Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 18
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In natural nucleotides,the 1' carbon is joined by a linkage to the base (in this caseadenine); both the base (blue) and the phosphate on the 5'hydroxyl (red) extend above the plane of the furanose ring.(b) Ribose and deoxyribose, the pentoses in RNA and DNA,respectively.NH2C6CHN4HN35CHC2C5C61NOCH3N3CCHOHHUracil (U)2415CH6CHNHThymine (T)Cytosine (C)▲ FIGURE 2-15 Chemical structures of the principal basesin nucleic acids. In nucleic acids and nucleotides, nitrogen 9 ofpurines and nitrogen 1 of pyrimidines (red) are bonded to the1 carbon of ribose or deoxyribose. U is only in RNA, and T isonly in DNA. Both RNA and DNA contain A, G, and C.the 1 carbon atom of the sugar (ribose or deoxyribose) isattached to the nitrogen at position 9 of a purine (N9) or atposition 1 of a pyrimidine (N1).
The acidic character of nucleotides is due to the phosphate group, which under normalintracellular conditions releases a hydrogen ion (H), leaving the phosphate negatively charged (see Figure 2-14a).Most nucleic acids in cells are associated with proteins,which form ionic interactions with the negatively chargedphosphates.Cells and extracellular fluids in organisms contain smallconcentrations of nucleosides, combinations of a base and asugar without a phosphate.
Nucleotides are nucleosides thathave one, two, or three phosphate groups esterified at the 5hydroxyl. Nucleoside monophosphates have a single esterified phosphate (see Figure 2-14a); diphosphates contain apyrophosphate group:O1H3OOOOPOOPOPyrophosphateand triphosphates have a third phosphate. Table 2-2 lists thenames of the nucleosides and nucleotides in nucleic acids andthe various forms of nucleoside phosphates. The nucleosidetriphosphates are used in the synthesis of nucleic acids,which we cover in Chapter 4. Among their other functions inthe cell, GTP participates in intracellular signaling and actsas an energy reservoir, particularly in protein synthesis, andATP, discussed later in this chapter, is the most widely usedbiological energy carrier.2.2 • Chemical Building Blocks of CellsTABLE 2-241Terminology of Nucleosides and NucleotidesBasesPurinesPyrimidinesAdenine (A)Guanine (G)Cytosine (C)Uracil (U)Thymine [T]in RNAAdenosineGuanosineCytidineUridinein DNADeoxyadenosineDeoxyguanosineDeoxycytidineDeoxythymidinein RNAAdenylateGuanylateCytidylateUridylatein DNADeoxyadenylateDeoxyguanylateDeoxycytidylateDeoxythymidylateNucleoside monophosphatesAMPGMPCMPUMPNucleoside diphosphatesADPGDPCDPUDPNucleoside triphosphatesATPGTPCTPUTPDeoxynucleoside mono-,di-, and triphosphatesdAMP, etc.NucleosidesNucleotides{{Monosaccharides Joined by Glycosidic BondsForm Linear and Branched PolysaccharidesThe building blocks of the polysaccharides are the simplesugars, or monosaccharides.
Monosaccharides are carbohydrates, which are literally covalently bonded combinations ofcarbon and water in a one-to-one ratio (CH2O)n, where nequals 3, 4, 5, 6, or 7. Hexoses (n = 6) and pentoses (n = 5)are the most common monosaccharides. All monosaccharides contain hydroxyl (OOH) groups and either an aldehyde or a keto group:OCCOHAldehydeD-GlucoseCC(a)H65HCOH O4HOHHHH32H1HOCHCOHHD-Glucofuranose6C2345HOH4HCH2OH5OHOH HHOHOHH1OH3OH2OHD-Glucopyranose6(rare)(common)CH2OHD-Glucose(b)(C6H12O6) is the principal external source ofenergy for most cells in higher organisms and can exist inthree different forms: a linear structure and two differenthemiacetal ring structures (Figure 2-16a).
If the aldehydegroup on carbon 1 reacts with the hydroxyl group on carbon5, the resulting hemiacetal, D-glucopyranose, contains a sixmember ring. In the anomer of D-glucopyranose, the hydroxyl group attached to carbon 1 points “downward” fromthe ring as shown in Figure 2-16a; in the anomer, this hydroxyl points “upward.” In aqueous solution the and anomers readily interconvert spontaneously; at equilibriumthere is about one-third anomer and two-thirds , withvery little of the open-chain form.
