B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 39
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Pure watercontains a steady-state concentration of hydrogen ions and–7hydroxyl ions (both 10 M).pHBASES(weak acid)_110ACIDICThe acidity of asolution is definedby the concentrationof H+ ions it possesses.For convenience weuse the pH scale, whereH+conc.moles/litermoles/liter1010_2_3_4_5_6_10 7_10 8ALKALINE_7[H+] = 101010pH = _log10[H+]For pure water10101010101010_9_10_11_12_13_14pH1234567891011121314Substances that reduce the number of hydrogen ions insolution are called bases. Some bases, such as ammonia,combine directly with hydrogen ions.NH3ammoniaH+NH4+hydrogen ionammonium ionOther bases, such as sodium hydroxide, reduce the number of+–H ions indirectly, by making OH ions that then combine+directly with H ions to make H2O.Na+NaOHsodium hydroxide(strong base)sodiumionOH–hydroxylionMany bases found in cells are partially associated with H+ ionsand are termed weak bases.
This is true of compounds thatcontain an amino group (–NH2), which has a weak tendency toreversibly accept an H+ ion from water, increasing the quantityof free OH– ions.–NH2H+–NH3+PANEL 2–3: The Principal Types of Weak Noncovalent Bonds that Hold Macromolecules Together94WEAK NONCOVALENT CHEMICAL BONDSVAN DER WAALS ATTRACTIONSOrganic molecules can interact with other molecules through threetypes of short-range attractive forces known as noncovalent bonds:van der Waals attractions, electrostatic attractions, and hydrogenbonds. The repulsion of hydrophobic groups from water is alsoimportant for the folding of biological macromolecules.If two atoms are too close together they repel each othervery strongly. For this reason, an atom can often betreated as a sphere with a fixed radius.
The characteristic“size” for each atom is specified by a unique van derWaals radius. The contact distance between any twononcovalently bonded atoms is the sum of their van derWaals radii.weaknoncovalentbondHYDROGEN BONDSAs already described for water (see Panel 2–2),hydrogen bonds form when a hydrogen atom is“sandwiched” between two electron-attracting atoms(usually oxygen or nitrogen).Hydrogen bonds are strongest when the three atoms arein a straight line:HONHCNO0.12 nmradius0.2 nmradius0.15 nmradius0.14 nmradiusAt very short distances any two atoms show a weakbonding interaction due to their fluctuating electricalcharges. The two atoms will be attracted to each otherin this way until the distance between their nuclei isapproximately equal to the sum of their van der Waalsradii. Although they are individually very weak, van derWaals attractions can become important when twomacromolecular surfaces fit very close together,because many atoms are involved.Note that when two atoms form a covalent bond, thecenters of the two atoms (the two atomic nuclei) aremuch closer together than the sum of the two van derWaals radii.
Thus,Weak noncovalent chemical bonds have less than 1/20 the strengthof a strong covalent bond. They are strong enough to providetight binding only when many of them are formed simultaneously.OH0.4 nmtwo non-bondedcarbon atomsO0.15 nmsingle-bondedcarbon atoms0.13 nmdouble-bondedcarbon atomsExamples in macromolecules:Amino acids in a polypeptide chain can be hydrogen-bondedtogether. These stabilize the structure of folded proteins.RCRHCOHCOHNCHHCHYDROGEN BONDS IN WATERAny molecules that can form hydrogen bonds to each othercan alternatively form hydrogen bonds to water molecules.Because of this competition with water molecules, thehydrogen bonds formed between two molecules dissolvedin water are relatively weak.RNCCHNCHOCCNCNNHHNHNCCCOCHNNCCOCHHCCO2H2OCCNHOCNHHNCOHHO2H2OHTwo bases, G and C, are hydrogen-bonded in aDNA double helix.HpeptidebondONHCC95HYDROPHOBIC FORCESWater forces hydrophobic groups together,because doing so minimizes their disruptiveeffects on the hydrogen-bonded waternetwork.
