A Phylogenetic Study of Cycads (1915)(en)(5s) by Chamberlain C.J.

By Chamberlain C.J.

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Key concepts: Porphyrin and nucleotide metabolism (Murray, pp 270–302. ) • Porphyrins consist of four pyrrole rings containing a nitrogen and four carbons; porphyrins are synthesized from the condensate of succinyl CoA and glycine to form aminolevulinic acid, porphobilinogen, protoporphyrin, and (after incorporation of iron), heme. • Porphyrias result from deficiencies of the enzymes responsible for porphyrin synthesis; deficiencies of hepatic enzymes (acute intermittent porphyria, porphyria cutanea tarda, variegate porphyria) produce abdominal pain, photosensitivity, and neuropsychiatric symptoms while those of erythrocyte enzymes (X-linked sideroblastic anemia, congenital erythropoietic porphyria, protoporphyria) produce anemia and skin changes with or without photosensitivity.

Reproduced with permission, from Murray RK, Granner DK, Mayes PA, Rodwell VW: Harper’s Biochemistry. 26/e. ) High-Yield Facts 17 • Enzyme assays use initial rate conditions with excess substrate such that the initial velocity (vi) of substrate reaction is proportionate to enzyme concentration. • For a given amount of enzyme, the relation between reaction velocity V and substrate concentration S is given by the Michaelis-Menton equation and its reciprocal (plotted in Fig. 9): Vi = Vmax[S] 1 K m + [S] 1 K 1 1 = = m × + and or K m + [S] Vi Vmax[S] Vi Vmax [S] Vmax The Michaelis constant Km is defined as the substrate concentration giving one-half the maximal reaction velocity, and can be viewed as a binding constant—substrates with high affinity for the enzyme catalytic site have low Km values and vice versa.

A microsomal fatty acid elongase extends C10 and longer fatty acids through direct condensation of fatty acyl and acetyl CoA. Fatty acids are thus made using acetyl CoA building blocks, with similar steps of 2-carbon addition for the synthase or elongation pathways (Fig. 15). • Linoleic (C18 with two double bonds), α-linolenic (C18 with three double bonds), and arachidonic acids (C20 with four double bonds and precursor to prostaglandins) cannot be synthesized in humans and must be supplied in the diet (essential fatty acids); essential fatty acid deficiencies can occur with intestinal malabsorption (cystic fibrosis, Crohn disease) or liver disease (cirrhosis, alcoholism, immature neonatal liver) while excess trans-unsaturated fatty acids are associated with atherosclerosis.

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