Imidazoleglycerol-phosphate dehydratase is an enzyme in the histine biosynthesis pathway. It catalyzes the E1 dehydration of D-erthyro-imidazole-glycerol phosphate to imidazole acetol-phosphate. This is a rare example of a biological E1 reaction, as most biological elimination reactions occur through E1cB instead. In this reaction, D-erthyro-imidazole-glycerol phosphate is first protonated to form a good leaving group. Then, the leaving group is ejected to form the resonance-stabilized carbocation below. Draw curved arrows forming the most stable resonance structure to explain why this reaction goes through an E1 mechanism.

fertilization, pictured in figure 24.23a is the process in which gametes (an egg and sperm) fuse to form a zygote. the egg and sperm each contain one set of chromosomes. to ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. in mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. when a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. in placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as illustrated in figure 24.23b. the fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. the nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome.

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