Intermediary Metabolism (Basic Notes)

One of the major reasons for studying Biochemistry is to understand how living organisms utilize the chemical energy in their environment to carry out their biochemical activities. It is called intermediary metabolism.

This requires an understanding of the simpler principles of physical chemistry and thermodynamics as they apply to live organisms.

Also required is an appreciation of the so-called “energy-rich compounds’ that permit the living organism to trap and subsequentially utilize the chemical energy contained in the food materials it consumes.

What is Metabolism?

The sun is the ultimate source of energy for all life on the planet Earth. That energy, like sunlight, is trapped by photosynthetic organisms and used to convert CO2 into the organisms’ cellular material, composed mainly of proteins, carbohydrates, and lipids, but also smaller amounts of nucleic acids, vitamins, coenzymes, and other compounds.

Some of those products of photosynthesis (carbohydrates and lipids) are, in turn, utilized by non-photosynthetic organisms, mainly animals, as a source of energy for growth, development, and reproduction.

Types of Metabolic Reactions

There are THREE types of reactions are present in Metabolism. They are

Bonds Cleavage and its types

In intermediary metabolism, a covalent bond (σ-bond) is formed with the help of two atoms by the sharing of a pair of electrons. When the two atoms are separated from each other, bond fusion (or cleavage) is said to have taken place.

The cleavage process can occur in two ways: a) homolytic cleavage and   b) Heterolytic cleavage.

a) Homolytic cleavage

When one electron of the bonding pair goes with each of the departing atoms, the fission is symmetrical (0r) and homogenous, and is called “Homolytic cleavage” (or) “Homolysis”.

A-B (or) A:B    –>  A^. + .B

• The two fragments that are produced as a result of hemolytic fission carry an odd electron each and are called “Free Radicals”.
• These are transitory and at once react with other radicals (or) molecules by giving one more electron to restore the stable bonding pair.

b) Heterolytic Cleavage

When a covalent bond breaks in a fashion that both the bonding electrons are appropriated by one of the two departing fragments (atoms or groups), it is said to have undergone “Heterolytic cleavage” (or) “Heterolysis“.

A-B (or) A:B –> A:^– + B⁺

• The heterolytic cleavage yields one positive & one negative ion.
• These reactions take place more readily in polar solvents like water and are catalyzed by the presence of ionic catalysts (e.g.: acidic or basic).

Bond Length

Atoms involved in the formation of a bond cannot come any closer to each other than a certain distance where the potential energy is at its minimum.

The average distance between the nuclei of two atoms bonded to each other is used under the name of bond length” or “bond distance.”

Thus, bond length may be defined as the average distance between the centers of the nuclei of the two bonded atoms. It is expressed in angstrom (A0) units (1A0 = 10^-8 cm).

Points to remember:

• Multiple bonds (double or triple bonds between two atoms) are always shorter than the corresponding single bond.
• Bond length decreases with the increase in “s” character, since an s-orbital is smaller than a p-orbital. Thus

1. sp³ C-H = 1.093 A^o (in alkanes)
2. sp²C-H = 1.087 A^o (in Alkenes)
3. sp C-H = 1.057 A^o (in Alkynes)

• Since bond distance is the sum of the ionic or covalent (atomic) radii of the two concerned atoms, the factors, and trends observed in the ionic or atomic radii will apply on the bond distance.

Bond Angle

In the molecules are made up of three or more atoms, the average angle between the bonded orbitals is known as a bond angle, Ф. Fundamentally, the value of the bond angle largely depends on the nature of the bonds concerned. The bond angle in the water molecule formed by the overlapping of two s-orbitals of hydrogen atoms with one 2p orbital of an oxygen atom should be 900. The actual H-O-H bond angle is 1040 31’. Bong angles are important for analyzing the behavior of molecules in intermediary metabolism.

Points to remember

• Repulsion between atoms or groups attached to the central atoms: the positive charge, developed due to the high electronegativity of oxygen, on the two hydrogen atoms in water causes repulsion among themselves, which increases the bond angle, H-O-H from 900 to 1050.
• Hybridization of bonding orbitals: hybridization of bonding orbitals also plays a very important role in determining the values of bond angles.

• Repulsion due to non-bonded electrons: the deviation of the normal tetrahedral bond angle of 1090 28’ in H2O (1050), NH3 (1070), etc. although their central atoms are in an sp3 hybridized state.

Classification of Metabolic Reagents

Now let us discuss the metabolic reagents in intermediary metabolism.  The presence of a charge on the reactant certainly helps the attack of the reagent on the reactant, but it is far from essential.

Indeed, the requisite unsymmetrical charge distribution may be induced by the mutual polarization of the reagent and reactant on their close approach, as when bromine is added to ethylene.

Thus, because of the above principle, most of the reagents can be classified into the following two types.

1.Electrophilic reagents (Electrophiles)

The name implies electrophilic (electro=electron, phile=love) reagents are electron seeking (or) loving and thus attack the substrate at the point of maximum electron density. Thus, an electrophile is a species having an electron-deficiency atom (or) center.

The electrophilic reagent may be a positively charged species or a neutral molecule with an electron-deficient center. Some important electrophiles are given below:

E.g.: H⁺, Br⁺, NH₄⁺, SO₃, R-N⁺≡N,

It is interesting to note that since the electrophiles are capable of accepting electrons pair, they are Lewis acids.

Reactions involving the attack of electrophiles are known as an electrophilic reaction.

2. Nucleophilic reagents (Nucleophiles)

The reagent possessing at least one lone pair of electrons is known as nucleophilic reagents (or) nucleophiles (nucleo=nucleus; phile=love).