Hydrophobic groups heldtogether in this way are sometimes saidto be held together by “hydrophobicbonds,” even though the apparent attractionis actually caused by a repulsion from thewater.HHCCHHHHELECTROSTATIC ATTRACTIONS INAQUEOUS SOLUTIONSHHHCHHCharged groups are shielded by theirinteractions with water molecules.Electrostatic attractions are thereforequite weak in water.CHHOHOOPHHOHHHOHOC+O+ClClHHNaCl+NHClsubstrate+–enzyme1 mmHClDespite being weakened by water and salt,electrostatic attractions are very important inbiological systems. For example, an enzyme thatbinds a positively charged substrate will oftenhave a negatively charged amino acid side chainat the appropriate place.Na+a crystal ofsalt, NaClONa+NaCl–O + MgHNaNa++In the absence of water, electrostatic forces are very strong.They are responsible for the strength of such minerals asmarble and agate, and for crystal formation in commontable salt, NaCl.O+ HSimilarly, ions in solution can cluster aroundcharged groups and further weakenthese attractions.δ–The force of attraction between the two charges, δ+and δ–, falls off rapidly as the distance between thecharges increases.HOHHHELECTROSTATIC ATTRACTIONSHOHOδ+OHHAttractive forces occur both between fully chargedgroups (ionic bond) and between the partially chargedgroups on polar molecules.HHHOOOPANEL 2–4: An Outline of Some of the Types of Sugars Commonly Found in Cells96MONOSACCHARIDESMonosaccharides usually have the general formula (CH2O) n, where n can be 3, 4, 5, 6, 7, or 8, and have two or more hydroxyl groups.OThey either contain an aldehyde group ( C H ) and are called aldoses, or a ketone group ( C O ) and are called ketoses.3-carbon (TRIOSES)5-carbon (PENTOSES)6-carbon (HEXOSES)OHOHALDOSESCOHCCHCOHHCOHHOCHHCOHHCOHHCOHHCOHHCOHHCOHHCOHHCOHHHHglyceraldehyderiboseglucoseHHHKETOSESHHHCOHCOHOCHHCOHCOCOHHCOHHCOHCOHCOHHCOHCOHHCOHHCOHHHHdihydroxyacetoneribulosefructoseRING FORMATIONISOMERSIn aqueous solution, the aldehyde or ketone group of a sugarmolecule tends to react with a hydroxyl group of the samemolecule, thereby closing the molecule into a ring.Many monosaccharides differ only in the spatial arrangementof atoms—that is, they are isomers.
For example, glucose,galactose, and mannose have the same formula (C6H12O6) butdiffer in the arrangement of groups around one or two carbonatoms.CH2OHOHOOHHCH2OHHOHCH2OHHHOHOHOHHOHHOHHHOHgalactoseOHOHHHOHHOHOHHHglucoseOH1 CHC2HO C3HHOHHCOHCOH45CH2OH6HO1CH 2CH 3C5HHOH4HOOHOHO1glucoseH2OHCH2OHO5HOHH3H4H 4C OHCH2OH5CH2OH6H3OHOHH1ribose2 HOHNote that each carbon atomhas a number.mannoseThese small differences make only minor changes in thechemical properties of the sugars. But they are recognized byenzymes and other proteins and therefore can have majorbiological effects.97α AND β LINKSSUGAR DERIVATIVESThe hydroxyl group on the carbon that carries thealdehyde or ketone can rapidly change from oneposition to the other. These two positions arecalled α and β.The hydroxyl groups of a simplemonosaccharide such as glucosecan be replaced by other groups.For example,HOCH2OHOOHOOOHβ hydroxylOHHOOHHN-acetylglucosamineAs soon as one sugar is linked to another, the α orβ form is frozen.DISACCHARIDESCOHOHHNH2OOHCglucosamineONHα hydroxylCH2OHOglucuronic acidOHOOHCH3CH2OHα glucoseThe carbon that carries the aldehydeor the ketone can react with anyhydroxyl group on a second sugarmolecule to form a disaccharide.The linkage is called a glycosidicbond.OHβ fructoseO+OHHOOHHOHOOHH2OOmaltose (glucose + glucose)lactose (galactose + glucose)sucrose (glucose + fructose)HOCH2OHHOOHOHOOThe reaction forming sucrose isshown here.CH2OHOHCH2OHThree common disaccharides areOHOCH2CH2OHOHsucroseOLIGOSACCHARIDES AND POLYSACCHARIDESLarge linear and branched molecules can be made from simple repeating sugarsubunits.
