Key Concepts of Metabolism --Lecture 1

Lecture 1 - Key Concepts of Metabolism

                             

Part I: Metabolism:

•   The sum of all the chemical processes whereby is made available and used by the cells of the body
•   Metabolism is the sum of all proce
  sses that allow a cell or organism to (i) extract energy from its environment, and (ii) to synthesize the macromolecules required for life.
  Metabolic Pathways:

•   Metabolic pathways are categories into
•   Anabolic (endergonic) pathways  Catabolic pathways (exoergonic)

 Amphibolic pathways occurs in the crossroads of metabolism

 Normal metabolism include adaptation to starvation, exercise, pregnancy and lactation

 Abnormal metabolism may result from nutritional deficiency, enzyme deficiency, abnormal secretion of hormones or action of drugs and toxins

Types: Catabolism, Anabolism

 Metabolism can be divided into two major classes: catabolism, converting fuel molecules into usable energy, and anabolism, utilizing this energy to generate specialized molecules, building blocks for macromolecules, and the macromolecules themselves.

Comparison Between Anabolism and Catabolism:

Figure 1: overview of metabolism pathways 1

Catabolism:

Stage 1: Hydrolysis of complex molecules to their building blocks. Proteinsamino acids Polysaccharidessimple sugars Fatsglycerol, fatty acids Preparatory, no energy capture. Stage 2: Conversion of building blocks Insignificant ATP generation. Stage 3: ATP is produced from complete oxidation of acetyl CoA unit. Citric acid cycle and oxidative phosphorylation are the final common pathways of oxidation of fuel molecules.

Regulation of Metabolism:

A) Signals within the cells: three major ways:

1. Amount of enzymes (regulated via rate of transcription).
2. Catalytic enzymes activity (Allosteric control, phosphorylation, feedback inhibition).

3. Accessibility of substrates (transfer of substrates into cell compartments)

compartimentalization of opposed reactions.

B) Communications between cells:

- Signal transduction mechanisms (will be covered in the next lecture). 


Part II: Bioenergetics:

• It describes the transfer and utilization of energy in biologic systems (unlike kinetics that measures how fast the reaction occurs).
• It concerns only the initial and final energy states of reaction components, neither the mechanism nor how much time is needed for the chemical change to take place.
  AB
• It predicts if a chemical reaction is possible.

Free Energy (Gibb’s free energy [G]):

 It is the energy available to do work.

 It predicts the direction in which a chemical reaction will spontaneously proceed.

Free Energy AB

The direction of the reaction: from A to B

AB Free Energy

The direction of the reaction: from B to A

•   Adult human require 2400 – 2900 Kcal (1 kcal = 4.18 Joule ) from metabolic fuel each day.
•   Calori:

 It is defined as an energy required to heat 1 g of water from 14.5-15.5oC

•   40 -60 % from carbohydrate
•   30 -40 % lipids
•   10 – 15 % proteins.
•   In starving state glucose spared for use by the central nervous system and
  erythrocytes.
  Free Energy Change:
   Free Energy Change (ΔG) :
  •   It is the change in free energy when reactants and products are present at any
    specified concentration
  •   ΔG = free energy of products - free energy of reactants.
  •   It predicts the direction of a reaction at any specified concentration of reactants
    and products (according to the sign of ΔG).
    
    
     Standard Free Energy Change (ΔGO) :
     It is the change in free energy when reactants and products are present at a
    
    
    
    concentration of 1 mol/L.
     It is useful in comparing the energy changes of different reactions.
    
    

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Types of Reaction-Based Bioenergetics (Sign of ΔG):

 It predicts the direction of a reaction at constant temperature and pressure, thus for the following reaction;

AB

1. Exergonic: If ΔG is a negative number i.e. there is a net loss of free energy, the reaction goes spontaneously from A to B.
2. Endergonic: If ΔG is a positive number (i.e. there is a net gain of free energy, the reaction does not go spontaneously from A to B). So energy must be added to the system to make the reaction go from A to B.
3. Isoergonic: If ΔG is zero (0); the reactants are in equilibrium

ΔG of the Forward and Backward Reaction:

 ΔG of the forward reaction (A B) is equal in magnitude but opposite in sign to that of the backward reaction (B A). For example, if ΔG of the forward reaction is - 70 cal/mol, then ΔG of the backward reaction is + 70 cal/mol.

Free energy

A Forward reaction B ΔG

(100 cal/mol)

Free energy

(30 cal/mol) - 70 cal/mol

Backward reaction

B AΔG

(30 cal/mol) (100 cal/mol) + 70 cal/mol

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Biochemistry 2 – Dental Students 2018/2019

Dr. Khaled A. Ahmed

ΔGs of a Pathway are Additive:

 In the pathway A→B→ C →D, as long as the sum of the ΔGs of the individual reactions is negative, the pathway can proceed even if some individual reactions of the pathway have a positive AG.

- Example:

Energy Transfer in Metabolism:

1. 1)  ATP-ADP Cycle
2. 
   
3. 2)  Electron Transport Chain
4. 
   
   
   
   

Why do organisms need energy?

 Performance of mechanical work

 Active transport of molecules and ions

 Synthesis of macromolecules and biomolecules from simple precursors

Photosynthesis:

•   It is the process by which plants and some bacteria use the energy from sunlight to produce sugar, which cellular respiration converts into ATP, the "fuel" used by all living things.
•   The conversion of unusable sunlight energy into usable chemical energy is associated with the actions of the green pigment chlorophyll.