s: 9U R T H E m Irma:-
`
`
`
`Luber't Stryer
`
`PGR2020-00009
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`Petitioner Ex. 1082 - Page 1
`
`

`

`r
`
`FOURTH EDITION
`
`Lubert Stryer
`
`STANFORD UNIVERSITY
`
`•=
`
`W. H. Freeman and Company
`New York
`
`PGR2020-00009
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`Petitioner Ex. 1082 - Page 2
`
`

`

`Library of Congress Cataloging-in-Publication Data
`
`Stryer, Lubert.
`Biochemistry/Lubert Stryer.-4th ed.
`
`Includes index.
`ISBN 0-7167-2009-4
`I. Title.
`1. Biochemistry.
`QP514.2.S66 1995
`574.19'2-dc20
`
`94-22832
`
`©1975, 1981, 1988, 1995 by Lubert Stryer
`
`No part of this book may be reproduced by any mechanical,
`photographic, or electronic process, or in the form of a phonographic
`recording, nor may it be stored in a retrieval system, transmitted, or
`otherwise copied for public or private use, without written permission
`from the publisher.
`
`Printed in the United States of America
`
`Second printing 1995, KP
`
`PGR2020-00009
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`Petitioner Ex. 1082 - Page 3
`
`

`

`CHAPTER
`
`18
`
`Carbohydrates
`
`Before embarking on our metabol_icjourney, let us take an overview of
`
`carbohydrates, one of the four major classes of biomolecules. We
`have already considered the other three-proteins, nucleic acids, and
`lipids. Carbohydrates are aldehyde or ketone compounds with multiple
`hydroxyl groups. They make up most of the organic matter on earth
`because of their multiple roles in all forms of life. First, carbohydrates
`serve as energy stores, fuels, and metabolic intermediates. Starch in plants and
`glycogen in animals are polysaccharides that can be rapidly mobilized to
`yield glucose, a prime fuel for the generation of energy. ATP, the univer(cid:173)
`sal currency of free energy, is a phosphorylated sugar derivative, as are
`many coenzymes. Second, ribose and deoxyribose sugars form part of the
`structural framework of RNA and DNA. The conformational flexibility of
`these sugar rings is important in the storage and expression of genetic
`information. Third, polysaccharides are structural elements in the cell walls of
`bacteria and plants, and in the exoskeletons of arthropods. In fact, cellulose, the
`main constituent of plant cell walls, is one of the most abundant organic
`compounds in the biosphere. Fourth, carbohydrates are linked to many
`proteins and lipids. For example, the sugar units of glycophorin, an integral
`membrane protein, give red blood cells a highly polar anionic coat.
`
`opening Image: Structure of a carbohydrate-bearing protein. The seven-residue
`oligosaccharide projects from the surface of the protein. Four kinds of carbohydrate units(cid:173)
`N-acetylglucosamine (yellow), mannose (green), fucose (red), and xylose (orange)-are
`present in this branched chain. The oligosaccharide chain is attached to an asparagine
`residue (blue) of the protein. [Drawn from 1 lte.pdb. B. Shaanan, H. Lis, and N.
`Sharon. Science 254(1991):862.J
`
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`

