cellular biochemistry and metabolism (cls 331) dr. samah kotb nasr eldeen
TRANSCRIPT
Cellular Biochemistry and Metabolism(CLS 331)
Dr. Samah Kotb Nasr Eldeen
2Dr Samah KotbLecturer of Biochemistry
CHAPTER 8 BIOENERGETICS & ATP
• Bioenergetics is basically how living systems make use of free energy.
Bioenergetics & ATP
Bioenergetics & ATP
There are 2 types of energy that can be used by systems to do work:-
1. Free Energy
2. Heat Energy
Free energy is the kind of energy that can be
used to do work under conditions of
constant temperature & pressure.
Heat energy can be used to do work only through a change of temperature.
Bioenergetics & ATP:
Heat is not a significant source of energy for
living cells because heat can only do work as it
passes from a zone at one temperature to
another at a lower temperature.
Since living cells have the same temperature
throughout, they cannot make use of heat
energy.
Cells use free energy (G) which can work at
constant temperature and pressure. Free
energy is obtained by animal cells from the
catabolism of energy rich nutrient molecules
whereas plant cells obtain it from solar
radiant.
What do you know about
Anabolic Pathways
And
Catabolic Pathways
Anabolic Pathways
reactions that result in
the synthesis of
biomolecules using
basic unit components
and require an input of
energy to take place .
Catabolic Pathways
reactions through which
energy rich nutrient
molecules are broken
down by chemical
reactions into simple
end products. As a
result of catabolic
pathways energy is
produced and released
to the cell.
Standard free energy change (G) of a chemical reaction:
G is the difference between the free energy
content of the reactants and that of the
products under standard conditions of
temperature and pressure (298k & 1
atmospheric pressure).
• When a reaction results in release of energy
It means that the products contain less free
energy than the reactants. Here G for the
reaction will have a negative value and the
reaction will be catabolic in nature.
A reaction is anabolic and will have a
positive G value if the products contain
more free energy than the reactants. Energy
has to be put into the reaction for it to
proceed.
The free-energy change of a reaction (ΔG) divided into 3 types:
• 1. A reaction can occur only if ΔG is negative. An output of free energy is required to drive such a reaction, Such reactions are said to be exergonic.
• 2. A system is at equilibrium and no net change can take place if ΔG is zero.
• 3. A reaction can occur if ΔG is positive. An input of free energy is required to drive such a reaction. These reactions are termed endergonic.
It means that the products contain less free
energy than the reactants. Here G for the
reaction will have a negative value and the
reaction will be catabolic in nature.
A reaction is anabolic and will have a
positive G value if the products contain
more free energy than the reactants. Energy
has to be put into the reaction for it to
proceed.
Standard free energy change (G) of a chemical reaction:
For every reaction G can be calculated using:-
G = - 2.303 RT log Keq
While:
R = Gas constant
T = Absolute Temp.
Keq = Equilibrium constant
Note: G indicates constant temperature & pressure and
physiological pH 7.2 for cells. Unites of free energy = calorie (cal) or kilocalorie (kcal) /mole.
Units of energy• A calorie (cal) is equivalent to the amount of heat required to
raise the temperature of 1 gram of water from 14.5°C to 15.5°C.
• A kilocalorie (kcal) is equal to 1000 cal.
• A joule (J) is the amount of energy needed to apply a 1-newton force over a distance of 1 meter.
• A kilojoule (kJ) is equal to 1000 J.• 1 kcal = 4.184 kJ.
• The kilocalorie (abbreviated kcal) and the kilojoule (kJ) will be used as the units of energy. One kilocalorie is equivalent to 4.184 kilojoules.
• Consider the reaction
• The ΔG of this reaction is given by
• In which ΔG° is the standard free-energy change, R is the gas constant, T is the absolute temperature, and [A], [B], [C], and [D] are the molar concentrations (more precisely, the activities) of the reactants.
