What is Biosynthesis?

Biosynthesis is an enzymatic process, where simple compounds are used to synthesize macromolecules inside the living cells. It is a multistep process. Products of biosynthesis are essential for the cellular activity, growth, development and survival of living organisms.

It is an anabolic process of metabolism. Examples include Sugar biosynthesis by Calvin cycle (in plants) or Gluconeogenesis, Protein synthesis by Translation, Fatty acid, Lipid and Cholesterol biosynthesis, etc.

Biosynthesis is a major scientific research area, where synthesis is done in vitro using chemicals and also by recombinant methods in microorganisms using enzymes and substrates.

Table of Contents

Biosynthesis Key Features

  • Biosynthesis refers to anabolism.
  • Biosynthesis is a multi-step and multi-enzymatic process.
  • Biosynthesis may occur in one or multiple cell organelles.
  • Biosynthesis is an energy-driven process. Chemical energy is utilised in the process, which we get when a bond is broken. E.g. a terminal high energy phosphate group is hydrolysed from ATP to generate energy.
  • Multiple enzymes are involved in the process. Enzymes catalyse the reaction by decreasing activation energy thereby increasing the rate of reaction.
  • Many enzymes require cofactors for proper functioning such as prosthetic groups, e.g. metal and non-metal ions and coenzymes, e.g. NAD, NADP, FAD, etc.
  • Macromolecules are generally polymers of monomers, so the synthesis of monomers is an important step, e.g. amino acids for protein synthesis, nucleotides for DNA, RNA synthesis.
  • Biosynthesis is controlled at each step. Many enzymatic activities are regulated by a feedback mechanism.

Let us now learn about various important biosynthetic processes.

Sugar Biosynthesis or CO2 Fixation

It is a very important step for living organisms. Most of the energy comes from food from plants. Being primary producers, plants fix atmospheric carbon dioxide to form organic compounds such as sugar. Two of the main pathways of sugar biosynthesis are

  • Calvin Cycle- Carbon fixation, reduction of C from CO2 to triose phosphate, a sugar.
  • Gluconeogenesis- Formation of glucose from non-carbohydrate precursors, such as lactate, glycerol, pyruvate, etc.

Calvin Cycle

Plants and some microorganisms fix atmospheric carbon to synthesize sugar. Calvin cycle is the light-independent or dark reaction of photosynthesis. It occurs in all the photosynthetic organisms (C3, C4, CAM). It is not directly dependent on light but utilizes ATP and NADPH produced in the light reaction.

It is a three-step process:

  1. Carboxylation or CO2 Fixation- It is the first step, here CO2 is fixed by a ketopentose sugar-phosphate RuBP (Ribulose bisphosphate) to form 3-PGA (3-phosphoglycerate). The enzyme RuBisCO (Ribulose bisphosphate carboxylase oxygenase) catalyzes this reaction.
  2. Reduction- It involves a series of steps reducing 3PGA to triose phosphates such as G3P (glyceraldehyde-3-phosphate) and Dihydroxyacetone phosphate (DHAP), which are further converted to Fructose, Glucose, Starch or Sucrose.
  3. Regeneration- In the last step, RuBP is regenerated back to continue the cycle.

💡 Did you know? RuBisCO is the most abundant protein present on earth.

Gluconeogenesis

It is the process of sugar synthesis from non-carbohydrate precursors. It is present in microorganisms, plants, fungi as well as animals.

In the gluconeogenesis process, substrates coming from breakdown of proteins, lipids like glycerol, lactate, glucogenic amino acids are converted to pyruvate, which is then converted to glucose.

Protein Biosynthesis

Protein synthesis takes place by the process of translation from mRNA. The genetic information contained in DNA gets translated into polypeptide chains, which are folded to form proteins.

Translation starts with the charging or aminoacylation of tRNA. The enzyme catalysing the reaction is aminoacyl tRNA synthetase. First, an amino acid attaches to ATP forming aminoacyl-AMP and the amino acid gets transferred to tRNA.

Amino acid + ATP ⇌ Aminoacyl-AMP + PPi

Aminoacyl-AMP + tRNA ⇌ Aminoacyl-tRNA + AMP

Proteins are the polymers of amino acids. The amino acid sequence in a polypeptide chain is governed by genetic codes present in mRNA. It is a three-step process:

  1. Initiation: Binding of the ribosome to mRNA at the start codon and binding of initiator tRNA to the ribosome.
  2. Elongation: It includes elongation of the polypeptide chain by forming peptide bonds between the growing polypeptide chain and the new amino acid carried by tRNA. This involves the interaction of codon in mRNA and anticodon present on tRNA.
  3. Termination: In the end, binding of a release factor to the stop codons terminates the translation process. The polypeptide chain is released from the ribosome.