`

`464
`
`Part Ill
`
`METABOLIC ENERGY
`
`A
`
`Re~ent studies ha:~ revealed that c~~boh~drate U'r':its o~ cell surf~cesplay key
`roles zn cell-cell recognztzon processes. Fert1hzat1on begms with the bmding of a
`sperm to a specific oligosaccharide on the surface of an egg. The adhe(cid:173)
`sion of leukocytes to the lining of injured blood vessels and the return of
`lymphocytes to their sites of origin in lymph nodes further illustrate the
`importance of carbohydrates in recognition processes. Carbohydrates
`have entered the limelight as information-rich molecules, full of signifi(cid:173)
`cance in development and repair.
`
`MONOSACCHARIDES ARE ALDEHYDES OR KETONES
`WITH MULTIPLE HYDROXYL GROUPS
`
`Monosaccharides, the simplest carbohydrates, are aldehydes or ketones
`that have two or more hydroxyl groups; the empirical formula of many is
`(CH20)n- The smallest ones, for which n = 3, are glyceraldehyde and
`dihydroxyacetone. They are trioses. Glyceraldehyde is also an aldose be(cid:173)
`cause it contains an aldehyde group, whereas dihydroxyacetone is a ketose
`because it contains a keto group.
`
`0~ H
`~c/
`I
`H-c~oH
`I
`CH20H
`o-Glyceraldehyde
`(An aldose)
`
`0
`H
`~ /
`C
`I
`HO-C-H
`I
`CH20H
`L-Glyceraldehyde
`(An aldose)
`
`CH20H
`I
`C=O
`I
`CH20H
`Dihydroxyacetone
`(A ketose)
`
`Figure 18-1
`(A) Fischer representation of a tetra(cid:173)
`hedral carbon atom with substituents
`A, B, C, and D; and· (B) a model
`showing the stereochemistry denoted
`by this projection.
`
`Glyceraldehyde has a single asymme tric carbon. Thus, there are two
`stereoisome5s of this three-carbon aldose, o-glyceraldehyde and L-glycer(cid:173)
`aldehyde. Th~ prefixes n and L designate the absolute configuration. Re(cid:173)
`call that in a Fischer projection of a molecule, atoms joined to an asym(cid:173)
`metric carbon atom by horizontal bonds are in front of the plane of the
`page, and those joined by vertical bonds are behind (Figure 18-1).
`
`Figure 18-2
`Model showing the absolute configu(cid:173)
`ration of n-glyceraldehyde. Fischer's
`arbitrary assignment of the D configu(cid:173)
`ration to this stereoisomer was shown
`years later, by x-ray crystallography,
`to be correct.
`
`TH 20H
`C=O
`I
`HO-C-H
`I
`H-C-OH
`I
`H-C-OH
`I
`CH 20H
`o-Fructose
`(A ketose)
`
`Aldoses with 4, 5, 6, and 7 carbon atoms are called tetroses, pentoses,
`hexoses, and heptoses. Two common hexoses are o-glucose (an aldose) and
`o-Jructose (a ketose). For sugars with more than one asymmetric carbon
`atom, the symbols n and L refer to the absolute configuration of the
`asymmetric carbon farthest from the aldehyde or keto group. These hex(cid:173)
`oses belong to the n series because their configuration at C-5 is the same
`as that in n-glyceraldehyde.
`In general, a molecule with n asymmetric centers and no plane of sym(cid:173)
`metry has 2n stereoisomeric forms. For aldotrioses, n = l , and so there
`are two stereoisomers, o- and L-glyceraldehyde. They are enantiomers (mir-
`
`2
`
`3
`
`4
`
`5
`
`6
`
`0~
`
`/ H
`
`?
`H-C-OH
`I
`HO-C-H
`I
`H-C-OH
`I
`H-C-OH
`I
`CHPH
`o-Glucose
`(An aldose)
`
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`