Short chains are called oligosaccharides, while long chains are calledpolysaccharides. Glycogen, for example, is a polysaccharide made entirely ofglucose units joined together.glycogenbranch pointsCH2OHCOMPLEX OLIGOSACCHARIDESIn many cases a sugar sequenceis nonrepetitive. Many differentmolecules are possible. Suchcomplex oligosaccharides areusually linked to proteins or to lipids,as is this oligosaccharide, which ispart of a cell-surface moleculethat defines a particular blood group.CH2OHHOCH2OHOHOOOOONHCOOCH3OOHOOHCH3HOOHNHCOCH3OHPANEL 2–5: Fatty Acids and Other Lipids98COMMON FATTYACIDSThese are carboxylic acidswith long hydrocarbon tails.COOHCOOHCOOHCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CHCH2CH2CHCH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3CH2CH2palmiticacid(C16)CH2CH3stearicacid (C18)Hundreds of different kinds of fatty acids exist.
Some have one or more double bonds in theirhydrocarbon tail and are said to be unsaturated. Fatty acids with no double bonds are saturated.–O–OOOCCThis double bondis rigid and createsa kink in the chain.The rest of the chainis free to rotateabout the other C–Cbonds.oleicacidspace-filling modelSATURATEDTRIACYLGLYCEROLSFatty acids are stored as an energy reserve (fats andoils) through an ester linkage to glycerol to formtriacylglycerols, also known as triglycerides.OOHCOH2COCH3oleicacid (C18)carbon skeletonUNSATURATEDH2CCARBOXYL GROUPstearicacidCOH2COHCHCOHOH2COHCglycerolPHOSPHOLIPIDSPhospholipids are the major constituentsof cell membranes.If free, the carboxyl group of afatty acid will be ionized.cholineOChydrophilicheadO_OOOP_OBut more usually it is linked toother groups to form either estersCH2OCHCH2COCor amides.Ohydrophobicfatty acid tailsCNHgeneral structureof a phospholipidspace-filling model ofthe phospholipidphosphatidylcholineIn phospholipids, two of the –OH groups in glycerol arelinked to fatty acids, while the third –OH group is linkedto phosphoric acid.
The phosphate is further linked toone of a variety of small polar groups, such as choline.99POLYISOPRENOIDSLIPID AGGREGATESlong-chain polymers of isopreneFatty acids have a hydrophilic headand a hydrophobic tail.O–micelleIn water they can form a surface filmor form small micelles.OPO–OTheir derivatives can form larger aggregates held together by hydrophobic forces:Triacylglycerols (triglycerides) can formlarge spherical fat droplets in the cellcytoplasm.Phospholipids and glycolipids form self-sealing lipidbilayers that are the basis for all cell membranes.200 nmor more4 nmOTHER LIPIDSSTEROIDSLipids are defined as the water-insolublemolecules in cells that are soluble in organicsolvents.
Two other common types of lipidsare steroids and polyisoprenoids. Both aremade from isoprene units.CH3CCHCH2CH2isopreneSteroids have a common multiple-ring structure.OHHOOcholesterol—found in many membranestestosterone—male steroid hormoneGLYCOLIPIDSLike phospholipids, these compounds are composed of a hydrophobicregion, containing two long hydrocarbon tails and a polar region,which contains one or more sugars and, unlike phospholipids,no phosphate.OHHCCHCHHCC NHOgalactoseOCH2sugara simpleglycolipiddolichol phosphate—usedto carry activated sugarsin the membrane-associatedsynthesis of glycoproteinsand some polysaccharidesPANEL 2–6: A Survey of the Nucleotides100OBASESNH2CNH2HCHCCHCHCCUcytosineCNHCNHNThe bases are nitrogen-containing ringcompounds, either pyrimidines or purines.uracilH3C4HCthymineNHT53N621NN751N429N3NNHPYRIMIDINEOPURINENUCLEOTIDESThe phosphates are normally joined tothe C5 hydroxyl of the ribose ordeoxyribose sugar (designated 5').
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