Because enzymes can distinguish between the and anomers of D-glucose, theseforms have distinct biological roles. Condensation of the hydroxyl group on carbon 4 of the linear glucose with its alde-COHHO1HCCKetoO1CCH2OHHOHOHHCCCCO1C2345HHOHOH6CH2OHD-MannoseHHOHOHCCCC2345OHHHOH6CH2OHD-Galactose▲ FIGURE 2-16 Chemical structures of hexoses. All hexoseshave the same chemical formula (C6H12O6) and contain an aldehydeor keto group. (a) The ring forms of D-glucose are generated fromthe linear molecule by reaction of the aldehyde at carbon 1 withthe hydroxyl on carbon 5 or carbon 4. The three forms are readilyinterconvertible, although the pyranose form (right) predominatesin biological systems.
(b) In D-mannose and D-galactose, theconfiguration of the H (green) and OH (blue) bound to one carbonatom differs from that in glucose. These sugars, like glucose, existprimarily as pyranoses.42CHAPTER 2 • Chemical Foundationspolymers. Glycosidic bonds are usually formed between a covalently modified sugar and the growing polymer chain.Such modifications include a phosphate (e.g., glucose 6phosphate) or a nucleotide (e.g., UDP-galactose):hyde group results in the formation of D-glucofuranose, ahemiacetal containing a five-member ring. Although all threeforms of D-glucose exist in biological systems, the pyranoseform is by far the most abundant.Many biologically important sugars are six-carbon sugars that are structurally related to D-glucose (Figure 2-16b).Mannose is identical with glucose except that the orientationof the groups bonded to carbon 2 is reversed.
Similarly,galactose, another hexose, differs from glucose only in theorientation of the groups attached to carbon 4. Interconversion of glucose and mannose or galactose requires the breaking and making of covalent bonds; such reactions are carriedout by enzymes called epimerases.The pyranose ring in Figure 2-16a is depicted as planar. Infact, because of the tetrahedral geometry around carbonatoms, the most stable conformation of a pyranose ring has anonplanar, chairlike shape. In this conformation, each bondfrom a ring carbon to a nonring atom (e.g., H or O) is eithernearly perpendicular to the ring, referred to as axial (a), ornearly in the plane of the ring, referred to as equatorial (e):HaeeOaHOeaHOeHHa1HOHH-D-GlucopyranosePyranosesThe enzymes that make the glycosidic bonds linkingmonosaccharides into polysaccharides are specific for the or anomer of one sugar and a particular hydroxyl group onthe other.
In principle, any two sugar molecules can be linkedin a variety of ways because each monosaccharide has multiple hydroxyl groups that can participate in the formation ofglycosidic bonds. Furthermore, any one monosaccharide hasthe potential of being linked to more than two other monosaccharides, thus generating a branch point and nonlinearHO1OH HHH4OH1HHOH1HOHHOHHOOOOPUridineOOHOPOUDP-galactoseCH2OHOHOH HOHH2OCH2OHOHOH HH1HHHOHOCH2OHOHOH HO4OHHH▲▲ FIGURE 2-17 FormationCH2OHOHOH H6CH2OHOHThe epimerase enzymes that interconvert different monosaccharides often do so using the nucleotide sugars rather thanthe unsubstituted sugars.Disaccharides, formed from two monosaccharides, arethe simplest polysaccharides. The disaccharide lactose, composed of galactose and glucose, is the major sugar in milk;the disaccharide sucrose, composed of glucose and fructose,is a principal product of plant photosynthesis and is refinedinto common table sugar (Figure 2-17).Larger polysaccharides, containing dozens to hundredsof monosaccharide units, can function as reservoirs for glucose, as structural components, or as adhesives that helphold cells together in tissues.
The most common storage carbohydrate in animal cells is glycogen, a very long, highlybranched polymer of glucose. As much as 10 percent byweight of the liver can be glycogen. The primary storage carbohydrate in plant cells, starch, also is a glucose polymer. Itoccurs in an unbranched form (amylose) and lightlybranched form (amylopectin).
Both glycogen and starch arecomposed of the anomer of glucose. In contrast, cellulose,the major constituent of plant cell walls, is an unbranchedpolymer of the anomer of glucose. Human digestive enzymes can hydrolyze the glycosidic bonds in starch, but notthe glycosidic bonds in cellulose. Many species of plants,bacteria, and molds produce cellulose-degrading enzymes.OHHO3OHOPO32OHGlucose 6-phosphate2HHHO6CH2OH45ae6CH2of the disaccharides lactoseand sucrose. In any glycosidiclinkage, the anomeric carbonof one sugar molecule (ineither the or conformation)is linked to a hydroxyl oxygenon another sugar molecule.
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