Disulfides sequences

There are several levels of pepfide sfrucfure The primary structure is the ammo acid sequence plus any disulfide links With the 20 ammo acids of Table 27 1 as building blocks 20 dipeptides 20 tripeptides 20" tetrapeptides and so on are possible Given a peptide of unknown structure how do we determine its ammo acid sequence" ... 

Sanger also determined the sequence of the A chain and identified the cysteine residues involved m disulfide bonds befween fhe A and B chains as well as m fhe disulfide linkage wifhin fhe A chain The complefe insulin sfruefure is shown m Figure 27 11 The sfruefure shown is fhaf of bovine insulin (from cattle) The A chains of human insulin and bovine insulin differ m only fwo ammo acid residues fheir B chains are identical except for the ammo acid at the C terminus... 

The primary structure of a peptide is given by its ammo acid sequence plus any disulfide bonds between two cysteine residues The primary structure is determined by a systematic approach m which the protein is cleaved to smaller fragments even individual ammo acids The smaller fragments are sequenced and the mam sequence deduced by finding regions of overlap among the smaller peptides... 

The amino acid sequences of hCS-A, hCS-B, and hCS-V are shown in relation to GH in Figure 1. The sequence of hCS-V is predicted from the DNA coding sequence and apparentiy does not possess amino acids 8—55 relative to GH and the other hCS molecules. It is not certain whether hCS-V is expressed or what function it may have. Human CS-A and hCS-B share approximately 85% identity with GH and also possess the disulfide bonds between Cys 53—165 and Cys 182—189 which produce the long and short S—loops characteristic of the PRL/GH family. 

Fig. 1. Primary stmcture of hGH showing the amino acid sequence and the disulfide bonds.

Fig. 1. Amino acid sequence for the A- and B-chains of human iasulin [11061-68-0] where soHd lines denote disulfide bonds. Porciae iasulin [12584-58-6] differs by one amino acid ia the B-chaia where alanine replaces threonine at positioa 30. Boviae iasulia [11070-73-8] differs by three amino acids. la the A-chain alanine replaces the threonine at positioa 8 and valine replaces the isoleuciae at position 10. In the B-chain there is an alanine at position 30.

Thermal Stability. Dimethyl sulfoxide decomposes slowly at 189°C to a mixture of products that includes methanethiol, formaldehyde, water, bis(methylthio)methane, dimethyl disulfide, dimethyl sulfone, and dimethyl sulfide. The decomposition is accelerated by acids, glycols, or amides (30). This product mixture suggests a sequence in which DMSO initially undergoes a Pummerer reaction to give (methylthio)methano1, which is labile and reacts according to equations 1—3. Disproportionation (eq. 4) also occurs to a small extent ... 

Thaumatin. Thaumatin [53850-34-3] is a mixture of proteins extracted from the fmit of a West African plant, Thaumatococcus daniellii (Beimett) Benth. Work at Unilever showed that the aqueous extract contains two principal proteins thaumatin I and thaumatin II. Thaumatin I, mol wt 22,209, contains 207 amino acids in a single chain that is cross-linked with eight disulfide bridges. Thaumatin II has the same number of amino acids, but there are five sequence differences. Production of thaumatins via genetic engineering technology has been reported (99). 

Streptokinase has a molecular weight of about 47,000 with a single chain of 415 amino acids there are no intramolecular disulfide bonds (64). The complete nucleotide sequence of the gene encoding the RNA for this protein has been reported (65,66). 

Scheme 68 shows the conversion of the phenoxymethylpenicillin-derived disulfide (see Scheme 10) to penem derivative (91) (78JA8214). Of particular interest in this sequence is the reductive acylation step to afford (89) and the Wittig ring closure to give (90). The rate of the latter reaction was found to be greatly infiuenced by the steric and electronic character of both the thiol ester and the carboxyl blocking group. 

A prior distribution for sequence profiles can be derived from mixtures of Dirichlet distributions [16,51-54]. The idea is simple Each position in a multiple alignment represents one of a limited number of possible distributions that reflect the important physical forces that determine protein structure and function. In certain core positions, we expect to get a distribution restricted to Val, He, Met, and Leu. Other core positions may include these amino acids plus the large hydrophobic aromatic amino acids Phe and Trp. There will also be positions that are completely conserved, including catalytic residues (often Lys, GIu, Asp, Arg, Ser, and other polar amino acids) and Gly and Pro residues that are important in achieving certain backbone conformations in coil regions. Cys residues that form disulfide bonds or coordinate metal ions are also usually well conserved. 

CH2SH + 1/2 O2 -CH2-S-S-CH2 + H2O Disulfide bonds form between the side chains of two cysfeine residues. Two SH groups from cysteine residues, which may be in different parts of the amino acid sequence but adjacent in the three-dimensional structure, are oxidized to form one S-S (disulfide) group. 

The C-terminal domain of phosducin is a five-stranded mixed p sheet with a helices on both sides, similar to the thioredoxin fold of disulfide iso-merase DsbA described in Chapter 6. Despite significant sequence homology to thioredoxin, the phosducin domain, unlike other members of this family. 

Lysozyme from bacteriophage T4 is a 164 amino acid polypeptide chain that folds into two domains (Figure 17.3) There are no disulfide bridges the two cysteine residues in the amino acid sequence, Cys 54 and Cys 97, are far apart in the folded structure. The stability of both the wild-type and mutant proteins is expressed as the melting temperature, Tm, which is the temperature at which 50% of the enzyme is inactivated during reversible beat denat-uration. For the wild-type T4 lysozyme the Tm is 41.9 °C. 

Insulin has 51 anino acids, divided between two chains. One of these, the A chain, has 21 amino acids the other, the B chain, has 30. The A and B chains are joined by disulfide bonds between cysteine residues (Cys-Cys). Figure 27.10 shows some of the information that defines the amino acid sequence of the B chain. 

FIGURE 5.17 The hormone insulin consists of two polypeptide chains, A and B, held together by two disulfide cross-bridges (S—S). The A chain has 21 amino acid residues and an intrachain disulfide the B polypeptide contains 30 amino acids. The sequence shown is for bovine insulin. 

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