or Cu as cofactors

Dehydrogenases – transfer of hydrogen

atoms from one substrate to another

substrate (not O2)

– Major coenzymes used is nicotinamide

adenine dinucleotide (NAD+) or nicotinamide

adenine dinucleotide phosphate (NADP+)

made from niacin (vitamin B3) and flavin

adenine dinucleotide (FAD) or flavin

mononucleotide (FMN) made from riboflavin

(vitamin B2)

– Produces NADH or NADPH or FADH2 or

FMNH2 (reduced coenzymes)

Oxidoreductase: Hydroperoxidases

Hydroperoxidases – use hydrogen

peroxide or other organic peroxides as a

substrate and produces H2O

– Catalase

• 2H2O2 → 2H2O + O2

– Peroxidases: destruction of peroxides through

the conversion of reduced glutathione to

oxidized glutathione

• H2O2 + AH2 → 2H2O + A

Oxidoreductases: oxygenases

Oxygenases – catalyzes the incorporation

of O2 (either both atoms or one) into

another substrate

– Mixed function oxidases (monooxygenases)

• AH + O2 + ZH2 → AOH + H2O + Z

– Liver cytochrome P450 enzymes (heme-

containing monooxygenases) used for

detoxifying and metabolism of drugs and

hydroxylation of steroids

– Superoxide dismutase (prevents oxygen

toxicity due to superoxide anion free radical,

O2-, formation)