Since they possess a higher electron density, they attack the substance at the point of minimum electron density. The nucleophilic reagent may be a negatively charged species or neutral molecule with free electron pair (s).

Some important nucleophiles are given below:

E.g.: OH^–, Br^–, CN^–, C00^–, R.C≡C^–, CH₃COC^–H₂, H-O-H, R-O-R, R-O-H, R-S-H, R-NH2, R2-NH2, :NH₃, LiAlH₄.

It is interesting to note that since the nucleophiles are capable of donating electrons pair, they are Lewis bases.

The reaction involving the attack of nucleophiles is referred to as a nucleophilic reaction.

Metabolic Phases

Thousands of chemical reactions are taking place inside a cell in an organized, well co-organized, and purposeful manner; all these are collectively called Intermediary Metabolism. The term Metabolism came from the Greek language (metabole^G = Change; ballein^G= to throw).

In living organisms, there are three different phases of metabolic phases done. These are :

1. Primary Metabolism: The primary metabolic state
   includes digestion.
2. Secondary Metabolism: The secondary metabolism is also called the intermediary metabolism. It includes catabolism (breakdown process of biomolecules) and anabolism. (Synthesis process of
   biomolecules).
3. Tertiary Metabolism: Biological oxidation and Oxidative phosphorylation are included in this type of metabolism.

Intermediary metabolism serves the following purposes

Metabolism is the overall process through which living systems acquire and utilize the free energy they need to carry out their functions. The terms “catabolism” and “anabolism” were coined by the physiologist Gasket in 1886.

• Chemical energy is obtained from the degradation of energy-rich nutrients.
• Food materials are converted into the building block precursors of cellular macromolecules. These building blocks are later made into macromolecules, such as proteins, nucleic acids, polysaccharides, etc.
• Biomolecules required for specialized functions of the cell are synthesized. Specialized functions of the cell are synthesized.
• Metabolic pathways are taking place with the help of sequential enzyme systems. These pathways are regulated at three levels:

1. Regulation through the action of allosteric enzymes, which increases or decreases in the activity under the of effector molecules.
2. Hormonal regulation: Hormones are chemical messengers secreted by different endocrine glands.
3. Regulation at the DNA level: The concentration of the enzyme is changed by regulation at the level of synthesis of the enzyme.

• Metabolism is broadly divided into TWO types: a) Catabolism and b) Anabolism

a) Catabolism

It is a degradation process. A degradative process in which complex molecules are broken down into simpler ones; includes processes such as Cellular Respiration and digestion. The meaning of catabolism came from Greek (katabole^G = Throwing down; ballein^G= to throw). It is also spelled as katabolism. The energy released during this process is trapped chemical energy, usually ATP. Eg: Cellular Respiration

b) Anabolism

It is a biosynthesis process. A constructive process in which complex molecules are synthesized from simpler ones; consumes rather than produces cellular energy; includes processes such as photosynthesis and assimilation; the opposite of catabolism. The meaning of the word “anabolism” comes from the Greek (Anabole^G = Throwing up; ballein^G= to throw). This needs energy. Eg: Photosynthesis

Metabolic Pathways

An array of enzyme-catalyzed chemical reactions that bring about transformations of certain organic compounds vital to the organism, constitute “Metabolic pathways” (or) “Metabolic routes”.

Three Principle characteristics of metabolic pathways, there are Irreversible pathways, Typical irreversible metabolic pathways, and Branched metabolic pathways.

1. Irreversible pathways

These are highly exergonic reactions. If two metabolites are metabolically inter-convertible, the pathway from the first to the second must differ from the pathway from the second back to the first. Example of irreversible pathways is Carbohydrate Oxidation, Fatty acid Oxidation, and Heme Biosynthesis.

2. Typical irreversible metabolic pathways

To obtain the product, a metabolic sequence should be essentially irreversible. Examples fro typical metabolic pathways are Glycolysis, Synthesis and Degradation of liver glycogen from (or) to Glucose.

3. Branched metabolic pathways

Some metabolic sequences may have a common path for many steps and then branch into two (or) more separate paths. Examples to the Branched metabolic pathways are, in carbohydrates metabolism, “Glucose-6-Phosphate” act as starting precursor molecule for the following pathways. Glycogen synthesis, EMP pathway (or) Glycolysis, HMP shunt.

Intermediary Metabolism

Metabolism is the term used to describe the interconversion of chemical compounds in the body, the pathways taken by individual molecules, their interrelationships and the mechanisms that regulate the flow of metabolites through the pathways.

All the metabolic activities take place in the Liver. Metabolic pathways fall into THREE categories:

1. Anabolic pathways-These are involved in the synthesis of larger molecules from smaller molecules. E.g.: Proteins from amino acids; Polysaccharides from monosaccharide.
2. Catabolic pathways: These are involved in the breakdown of larger molecules into smaller molecules E.g.: Amino acids from Proteins and Monosaccharide from Polysaccharides.
3. Amphibolic pathways: which occur at the “crossroads” of metabolism, acting as links between the anabolic and catabolic pathways. E.g.: Citric acid cycle.

Pathways that process the Major Products of Digestion

The nature of the diet sets the basic pattern of metabolism. There is a need to process the products of digestion of dietary carbohydrates, lipids, and protein.

These are mainly glucose, fatty acids and glycerol, and amino acids, respectively. All the products o digestion are metabolized to a common product, acetyl~coA, which is then oxidized by the citric acid cycle.

Metabolism can be considered in different types based on the Biomolecule:

1. Carbohydrate Metabolism
2. Amino acid Metabolism
3. Lipids Metabolism
4. Nucleotide Metabolism
5. Porphyrin Metabolism
6. Vitamin Metabolism
7. Mineral Metabolism