Mechanochemistry: The Motor Molecules and the Process of ATP Synthesis, Lecture notes of Biology

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Mechanochemistry

• (^) Mechanochemistry is the coupling of the mechanical and the chemical phenomena on a molecular scale.
• (^) Molecular motors are biological molecular machines that are the essential agents of movement in living organisms.
• (^) A motor may be defined as a device that consumes energy in one form and converts it into motion or mechanical work; for example, many protein-based molecular motors harness the chemical free energy released by the hydrolysis of ATP in order to perform mechanical work

The Motor of Life

• (^) An enzyme within our body's cells called an ATP Synthase.
• (^) Like any other motor it rotates, and surprisingly fast - in fact at about 6,000 revs per minute!
• (^) Further, it is the last word in ultra-miniaturisation, being 200, times smaller than a pinhead!
• (^) We have some 100 trillion (1 followed by 14 zeros) cells, there are in excess of 10 quadrillion (1 followed by 16 zeros) of these amazing ultra-tiny little motors which drive our bodies and upon which our very lives depend!
• (^) The ATP Synthase motor's job is to manufacture a little molecule called ATP - short for Adenosine triphosphate - which is of enormous importance for the successful functioning of our bodies.

The food we eat is ultimately converted into energy

Mitochondrion:

Site for ATP synthesis

The Respiratory chain

• (^) An electron transport chain (ETC) couples electron transfer between an electron donor (such as NADH) and an electron acceptor (such as O 2 ) with the transfer of H+^ ions (protons) across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate.
• (^) If protons flow back through the membrane, they enable mechanical work, such as rotating bacterial flagella. ATP synthase, an enzyme converts this mechanical energy into chemical energy by producing ATP, which powers most cellular reactions.

• (^) Oxidative phosphorylation begins with the entry of

electrons into the respiratory chain via electron

carriers- nicotinamide nucleotides (NAD or NADP)

or flavin nucleotides (FMN or FAD).

• (^) NAD+^ + 2H+^ + 2e-^  NADH + H+
• (^) NADP+^ + 2H+^ + 2e-^  NADPH + H+
• (^) FMN or FAD can accept 1 e-^ + 1 H+^ to become

semiquinone form or 2 e-^ + 2 H+^ to form FMNH 2 or

FADH 2.

Respiratory chain consists of four complexes

• (^) Complex I (NADH coenzyme Q reductase): accepts electrons

from the Krebs cycle electron carrier nicotinamide adenine

dinucleotide (NADH), and passes them to coenzyme UQ

(ubiquinone)

• (^) Complex II (succinate dehydrogenase): also passes electrons

to UQ.

• (^) Complex III (cytochrome bc1 complex): passes electrons to

cyt c

• (^) Complex IV (cytochrome c oxidase) recieves electrons from

cyt c and uses the electrons and hydrogen ions to reduce

molecular oxygen to water.

Complex I

• Two electrons are removed from NADH and

transferred to ubiquinone (Q). The reduced

product, ubiquinol (QH 2 ) freely diffuses within

the membrane, and Complex I translocates

four protons (H+) across the membrane, thus

producing a proton gradient.

Complex II

• Additional electrons are delivered into the

quinone pool (Q) originating from succinate

and transferred (via FAD) to Q.

Complex IV

• four electrons are removed from four

molecules of cytochrome c and transferred to

molecular oxygen (O 2 ), producing two

molecules of water. At the same time, four

protons are translocated across the

membrane, contributing to the proton

gradient.

Proton gradient powers synthesis

of ATP

• Flow of electrons from NADH to oxygen is an

exergonic process which is coupled to ATP

synthesis, an endergonic process.

ATP motors

• (^) ATP synthase ( mitochon-drial ATPase or F1-F ATPase or Complex V) is an important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate (ATP).
• (^) ATP is the most commonly used "energy currency" of cells from most organisms.
• (^) It is formed from adenosine diphosphate (ADP) and inorganic phosphate (Pi), and needs energy.
• ATP synthase + ADP + Pi → ATP Synthase + ATP

ATP synthase

• Is located within the mitochondria

• ATP synthase consists of 2 regions

• the FO portion is within the membrane.
• (^) The F 1 portion of the ATP synthase is above the membrane, inside the matrix of the mitochondria.