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From Proteins to Phenotypes Chapter 10 10.1 Proteins Are the Link Between Genes and the Phenotype Proteins are the visible end product of the gene expression pathway Some genes encode proteins that act as growth hormones, receptors, and growth factors Other genes encode proteins that act as biological catalyst called enzymes. 10.2 Enzymes and Metabolic Pathways Biochemical reactions in the cell are linked together to form metabolic pathways Metabolism Metabolism • The sum of all biochemical reactions by which cells convert and utilize energy 10.3 Phenylketonuria (PKU): A Mutation That Affects an Enzyme Phenylalanine is an essential amino acid and the starting point for a network of metabolic reactions Essential amino acids • Amino acids that cannot be synthesized in the body and must be supplied in the diet Phenylalanine Metabolism and PKU A mutation in a gene encoding the enzyme, phenalanine, that controls the first step in the breakdown of phenylalanine causes phenylketonuria (PKU). PKU affects about 1 in every 12,000 newborns PKU Phenylketonuria (PKU) • An autosomal recessive disorder of amino acid metabolism that results in mental retardation if untreated The phenotype is caused by the buildup of phenylalanine and the products of secondary reactions (such as phenylpyruvic acid) PKU Can Be Treated With Diet Infants with PKU can develop normally before birth, then develop neurological damage when fed a diet containing protein A PKU diet must contain phenylalanine levels high enough for normal development, but low enough to prevent mental retardation Genetic Journeys: Dietary Management and Metabolic Disorders For PKU, a formula is prepared from enzymatically digested proteins (or synthetic mixtures of amino acids), fats, carbohydrates, and vitamin and mineral supplements A typical lunch might include vegetable soup, crackers, fruit cocktail, and formula Keep In Mind Phenylketonuria and several other metabolic disorders can be treated by dietary restrictions 10.5 Genes and Enzymes of Carbohydrate Metabolism Mutations in genes encoding enzymes can affect the metabolic pathways of other biological molecules, including carbohydrates Galactosemia is a genetic disorder caused by lack of an enzyme in sugar metabolism Galactosemia is Caused by Enzyme Deficiency Galactosemia • A heritable trait associated with the inability to metabolize the sugar galactose • If left untreated, high levels of galactose-1phosphate accumulate, causing cataracts and mental retardation • Dietary treatment does not prevent long-term complications Studies of Sickle Cell Anemia Sickle cell anemia was the first example of inherited variation in protein structure • First direct proof that mutations result in a change in amino acid sequence in proteins • Evidence that a change in a single nucleotide can cause a genetic disorder Genetic Journeys: The First Molecular Disease Studies by Linus Pauling and James Neel concluded that a mutant gene involved in the synthesis of hemoglobin causes sickle cell anemia, and that a genetic disorder can be caused by a defect in a single molecule Keep In Mind Sickle cell anemia is caused by substitution of a single amino acid in beta globin 10.8 Pharmacogenetics Individual differences in reactions to therapeutic drugs represent a “hidden” set of phenotypes that are not revealed until exposure occurs The first pharmacogenetic trait was discovered in the 1930s when researchers at DuPont were trying to develop a sugar substitute. Pharmacogenetics Pharmacogenetics • A branch of genetics concerned with inheritance of differences in the response to drugs Variations in the amino acid sequences of proteins affect the way individuals react to prescription drugs and chemicals in the environment • For example, why some people can smoke for cigarettes for years & never develop lung cancer. Genetic Differences in Ability to Taste PROP compounds (related to PTC) in kale, cabbage, broccoli and Brussels sprouts taste bitter to some people Genetic Differences in Ability to Smell Two-thirds of people tested could smell pink Verbena flowers but not red ones; the rest could smell red flowers but not pink ones Drug Sensitivities are Genetic Traits Gene variations and breast cancer therapy • Cancer center treatment facilities are sequencing the genome of tumors to design treatments for their patients. Keep In Mind Small differences in proteins can have a large effect on our ability to taste, smell, and metabolize medicines 10.9 Ecogenetics Ecogenetics • The study of genetic variation that affects responses to environmental chemicals Some individuals may be sensitive or resistant to environmental chemicals such as pesticides • Important in research, medicine, and public policy