`

`CHO
`I
`HCOH
`I
`CH 20H
`o-Glyceraldehyde
`
`2
`
`3
`
`~
`
`2
`
`3
`
`4
`
`CHO ~
`I
`HCOH
`HtOH
`I
`CH 2OH
`o-Erythrose
`
`CHO
`I
`HOCH
`HtOH
`I
`CH 20H
`o-Threose
`
`/
`
`CHO
`I
`HCOH
`I
`HOCH
`H60H
`I
`CH 2OH
`o-Xylose
`
`I~
`
`CHO
`I
`HCOH
`I
`HCOH
`I
`HOCH
`I
`HCOH
`I
`CH 2OH
`· o-Gulose
`
`CHO
`I
`HOCH
`I
`HCOH
`I
`HOCH
`I
`HCOH
`I
`CH 2OH
`o-ldose
`
`~ CHO
`
`I
`HOCH
`I
`HOCH
`H60H
`I
`CH 20H
`o-Lyxose
`
`I ~
`
`CHO
`I
`HCOH
`I
`HOCH
`I
`HOCH
`I
`HCOH
`I
`CH 2OH
`o-Galactose
`
`CHO
`I
`HOCH
`I -
`HOCH
`I
`HOCH
`I
`HCOH
`I
`CH 2OH
`o-Talose
`
`/ ~
`
`CHO
`I
`HCOH
`I
`HCOH
`I
`HCOH
`I
`CH 2OH
`o-Ribose
`
`2
`
`3
`
`4
`
`5
`
`/ \
`
`CHO
`I
`HCOH
`I
`HCOH
`I
`HCOH
`I
`HCOH
`I
`CH 2OH
`o-Allose
`
`CHO
`I
`HOCH
`I
`HCOH
`I
`HCOH
`I
`HCOH
`I
`CH 2OH
`o-Altrose
`
`l
`
`2
`
`4
`
`~
`
`6
`
`CHO
`I
`HOCH
`I
`HCOH
`I
`HCOH
`I
`CH 2OH
`o-Arabinose
`
`I~
`
`CHO
`I
`HCOH
`I
`HOCH
`I
`HCOH
`HtOH
`I
`CH 2OH
`o-Glucose
`
`CHO
`I
`HOCH
`I
`HOCH
`I
`HCOH
`I
`HCOH
`I
`CH 2OH
`o-Mannose
`
`Figure 18-3
`Stereochemical relations of n aldoses containing three, four, five, and six carbon
`atoms. These sugars are n aldoses because they contain an aldehyde group (shown
`in green) and have the configuration of n-glyceraldehyde at their farthest asym(cid:173)
`metric center (shown in red).
`
`ror images) of each other. Addition of an HCOH group gives four aldotet(cid:173)
`roses because n = 2. Two of them are o sugars and the other two are the
`enantiomeric L sugars. Let us follow the o sugar series (Figure 18-3). One
`of these four-carbon aldoses is o-erythrose and the other is o-threose.
`They have the same configuration at C-3 (because they are o sugars) but
`opposite configurations at C-2. They are diastereoisomers, not enantiomers,
`because they are not mirror images of each other.
`The five-carbon aldoses have three asymmetric centers, which give 8
`(23
`) stereoisomers, 4 in the D series. o-Ribose belongs to this group. The
`six-carbon aldoses have four asymmetric centers, and so there are 16 (24 )
`stereoisomers, 8 in the o series. o-Glucose, o-mannose, and o-galactose
`are abundant six-carbon aldoses. Note that o-glucose and o-mannose dif(cid:173)
`fer only in configuration at C-2. o Sugars differing in configuration at a
`single asymmetric center are epimers. Thus, o-glucose and o-mannose are
`epimers at C-2; o-glucose and o-galactose are epimers at C-4. Emil Fisch(cid:173)
`er's elucidation in 1891 of the configuration of o-glucose was a remark(cid:173)
`able achievement that greatly stimulated the field of organic chemistry.
`
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`
`

`

`466
`
`Part Ill
`
`METABOLIC ENERGY
`
`Figure 18-4
`Stereochemical relations of D ketoses
`containing three, four, five, and six
`carbon atoms. These sugars are
`D ketoses because they contain a
`keto group (shown in green) and have
`the configuration of o-glyceraldehyde
`at their farthest asymmetric center
`(shown in red) .
`
`2
`
`3
`
`CH 20H
`t=o
`I
`CH 20H
`Dihydroxyacetone
`
`l
`CH 20H
`l=o
`I
`HCOH
`I
`CH 20H
`
`2
`
`3
`
`4
`
`rr
`
`o-Ribulose
`
`/ ~
`
`2
`
`6
`
`CH 20H
`I
`C=O
`I
`3 HCOH
`I
`4
`HCOH
`I
`5 HCOH
`I
`CH 20H
`o-Psicose
`
`CH 20H
`~=0
`I
`HOCH
`I
`HCOH
`I
`HCOH
`I
`CH 20H
`a-Fructose
`
`o-Xylulose
`
`/ ~
`CH 20H
`t=O
`I
`HCOH
`I
`HOCH
`I
`HCOH
`I
`CH 20H
`o-Sorbose
`
`CH 20H
`~= 0
`I
`HOCH
`I
`HOCH
`
`Ht OH
`I
`CH 20H
`o-Tagatose
`
`The stereochemical relations of o ketoses are shown in Figure 18-4.
`Dihydroxyacetone, the simplest of these sugars, is optically inactive. o(cid:173)
`Erythrulose is the sole four-carbon o ketose because ketoses have one
`fewer asymmetric center than do aldoses with the same number of carbon
`atoms. Hence, there are two five-carbon and four six-carbQn D ketoses.
`o-Fructose is the most abundant ketohexose.
`
`PENTOSES AND HEXOSES CVCLIZE TO FORM
`FURANOSE AND PVRANOSE RINGS
`
`The predominant forms of glucose and fructose in solution are not open
`chains. Rather, the open-chain forms of these sugars cyclize into rings. In
`general, an aldehyde can react with an alcohol to form a hemiacetal.
`
`,P
`R-Cf" + HOR'
`"-H
`
`Aldehyde
`
`Alcohol
`
`H
`I
`== R-C- OR'
`I
`OH
`Hemiacetal
`
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`

`

`The C-1 aldehyde in the open-chain form of glucose reacts with the C-5
`hydroxyl group to form an intramolecular hemiacetal. The resulting six(cid:173)
`membered ring is called pyranose because of its similarity to pyran.
`
`Chapter 18
`CARBOHYDRATES
`
`467
`
`~ 4
`
`~H H
`
`1
`OH
`
`HO
`
`~ Reducing group
`
`H
`
`H
`
`c /
`~ O
`
`2
`3
`OH
`H
`0
`~c/
`CH 20H ~ a-o-Glucopyranose
`I
`I
`2 H-C-OH
`H/y-OH
`I
`I H
`3 HO-C-H ~
`~""?H ~ /
`I
`4 H-C-OH
`HO C-C
`I
`I
`I
`5 H-C-OH
`OH
`H
`I
`CH 20H
`o-Glucose
`(Open-chain form)
`
`0
`
`Pyran
`
`6
`
`~
`
`~
`HO
`
`OH H
`
`H
`H
`
`Similarly, a ketone can react with an alcohol to form a hemiketal.
`
`H
`OH
`/3-0-Glucopyranose
`
`R
`I
`C=O + HOR"
`I
`R'
`
`OR"
`" /
`R
`/c"
`OH
`R'
`
`Ketone
`
`Alcohol
`
`Hemiketal
`
`The C-2 keto group in the open-chain form of fructose can react with the
`intramolecular hemiketal. This five(cid:173)
`C-5 hydroxyl group to form an
`membered ring is called Juranose because of its similarity to Juran.
`
`0 0
`
`Furan
`
`2
`
`CH 20H
`~=O
`I
`3 HO-C-H
`I
`H-C-OH
`I
`H-C-OH
`I
`CH 20H
`o-Fructose
`
`4
`
`5
`
`6
`
`a-o-Fructofuranose
`(A ring form of fructose)
`
`The depictions of glucopyranose and fructofuranose on this page are
`Haworth projections. In such a projection, the carbon atoms in the ring are
`not explicitly shown. The approximate plane of the ring is perpendicular
`to the plane of the paper, with the heavy line on the ring projecting
`toward the reader.
`An additional asymmetric center is created when glucose cyclizes. C-1,
`the carbonyl carbon atom in the open-chain form, becomes an asymmet(cid:173)
`ric center in the ring form of the sugar. Two ring structures can be
`
`PGR2020-00009
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`
`

`

`468
`
`Part Ill
`
`METABOLIC ENERGY
`
`formed: a-o-glucopyranose and /3-0-glucopyranose. For n sugars drawn
`as Haworth projections, the designation a means that the hydroxyl group at(cid:173)
`tached to C -1 is below the plane of the ring; /3 means that it is above the plane of
`the ring. The C-1 carbon is called the anomeric carbon atom, and so the a
`and /3 forms are anomers.
`The same nomenclature applies to the furanose ring form of fructose
`except that a and /3 refer to the hydroxyl groups attached to C-2, th~
`anomeric carbon atom. Fructose also forms pyranose rings. In fact, the
`pyranose form predominates in fructose free in solution, whereas the
`furanose form is the major one in most of its derivatives.
`
`Figure 18-5
`Model of /3-n-glucopyranose.
`
`6
`HOH 2C~O 1 CHpH
`2
`5
`H HO
`
`H
`
`OH
`
`3
`4
`OH H
`a-o-Fructofuranose
`
`HOH 2C~O OH
`H HO
`
`H
`
`CH 20H
`
`OH H
`/3-0-Fructofuranose
`
`5
`HO
`
`CH20H
`2
`OH
`
`H 0 H
`
`3
`H
`OH
`a-o-Fructopyranose
`
`HO OH
`H
`H
`
`OH
`
`CH 20H
`
`OH H
`13:0-Fructopyranos e
`
`HO
`~
`
`Figure 18-6
`Model of /3-n-fructofuranose.
`
`Five-carbon sugars such as o-ribose and 2-deoxy-o-ribose form furanose
`rings, as was exemplified by the structure of these units in RNA and DNA.
`
`Reducing sugars-
`Sugars containing a free
`aldehyde or keto group reduce
`indicators such as cupric ion
`(Cu2+) complexes to the
`cuprous form (Cu+). The
`reducing agent in these
`reactions is the open-chain
`form of the aldose or ketose.
`The reducing end of a sugar is
`the one containing a free
`aldehyde or keto group.
`
`5
`2
`
`
`HOC~0 OH
`H
`H
`4
`1
`H
`z
`
`H
`
`3
`
`OH
`HO
`/3-0-Ribofu ranose
`
`
`HOC~H0 OH
`2
`H
`H
`
`
`
`H
`
`H
`
`H
`HO
`2-Deoxy-p-o-ribofuranose
`
`In water, a-o-glucopyranose and /3-0-glucopyranose interconvert
`through the open-chain form to give an equilibrium mixture. This inter(cid:173)
`conversion was detected many years ago by following changes in optical
`rotation and was called mutarotation. An equilibrium mixture contains
`about one-third a anomer, two-thirds f3 anomer, and very little ( <1 % ) of
`the open-chain form. Likewise, the a and /3 anomers of both the pyranose
`and the furanose forms of fructose interconvert through the open-chain
`form. Some cells contain mutarotases that accelerate the interconversion
`of anomeric sugars. We shall use the terms glucose and fructose to refer to
`the equilibrium mixture of the open -chain and ring forms.
`
`CONFORMATION OF PYRANOSE AND FURANOSE RINGS
`
`The six-membered pyranose ring, like cyclohexane, cannot be planar
`because of the tetrahedral geometry of its saturated carbon atoms. In(cid:173)
`stead, pyranose rings adopt chair and boat conformations (Figure 18-7) .
`The substituents on the ring carbon atoms have two orientations: axial
`
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`