• ΔG° is the free energy change for this reaction under standard conditions that is, when each of the reactants A, B, C, and D is present at a concentration of 1.0 M (for a gas, the standard state is usually chosen to be 1 atmosphere).
• Thus, the ΔG of a reaction depends on the nature of the reactants (expressed in the ΔG° term of equation 1) and on their concentrations (expressed in the logarithmic term of equation 1).
• The ΔG of a reaction depends only on the free energy of the products (the final state) minus the free energy of the reactants (the initial state).
• The ΔG of a reaction is independent of the path (or molecular mechanism) of the transformation. The mechanism of a reaction has no effect on ΔG.
• The ΔG provides no information about the rate of a reaction.
G values of pathways can be calculated:
The G value of an overall pathway can be calculated as the
algebraic sum of the G values of the individual reactions
making the pathway:-
Gpathway = G1 + G2 + G3 + G4 + G5
ATP
Chemistry of ATP (Adenosine – tri – phosphate):
ATP is a nucleotide type molecule made of the following
components:-
1. The nitrogenous base adenine
2. The pentose sugar ribose
3. Three phosphate groups
Chemistry of ATP (Adenosine – tri – phosphate):
Thus:-
ATP ADP + Pi G = -7.3 kcal/mole
ADP AMP + Pi G = -7.2 kcal/mole
AMP Adenosine + Pi G = -3.2 kcal/mole
ATP, ADP and AMP are present in all forms of life. They
occur not only in the cytosol of cells but also in the
mitochondria & nucleus. In normal respiring cells ATP
makes up 80% of the three ribonucleotides. ADP & AMP
account for 20%.
Chemistry of ATP (Adenosine – tri – phosphate):
At pH 7, ATP occurs as the multiply charged anion ATP4-
whereas ADP occurs as ADP3-. This is because their
phosphate groups are completely ionized at the intracellular
PH. ATP and ADP occur inside cells as magnesium
complexes:-
ATP4- + Mg2+ (ATP-Mg)2-
ADP3- + Mg2+ (ADP-Mg)ــ
Chemistry of ATP (Adenosine – tri – phosphate):
Inside cells the concentration of ATP remains normally
relatively constantly high. Its rate of formation equals its
rate of hydrolysis. Thus the terminal phosphate group of
ATP undergoes continuous removal & replacement from
the pool of inorganic phosphate during cell metabolism.
G values for some characteristic reactions:-
Super high energy compounds are compounds generated during catabolism. They are phosphorylated compounds. Once formed along a catabolic pathway, they undergo immediate hydrolysis (dephosphorylation). As a result a large amount of energy is released this is used by the cell to synthesize ATP from ADP and the hydrolyzed inorganic phosphate.
G values for some characteristic reactions:-
The Bioenergetics of Muscle Contraction:
The contraction of muscle requires a large amount of
energy that cannot be fulfilled by the ATP stored inside
muscle tissue. In addition to ATP there is a super-high
energy compound stored in muscle cells that plays a
major role in the energetics of muscle. This super-high
energy compound is also present in large concentrations
in other contractile tissues such as brain & nerve tissue.
The Bioenergetics of Muscle Contraction:
This compound is PHOSPHOCREATINE. It serves as a
storage form of high energy phosphate groups. The G
value for the hydrolytic reaction of phosphocreatine is
highly negative (-10.3 kcal/mole). This is greater than
that of ATP. The energy released is sufficient to allow
coupled synthesis of ATP from ADP:-
The Bioenergetics of Muscle Contraction:
Phosphocreatine thus functions to keep the ATP
concentration in muscle cells at constantly high level
whenever some of the ATP of muscle cells is used for
contraction, ADP is formed. Through the action of creatine
kinase phosphocreatine is quickly hydrolyzed and donates
its phosphate group to ADP to form ATP.
The phosphocreatine level inside muscle is 3-4 times
greater than that of ATP and thus stores enough high
energy phosphate groups to keep the ATP level constantly
high during short periods of intense muscular contraction.
The Bioenergetics of Muscle Contraction:
