J. A. CAMPBELL Harvey Mudd College Claremont, Colifornio 9171 1
Questions Q142. Sanger received t h e Nobel Prize for determining t h e structure of beef insulin. I t is m a d e u p of two polypeptide chains: chain A contains 21 amino acid residues, chain B contains 29. Discuss how t h e differences in t h e 8.9.10 positions of t h e A chain might have arisen for t h e species below. These a r e the only differences in insulin for these species.
P Lond ~ Cattle
Sheep
Sperm Whale
Home
Whale
threonine alanine alanine threonine glycine serine glycine serine valine isoleucine isoleucine threonine 10 valine What changes i n the DNA probably lead t o these changes? W h a t would he t h e length of the RNA template specifying the A chain in insulin? Assuming the distance between nucleotide pairs is 0.34 m r , how might this RNA fit on a ribosome 2 0 m p in diameter. Q143. A common method of measuring rate of photosynthesis in aquatic conditions is t o suspend two identical 8 alanine 9 serine
closed systems (one in a black, the other i n a colorless glass bottle) in t h e appropriate place a n d measure change i n each time. Name one or two assumptions impliin [Oz] cit in the method t h a t may not b e valid. H20 Q144. T h e reactions, citrate = cis-aconitate isocitrate, are enzyme catalyzed and found in many living systems. Calculate t h e two K's and t h e over-all citrate = isocitrate K, knowing t h a t the equilibrium physiological system a t 25'C and pH 7.4 contains 90.9% citrate, 2.9% cis-aconitrate, 6.2% isocitrate. Calculate values of AG", and comment on the fact t h a t these reactions go in living systems.
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This column consists of questions (plus possible, hut certainly not uniquely satisfactory, answers) requiring no more than a concurrent first-year, college level course, a data handbook, and a willingness to apply fundamental chemical ideas to the systems which surround us lor even are inside us). Contributions for passible inclusion are solicited. Initiated in the January, 1972, issue of this Journal.
Answers A142. Position 8
Amino Acid alanine threonine
DNA codons GCU GCC ACU ACC
GCA ACA
GCG ACG
9
serine
UCU AGU GGU AUU GUU
UCA AGC GGA AUA GUA
UCG
10
dycine isoleucine valine
UCC AGU GGC AUC CUC
Alanine differs from a threonine in CH3- instead of CHJCHONSerine differs from glycine in HOCHZ- instead of HValine differs from isoleucine in ( C H h C H - instead of (C2HdCHz)CHClearly these differences do not prevent synthesis of the insulin nor its satisfactory functioning in the species involved. There are 21 amino acids in the A chain of insulin thus reI quiring 63 nucleotides in the guiding gene plus start and stop I or 65 at least. 65 x 0.34 mp = 22 mp. The RNA could be wrapped around about '/3 of the periphery af the ribosome which is unlikely or it could be contacting it over a 3 X 0.34 mp = 1 mp interval linking its three bases to the ribosome with the other 21 mp suspended in the surrounding solution. Since one ribosome may catalyze the reaction of a given RNA with many amino acids it is unlikely ribosomes link simultaneously to more than three bases on the RNA. Otherwise bases other than the three which dictate the next amino acid would have to match the ribosome. ~
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Change change G to A in first position to changealanine to threonine change C toU in 2nd position to change A to G in first position to change serine to glyeine
GGC GUG
=
change threonine to isoleucine change A to G in 1st position to change isoleucine to valine A143. Possibly invalid assumptions: (1) that only photosynthesis is affected by the presence or absence of photons, not, for example, its reverse; (2) that non-photosynthetic processes are independent of photosynthesis and its products; (3) that the environment (aqueous solution) is independent of the presence of photons, (4) that the container, especially the colorless one, has no effect on photosynthesis or on dark reactions, (5) that the change in [02]in a closed system has no feedback effect on the photosynthetic or dark reactions. A144. Citrate = cis-aeonitate + HzO = isocitrate, may he represented by ?
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ci t ca + H,O e IC. A @ = -RT In K = -1.365 lag K (kcal/rnole)
K,
= [cal/[cil,
K 2= [icl/[ctl]. K,,,,.,,,,
-
[icl/[cil
= K,K,
Since all the concentrations come in as simple rations, their ahsolute values have no effect on the K's, only the relative values are involved. We can use percentage values rather than concentrations to calculate the K's.
KI Kz K,,,,.l,
= [cal]/[ci] = 2.9J90.0 = 0.032 = [ic]J[ea] = 6.212.9 = 2.1 = KIK. = 0.067
ACI" AGz0 AG',,,.,,
= 2.0 kealJmole = -0.43 kcalJmole = 1.6 kcalJmole
Two of the three AGO'S are positive so the in vivo reactions must occur only in situations where the ratio of the reactant to product concentrations appreciably exceed unity a t the site of reaction. ( A ratio of unity gives A C = 0, a relatively larger reactant concentration gives mare negative values to AG.)
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