"One Gene, One Enzyme"

In Principle: Proteins are the products of genes.
                        Proteins catalyze biochemical reactions.
                        Such reactions produce phenotypes, either directly or indirectly.
                        Different alleles produce different phenotypes

Interaction between alleles in diploid organisms is the subject matter of Genetics

---

How do genotypes produce phenotypes?

Beadle & Tatum experiment (1940s) on haploid Neurospora bread mold [iGen3 04-02]
haploid organisms have one allele at each gene locus
    prototroph wild-type grows on simple medium
auxotroph mutants cannot grow on simple medium,
                         require supplementation with specific amino acids [iGen3 04-03]
          [AKA autotrophs ("self feeder") and heterotrophs ("other feeder"), respectively]

Hypothesis: "No-growth" phenotype results from a change in the genotype:
                            inability to synthesize amino acid is the result of loss of enzyme activity
                            each mutant corresponds to a defect in a particular enzyme:
"One gene, one enzyme" (Homework)
                            [ Remember: Beadle & Tatum did not know about DNA in 1940 ]
    Ex.:  arg^- mutants cannot grow without arginine, always grow with added arginine
particular mutant classes sometimes grow with other amino acids
                        (such as citrulline and/or ornithine)
                        [cf. methionine metabolism: iGen3 04-04]

 Growth response to added amino acids

mutants	none	ornithine	citrulline	arginine
arg^- 4	-	+	+	+
arg^- 2	-	-	+	+
arg^- 1	-	-	-	+

      => These other amino acids are involved in the arginine pathway:

               Enzyme defect blocks interconversion of precursors in the pathway.

 Inference of a haploid biosynthetic pathway

Each mutant class (arg4, arg2, & arg1) affects a different enzyme in arginine biosynthesis

---

Homework:

1.    In the above discussion,  I have used "mutant" but carefully avoided "mutation": WHY?

2.    Critique the following statements:
"arg^- mutants result in defective arginine."
"arg^- mutants are defects of arginine."
"arg^- mutants are due to absence of the gene for arginine."
"arg^- mutants are enzymes that block synthesis of arginine."

3. Would you expect to observe an arg mutant class with the following phenotype? Explain.

 Growth response to added amino acids

mutant	none	ornithine	citrulline	arginine
arg^- X	-	+	-	+

---

Biochemical Basis of Human Genetic Diseases

In a haploid organism, discovery of DNA shows nature of metabolic  "mutants":

Mutations affect the genes responsible for different enzymes:
     mutate the gene eliminate (or modify) the enzyme

What about diploid organisms?
Diploid organisms have two alleles at each gene locus: one from each parent
Interactions between alleles at a locus are the subject matter of genetics

"Online Mendelian Inheritance in Man" (OMIM) database
            examples from human biochemical genetics: "Inborn errors of metabolism" [iGen3 04-Table02]
            First three involve disruptions of phenylalanine metabolism [iGen3 04-01]

Phenylketonuria (PKU) (Folling 1934) (OMIM citation 261600)
phenylalanine accumulates in Central Nervous System mental retardation
       A defect of phenylalanine hydroxylase
phenyalanine metabolized to phenylpyruvic acid in alternative pathway

         Detection & treatment
             biochemical testing of new-borns: Guthrie Test
             phenylalanine-restricted diet corrects inborn condition  (Euphenics)
        Maternal PKU results from high fetal [phe] in treated, asymptomatic mothers
       [ Further information on PKU & related Inborn Errors of Metabolism ]

PKU arises from variation at the Phenylalanine Hydroxylase (PAH) gene locus
Important: This gene is not a gene "for" PKU: it is a gene "for" PAH
Diploid humans each have two alleles at this locus
Allelic variants produce different levels of PAH activity
       Consider three (hypothetical) alleles: A, B, & C :

Phenotypic consequences of interactions between alleles at the PAH locus

Genotype	PAH Activity	[phe] uM	PKU Phenotype
AA	100%	60	Standard
AB	30%	120	Standard
CC	5%	200 ~  300	Hyperphenylalanemia:  no special diet required
BB	0.3%	600 ~  2400	Classic PKU: special diet required

   [Alleles B & C) arise from DNA mutations in the PAH gene]

PKU is a classic example of a "recessive" genetic disease: What does this mean?

AB genotype shows same PKU phenotype as AA  genotype
         that is, A allele shows haplosufficiency: one 'dose' is sufficient to produce standard phenotype
         or, expression of A allele "masks" expression of B allele
A is therefore "dominant" to B in influencing PKU phenotype
B is therefore "recessive" to A

    but PAH activity phenotype of AB is intermediate between AA & BB
AB phenotype is closer to BB than AA (0% <  30% << 100%)
B is an "incomplete dominant" to A

and B produces a higher [phe] phenotype than A:
                Isn't B therefore "dominant" to A ?

    Be careful to distinguish molecular & phenotypic expression (Homework)

Homework

Predict the PAH activity, [phe], & PKU phenotypes of the AC and CB genotypes.

Explain your reasoning.

Would you expect to find a dominant mutation in this pathway?
Why or why not? What might be the nature of such a mutation?

Other molecular disorders of phenylalanine metabolism
Alkaptonuria (OMIM 203500)
Oculo-cutaneous Albinism (OMIM 203100)

---

---

All text material © 2013 by Steven M. Carr

---