`

`and equatorial. Axial bonds are nearly perpendicular to the average plane
`of the ring, whereas equatorial bonds are nearly parallel to this plane. As
`can be seen from Figure 18-7 (or even better, from actual molecular
`models in your hands), axial substituents emerge above and below the
`average plane of the ring, whereas equatorial substituents emerge at the
`periphery. Axial substituents sterically hinder each other if they emerge
`on the same side of the ring (e.g., 1,3-diaxial groups). In contrast, equato(cid:173)
`rial substituents are less crowded. The chair form of (3-v-glucopyranose pre(cid:173)
`dominates because all axial positions are occupied by hydrogen atoms. The bul(cid:173)
`kier -OH and -CH2OH groups emerge at the less hindered periphery. In
`contrast, the boat form of glucose is highly disfavored because it is so
`sterically hindered.
`Furanose rings, like pyranose rings, are not planar. They can be puck(cid:173)
`ered so that four atoms are nearly coplanar and the fifth is about 0.5 A
`away from this plane. This conformation is called an envelope form because
`the structure resembles an opened envelope with the back flap raised
`(Figure 18-8) . In the ribose moiety of most biomolecules, either C-2 or
`C-3 is out of plane on the same side as C-5. These conformations are
`called Crendo and C3,-endo. As will be discussed further in Chapter 31,
`the sugars in RNA are in the C3•-endo form, whereas the sugars in the
`Watson-Crick DNA double helix are in the C2•-endo form. Furanose rings
`can interconvert rapidly between different conformational states. They
`are more flexible than pyranose rings, which may account for their selec(cid:173)
`tion as components of RNA and DNA.
`
`H
`
`HO
`
`Figure 18-8
`An envelope form of /3-0-ribose. The
`C3,-endo conformation is shown. C-3 is
`out of plane on the same side as C-5.
`
`, OH
`H
`
`a
`
`e
`
`e
`
`OH
`
`a
`
`a
`A chair form
`of a pyranose
`(e = par.allel substituent;
`a = axial substituent)
`
`H
`
`HO
`
`H
`Stable chair form of
`f:1-o-glucopyranose
`
`H
`
`a
`
`e
`
`a
`
`a
`A boat form
`of a pyranose
`
`Figure 18-7
`Chair and boat conformations of
`pyranose rings. For sugars with
`large equatorial substituents, the chair
`form is energetically more favorable
`because it is less hindered.
`
`CARBOHYDRATES ARE JOINED TO ALCOHOLS AND AMINES
`BY GLYCOSIDIC BONDS
`
`When glucose is warmed in anhydrous methanol containing HCI, its ano(cid:173)
`meric carbon atom reacts with the hydroxyl group of the alcohol to form
`two acetals, methyl a-v-glucopyranoside and methyl (3-v-glucopyranoside. Acid
`facilitates removal of the - OH group by protonating the anomeric car(cid:173)
`bon atom.
`
`0-Glycosidic
`
`:H,OH0 H)"d
`
`OH
`
`H
`
`OCH3
`
`HO
`
`~
`
`OH
`H
`Methyl
`a·o-glucopyranoside
`
`CH 20H
`O OCH 3
`
`OH
`
`H
`
`H
`
`HO
`~
`
`H
`OH
`Methyl
`/J-o-glucopyranoside
`
`The new bond between C-1 of glucose and the oxygen atom of methanol
`is called a glycosidic bond-specifically, an 0 -glycosidic bond. Sugars can
`be linked to each other by 0-glycosidic bonds to form disaccharides and
`polysaccharides. In cellulose, for example, o-glucose residues are joined
`
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`
`

