Polypeptides subunits

The terms polypeptide and protein are used interchangeably in discussing single polypeptide chains. The term protein broadly defines molecules composed of one or more polypeptide chains. Proteins having only one polypeptide chain are monomeric proteins. Proteins composed of more than one polypeptide chain are multimeric proteins. Multimeric proteins may contain only one kind of polypeptide, in which case they are homomultimeric, or they may be composed of several different kinds of polypeptide chains, in which instance they are heteromultimeric. Greek letters and subscripts are used to denote the polypeptide composition of multimeric proteins. Thus, an ag type protein is a dimer of identical polypeptide subunits, or a homodimer. Hemoglobin (Table 5.1) consists of four polypeptides of two different kinds it is an hetero-multimer. 

If the protein of interest is a heteromultimer (composed of more than one type of polypeptide chain), then the protein must be dissociated and its component polypeptide subunits must be separated from one another and sequenced individually. Subunit associations in multimeric proteins are typically maintained solely by noncovalent forces, and therefore most multimeric proteins can usually be dissociated by exposure to pEI extremes, 8 M urea, 6 M guanidinium hydrochloride, or high salt concentrations. (All of these treatments disrupt polar interactions such as hydrogen bonds both within the protein molecule and between the protein and the aqueous solvent.) Once dissociated, the individual polypeptides can be isolated from one another on the basis of differences in size and/or charge. Occasionally, heteromultimers are linked together by interchain S—S bridges. In such instances, these cross-links must be cleaved prior to dissociation and isolation of the individual chains. The methods described under step 2 are applicable for this purpose. 

Hydrophobic-tailed tetramers Abundant form in the mammalian CNS. Anchored to plasma membranes by a hydrophobic, 20 kDalton length polypeptide subunit named PRiMA (Proline-Rich Membrane Anchor). 

The signal recognition particle (SRP) is a cytosolic ribonucleoprotein complex which binds to signal sequences of nascent membrane and secretory proteins emerging from ribosomes. The SRP consists of a 7S RNA and at least six polypeptide subunits (relative molecular masses 9, 14, 19, 54, 68, and 72 kD). It induces an elongation arrest until the nascent chain/ ribosome/SRP complex reaches the translocon at the endoplasmic reticulum (ER) membrane. 

Mitochondria are unique organelles in that they contain their own DNA (mtDNA), which, in addition to ribosomal RN A (rRNA) and transfer RN A (tRNA)-coding sequences, also encodes 13 polypeptides which are components of complexes I, III, IV, and V (Anderson et al., 1981). This fact has important implications for both the genetics and the etiology of the respiratory chain disorders. Since mtDNA is maternally-inherited, a defect of a respiratory complex due to a mtDNA deletion would be expected to show a pattern of maternal transmission. However the situation is complicated by the fact that the majority of the polypeptide subunits of complexes I, III, IV, and V, and all subunits of complex II, are encoded by nuclear DNA. A defect in a nuclear-coded subunit of one of the respiratory complexes would be expected to show classic Mendelian inheritance. A further complication exists in that it is now established that some respiratory chain disorders result from defects of communication between nuclear and mitochondrial genomes (Zeviani et al., 1989). Since many mitochondrial proteins are synthesized in the cytosol and require a sophisticated system of posttranslational processing for transport and assembly, it is apparent that a diversity of genetic errors is to be expected. 

This complex contains 11 polypeptide subunits of which only one is encoded by mtDNA. Defects of complex III are relatively uncommon and clinical presentations vary. Fatal infantile encephalomyopathies have been described in which severe neonatal lactic acidosis and hypotonia are present along with generalized amino aciduria, a Fanconi syndrome of renal insufficiency and eventual coma and death. Muscle biopsy findings may be uninformative since abnormal mitochondrial distribution is not seen, i.e., there are no ragged-red fibers. Other patients present with pure myopathy in later life and the existence of tissue-specific subunits in complex III has been suggested since one of these patients was shown to have normal complex 111 activity in lymphocytes and fibroblasts. 