Where do amino acids required for proteins come from? Let’s understand here in brief.

Biosynthesis of Amino acids

There are 20 amino acids, which are part of a protein structure. Humans cannot synthesize all the amino acids in their body. Amino acids, which can be synthesized in the body are called non-essential amino acids and those which have to be supplemented through diet are called essential amino acids.

The non-essential amino acids are synthesized from various intermediates of metabolic pathways such as Citric acid cycle.

Some of the examples are:

  • Many amino acids are synthesized from α-ketoacids and then transaminated to form other amino acids. Glutamate, Glutamine, Arginine and Proline synthesis starts with α-ketoglutarate, which is an intermediate in the citric acid cycle.
  • Glutamate is formed by the amination of α-ketoglutarate. The enzyme catalysing this reaction is glutamate dehydrogenase.

α-ketoglutarate + NH4+ ⇄ glutamate

α-ketoacid + glutamate ⇄ amino acid + α-ketoglutarate

  • Similarly, lysine, methionine, asparagine, threonine and isoleucine are synthesized from oxaloacetate (OAA) and aspartate. Oxaloacetate is transaminated to form aspartate.
  • Phenylalanine, tryptophan and tyrosine are synthesized from phosphoenolpyruvate (PEP) or erythrose-4-phosphate.
  • 3PGA is used to synthesize serine, glycine and cysteine.
  • Pyruvate, which is the end product of glycolysis is utilized to synthesize alanine, valine and leucine.

Also Read: Biochemical Pathways

Biosynthesis of Nucleic Acid (DNA and RNA)

DNA is a polymer of nucleotides joined by a phosphodiester linkage. DNA has a double helix structure, two strands of DNA are joined by hydrogen bonding between complementary nitrogenous bases.

DNA synthesis takes place in the nucleus. DNA replication is a semiconservative process. DNA polymerase catalyses the reaction.

RNA is synthesized from DNA by the process of transcription. The genetic information stored in DNA gets transcribed into RNA. RNA synthesis is catalysed by the enzyme RNA polymerase.

To know more about the structure of DNA, RNA, Replication and Transcription, check Molecular Basis of Inheritance.

DNA, RNA are polymers of nucleotides and nucleotides are made up of nitrogenous bases, pentose sugar and phosphate.

Lipid Biosynthesis

Lipid is an important constituent of the cell. The cell membrane is made up of a lipid bilayer. Energy is stored in the form of lipids, they are also important in signalling. Many hormones are made up of lipids. Fatty acids are the simplest types of lipids. Triglycerides, phospholipids, glycolipids, cholesterol are all lipids.

Biosynthesis of Fatty acid

Fatty acid synthesis takes place in the cytoplasm of the cell. Fatty acids are synthesized from Acetyl-CoA using NADPH, which is an intermediate of the glycolytic pathway. The enzyme fatty acid synthases catalyse the reaction.

Linoleic acid and α-linolenic acid are essential fatty acids as they cannot be synthesized in the body.

Biosynthesis of Triglycerides

Three fatty acids combine with glycerol to form triglycerides. Triglycerides are commonly known as fat. The process is known as lipogenesis. Excess carbohydrate gets converted into triglycerides.

Biosynthesis of Phospholipids

Phospholipids are an important constituent of cell membranes. They constitute the lipid bilayer surrounding cell organelles. Phospholipids are amphipathic, they have a hydrophilic or polar head and hydrophobic tail of long-chain hydrocarbons.

Here two fatty acids combine with glycerol and the third hydroxyl group is phosphorylated forming phospholipids.

Cholesterol Biosynthesis

Cholesterol is a type of lipid known as sterols. Cholesterol is synthesized in the body from Acetyl-CoA. Most of the cholesterol synthesis takes place in the liver and intestines.

Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are the common precursors in the biosynthesis of cholesterol, terpenes, carotenoids, etc.

In animals, these precursors are synthesized from acetyl-CoA by the mevalonate pathway, whereas plants and bacteria synthesize these from pyruvate and G3P by the non-mevalonate pathway.

In cholesterol synthesis, six molecules of isopentenyl phosphate condense to form squalene, which is converted to lanosterol and then to cholesterol by a series of enzymatic reactions.

Cholesterol is a precursor of Vit D and many steroid hormones like cortisol, estrogen, testosterone, etc.

As we say “too much of anything is good for nothing”, so even biosynthesis has to be controlled and regulated in order to produce only a specific and required amount of the compound. Biosynthesis is under strict genetic and neuroendocrine control.

A slight error in biosynthetic pathways may lead to deleterious effects and lead to various disorders.

This was all about Biosynthesis. Explore notes on Biochemical Pathways and other important concepts related to NEET, only at BYJU’S.

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