A gene includes a specific nucleotide sequence in the DNA. Many of the genes encode various products such as enzymes and proteins. Certain enzymatic structures are dependent on the gene sequence. A correct gene sequence, without involving any mutations properly encodes a protein or an enzyme functioning normally. However, the mutated gene produces a truncated protein product or faulty products such as defective enzymes, leading to harmful effects. Hence, certain enzymes function as per the gene which encoded them. Defective enzymes interfere with the biochemical pathways. The disorders associated with enzyme defects arising due to defective genes get classified under the inborn errors of metabolism.
Garrod’s hypothesis:
An English physician known as Archibald Garrod studied the phenomenon of enzymatic activity related to genetic disorders for the first time. He called such disorders as inborn errors of metabolism. Garrod and his colleagues worked on an inborn error of metabolism known as alkaptonuria. It is a human disease involving changes in coloration of the urine when exposed to air. The urine usually turns into a black color. These individuals excrete homogentisic acid in the urine, unlike normal people. This component turns the urine black when exposed to air. It is a hereditary disorder that follows autosomal recessive inheritance. It arises due to an enzyme deficiency. An enzyme known as homogentisate 1,2 dioxygenase (HGD) becomes deficient. This enzyme gets produced in the hepatocytes or the liver cells and kidney cells. The enzyme helps in the breakdown of homogentisic acid which is an intermediate in the tyrosine degradation pathway.
Image: Phenylalanine-tyrosine pathway
One gene one enzyme hypothesis:
Biochemical genetics is a branch of genetics involving the study of genes and their effects on the biochemical pathways. Beadle and Tatum contributed great discoveries in the field of biochemical genetics following the footsteps of Garrod. Beadle and Tatum worked with a haploid fungus known as Neurospora crassa. Since Neurospora spreads like a web, it is known as a mycelial fungus. The asexual spores of Neurospora are known as conidia. This fungus is commonly known as the orange bread mold since the asexual spores or conidia have an orange color. The asexual spores or mycelial pieces get isolated easily for growing them on a suitable medium. Apart from the vegetative or the asexual reproduction, Neurospora also shows the ability to reproduce sexually. This fungus typically shows two mating types. In a nitrogen-limiting medium, these mating types fuse with each other. The fusion of the mating types, later on, initiates other steps such as the fusion of two haploid nuclei forming a diploid nucleus which follows meiosis to produce four haploid nuclei. This step occurs in an elongated sac known as ascus. Each ascus consists of eight ascospores obtained after mitotic division and spore maturation. Beadle and Tatum exploited the above characteristics of Neurospora. They studied auxotrophs and prototrophs in Neurospora. The wild-type strains are known as prototrophs. These strains show their ability to grow on a minimal medium. Based on the chemicals present in the minimal medium, Neurospora synthesized other important functional molecules. Hence, it was possible to isolate mutants that required nutrients to grow (auxotrophs). Beadle and Tatum isolated auxotrophic mutants and induced gene mutations in them using X rays. Then, they crossed them with the wild-type strains. From the resultant crosses, one progeny spore per ascus was allowed to germinate in a nutritionally rich or a complete medium. They also checked whether they grew on a minimal medium. Next round involved the determination of the specific substance needed for an auxotroph to grow on a minimal medium. The strains inoculated in 20 different tubes involved supplementation with one of the 20 different amino acids. In this way, they identified the biochemical pathways. The wild-type strains not only utilized the amino acids for their needs but also converted them into other important compounds involved in various biochemical pathways.
Methionine biosynthetic pathway in Neurospora crassa:
The methionine auxotrophic mutants require an additional methionine supplementation in the minimal medium. Identification of four main genes involved in methionine biosynthesis paved the way in the genetic analysis. These genes include met 2+, met 3+, met 5+, and met 8+. Any of the above gene mutations result in auxotrophic mutants for methionine. The study then involved the growth of the mutants on a minimal medium supplemented with chemicals or the intermediates involved in a methionine biosynthetic pathway. These intermediates include O-acetyl homoserine, cystathionine, and homocysteine. None of them grew on an unsupplemented medium. The steps involved in a biochemical pathway involve important intermediates required for the strain to grow. If a mutant strain gets blocked in the initial steps of the biochemical pathway, a larger number of strains grow. A genetic block in the pathway leads to the accumulation of the intermediates. Different mutants require different intermediates. For example, the wild-type strains grow in the presence or absence of any intermediates. Met-5 mutant strains grow only in the presence of O-Acetylhomoserine, cystathionine, homocysteine, and methionine. The met-3 mutant strains grow on a medium supplemented with methionine, homocysteine, or cystathionine. The met-2 mutants grow only when a medium gets supplemented with methionine and homocysteine. The met-8 mutants grow only in a medium supplemented with methionine. From the above experiments, Beadle and Tatum proposed that each enzyme gets encoded by a gene. Hence, they established and introduced one-gene-one-enzyme hypothesis.
Application of one gene-one enzyme hypothesis in human genetics:
The human body involves various biochemical reactions controlled by various enzymes and intermediate products. Every step of a metabolic pathway gets controlled by one gene product. Any defects in the gene lead to complications in the specific enzymatic function. Hence, these type of metabolic errors leads to various disorders. Mutations either block particular steps in the pathway or alter the entire process. For example, alkaptonuria involves a deficiency of an enzyme known as homogentisate 1,2 dioxygenase (HGD). The enzyme controls a reaction involving homogentisic acid as an intermediate. Hence, each metabolic step requires proper functioning of the enzyme. Improper functioning or complete deficiency of an enzyme leads to metabolic disorders.
Following are the examples:
· Phenylketonuria
· Albinism
· Alkaptonuria
· Galactosemia
· Hurler’s syndrome
· Familial hypercholesterolemia
References:
[1] Science as a Way of Knowing: The Foundations of Modern Biology, John Alexander Moore
[2] Essential Genetics: A Genomics Perspective, Daniel Hartl
[3] Biology, Eldra Solomon, Charles Martin, Diana W. Martin, Linda R. Berg
[4] IGenetics, Peter Russell
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