IgG consists of four polypeptide subunits held together by disulphide bonds. Native immunoglobulins are rather resistant to proteolytic digestion but certain enzymes have been usefiil in elucidating their structure. Papain cleaves the molecule into three fragments of similar size ... 

Quaternary structure refers to the overall spatial arrangement of polypeptide subunits within a... 

ICE is an oligomeric enzyme (its active form may be a tetramer). It contains two distinct polypeptide subunits, p20 (20 kDa) and plO (10 kDa). These two subunit types associate very closely, and the protease s active site spans residues from both. plO and p20 are proteolytically derived from a single 45 kDa precursor protein. 

To the best of our knowledge, there is one host which conforms to the structure of an Archimedean dual. Harrison was the first to point out that the quaternary structure of ferritin, a major iron storage protein in animals, bacteria, and plants, corresponds to the structure of a rhombic dodecahedron. [45] This protein, which is approximately 12.5 nm in diameter, consists of 24 identical polypeptide subunits (Fig. 9.18), and holds up to 4500 iron atoms in the form of hydrated ferric oxide with... 

Fig. 9.18 Ferritin, a spherical host based upon the rhombic dodecahedron. X-ray structure of the polypeptide subunit.

Bovine heart cytochrome bci (PDB 1BE3 and PDB IBGY) as studied by Iwata et al. exists as a dimer in the asymmetric unit cell. Each monomer consists of 11 different polypeptide subunits (SU) with a total of -2165 amino acid residues and a molecular mass of -240 kDa. The protein subunits of the complex occupy three separate regions (1) the intermembrane space (p side) occupied by cytochrome Ci (subunit 4, SU4), the iron-sulfur protein (ISP, SU5) and subunit 8 (2) the transmembrane region occupied by cytochrome b (SU3), the transmembrane helices of cytochrome Ci and the ISP, and subunits 7,10, and 11 and (3) the matrix space (n side) occupied by two large core proteins (subunits 1 and 2) as well as subunits 6 and 9. Subunit 8 is often called the hinge protein and is thought to be essential for proper complex formation between cytochrome c (the exit point for some bci complex electrons) and... 

Many of the current ideas about the shape of the ferritin molecule are derived from the high resolution x-ray crystallographic studies of Harrison and coworkers (5,6) (Figure 1) on the protein coat of ferritin from the spleen of horses, in which essentially all (> 90%) of the polypeptide subunits are identical. However, protein coats of ferritin from other animals, and indeed from different cells and tissues in the same animal, can be composed of assemblages of similar, but distinct, subunits (3). 

One striking characteristic of the coupling ATPase of energy-transducing membranes, apart from the extraordinarily large number of different polypeptide subunits, is the existence of two different polypeptides involved in the response of the enzyme to the inhibitor oligomycin. One binds the inhibitor, the other, separated in space from the former by possibly as much as 10 to 15 A, confers oligomycin sensitivity to the entire enzyme complex. How could the transfer of information between these two polypeptide subunits and their concerted interaction with the ATPase proper be visualized ... 

The AChR is one of the best characterized of all cell-surface receptors for hormones or neurotransmitters (Figure 2-9). One form of this receptor is a pentamer made up of four different polypeptide subunits (eg, two chains plus one B, one 7, and one 5 chain, all with molecular weights ranging from 43,000 to 50,000). These polypeptides, each of which crosses the lipid bilayer four times, form a cylindrical structure that is 8 nm in diameter. When acetylcholine binds to sites on the subunits, a conformational change occurs that results in the transient opening of a central aqueous channel through which sodium ions penetrate from the extracellular fluid into the cell. 

EC 1.111.2.2]. Purified in Triton X-100 by solubilising the crude enzyme with Triton X-100, followed by hydroxylapatite and gel chromatography. The minimum unit contains nine polypeptide subunits of Mr 6000 -49000 kD. [Engel et al. Biochim Biophys Acta 592 211 1980]. 

---