N-terminal end

Amino acid analysis itself does not directly give the number of residues of each amino acid in a polypeptide, but it does give amounts from which the percentages or ratios of the various amino acids can be obtained (Table 5.2). If the molecular weight and the exact amount of the protein analyzed are known (or the number of amino acid residues per molecule is known), the molar ratios of amino acids in the protein can be calculated. Amino acid analysis provides no information on the order or sequence of amino acid residues in the polypeptide chain. Because the polypeptide chain is unbranched, it has only two ends, an amino-terminal or N-terminal end and a carboxyl-terminal or C-termuial end. 

The unique characteristic of each protein is the distinctive sequence of amino acid residues in its polypeptide chain(s). Indeed, it is the amino acid sequence of proteins that is encoded by the nucleotide sequence of DNA. This amino acid sequence, then, is a form of genetic information. By convention, the amino acid sequence is read from the N-terminal end of the polypeptide chain through to the C-terminal end. As an example, every molecule of ribonucle-... 

At the N-terminal end of the f loop, near the membrane lipid interface, there is an autoinhibitory domain, rich in both basic and hydrophobic residues and consisting of a 20-aminoacid sequence (219-238). This aminoacid sequence, named exchange inhibitory peptide (XDP), is involved in NCX activity regulation. 

PelZ is a hydrophilic protein of 420 amino acids with a short hydrophobic sequence at its N-terminal end which has Ae characteristics of the signal sequences of exported proteins. The signal peptide may be 24 amino acids long, which would corroborate wiA the usual length encountered in prokaryotes. The molecular cloning of the pelZ gene in an expression vector pT7-6 allowed for the specific 35S-cysteine-methionine raAo-labelling of PelZ in E. coli K38. We could detect, in crude extracts, the presence of a precursor and a mature form of PelZ. After cell fractionation, Ae mature form of PelZ could be localized in Ae periplasm of E. coli. So PelZ appears to be a protein exported by Ae Sec-dependent system of translocation. 

There is a second long interface stretching between the threefold and fourfold axes, involving both hydrophobic and hydrophilic interactions. Close to the threefold axis is an intersubunit salt bridge between Asp-139 of subunit I and His-128 in III, which links the N-terminal end of helix D (III) to a position near the kink in helix D (I). Further along the interface, N-terminal residues 6-12 of subunit III make several interactions with the C-helix of subunit I, including several which are mediated... 

In some heterologous production systems, improper removal of the signal peptide may occur during the expression of secreted proteins, which would result in the addition or removal of amino acids at the N-terminal end. In most cases, these modifi-... 

Prothrombin (factor II) is a 582 amino acid, 72.5 kDa glycoprotein, which represents the circulating zymogen of thrombin (Ha). It contains up to six y-carboxyglutamate residues towards its N-terminal end, via which it binds several Ca2+ ions. Binding of Ca2+ facilitates prothrombin binding to factor Xa at the site of vascular injury. The factor Xa complex then proteolytically... 

B Because it is a hexapeptide and there are five distinct amino acids, one amino acid must appear twice. The fragmentation pattern indicates that the doubled amino acid is glycine. The sequences fall into place if we begin with the N-terminal end. 

The N-terminal end of a polypeptide chain, with its free amino group, is conventionally known as beginning of the chain, while the last amino acid, with free carboxyl group, is the C-terminal end of the chain (see Figure 2.7). 

The sequence for delivery of copper ions to SOD1 passes from the copper transporter (Ctr) by an unknown pathway to the copper chaperone for SOD1 (CCS) and by a studied pathway from CCS to SOD1. The CCS protein has been studied structurally and found to be similar to other copper chaperones such as those discussed above—Atxl and Atoxl (Hahl). Copper chaperone for superoxide dismutase (CCS) differs from other copper metallochaperones in that it folds into three functionally distinct protein domains with the N-terminal end of domain I... 

Figure 7.4 Basic structure of an IgG molecule. Two heavy chains (440 residues) and two light chains (214 residues) are joined by disulphide bonds and each shows a relatively constant amino acid sequence in one section (C-terminal end) and a variable sequence section (N-terminal end). The variable regions of both heavy and light chains are involved in the formation of the antigen-binding site.

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