`

`{3-Glycosidic bond
`
`CH,OH
`
`(
`
`CH,OH
`
`vt->A~:~
`
`H6H~ 'LJ'\.,t=--}l'1
`
`o H
`
`H
`
`H
`
`OH
`
`OH
`Cellobiose
`(f:l•o-Glucopyranosyl-(1---->4)(cid:173)
`a-o-glucopyranose)
`
`NH2
`I
`W"'c'c---N\'-
`1
`II
`~CH
`HC:::::;:.w,c...__ N
`
`N-Glycosidic
`HOC ~ bond
`
`H
`
`H
`
`H
`
`HO
`OH
`Adenosine
`
`-o
`
`tb
`pl
`a
`Il'
`
`s
`A
`
`I
`a
`s
`l
`j
`
`by glycosidic linkages between C-1 of one sugar and the hydroxyl oxyge
`11
`atom on C-4 ofan adjacent sugar. The glycosidic bonds in cellulose have
`f3 configuration. In other words, the bond emerging from C-1 lies abov:
`the plane of the ring when viewed in the standard orientation. Hence
`glucose units in cellulose are joined by /3 (1-4) glycosidic linkages, which
`can be concisely denoted by the abbreviation /3-1,4. Recall that N-acetyl(cid:173)
`muramate and N-acetylglucosamine sugars in bacterial cell wall polysac(cid:173)
`charides also are joined by /3-1,4 linkages (p. 208). By contrast, amylose is
`an a-1,4 polymer of glucose.
`The anomeric carbon atom of a sugar can be linked to the nitrogen
`atom of an amine by an N-glycosidic bond. The crucial biological impor(cid:173)
`tance of this type of glycosidic linkage is evident in such central biomole(cid:173)
`cules as nucleotides, RNA, and DNA. N-glycosidic linkages in virtually all
`naturally occurring biomolecules have the f3 configuration.
`
`PHOSPHORYLATED SUGARS ARE KEY INTERMEDIATES
`IN ENERGY GENERATION AND BIOSYNTHESES
`
`Phosphorylated sugars are another important class of derivatives. In the
`next chapter, we shall see that the first step in glycolysis, the breakdown of
`glucose to obtain energy, is its conversion to glucose 6-phosphate. The trans(cid:173)
`fer of a phosphoryl group from ATP to the C-6 hydroxyl group of glucose
`is catalyzed by hexokinase. Several subsequent intermediates in this meta(cid:173)
`bolic pathway, such as dihydroxyacetone phosphate and glyceraldehyde
`3-phosphate, are phosphorylated sugars. In fact, one of the strategies of
`glycolysis is to form three-carbon intermediates that can transfer their
`phosphate groups to ADP to achieve a net synthesis of ATP.
`._,
`
`o-
`1
`-o-P=O
`I
`0
`I
`
`H~t
`
`H
`OH
`Glucose 6-phosphate
`
`0
`H
`~c/
`I
`H-C-OH
`I
`CH2
`I
`0
`- o-~=O
`I o-
`
`CH20H
`I
`C=O
`I
`CH2
`I
`0
`I
`-o-P=O
`I o-
`
`Glyceraldehyde 3-phosphate
`
`Dihydroxyacetone phosphate
`
`Phosphorylation also serves to make sugars anionic. The pK values of a sug(cid:173)
`ar phosphate group are about 2.1 and 6.8. Hence, at an intracellular pH
`of 7.4, the net charge of a sugar phosphate such as glucose 6-phosphate is
`about -1.8. Such a group can have strong electrostatic interactions with
`the active site of an enzyme. The negative charge contributed by phos(cid:173)
`phorylation also prevents these sugars from spontaneously crossing lipid
`bilayer membranes. Phosphorylation helps to retain biomolecules inside
`cells, an effect whimsically characterized as "the importance of being
`ionized.''
`Another function of phosphorylation is the creation of reactive intermedi(cid:173)
`ates for the formation of 0- and N-glycosidic linkages. For example, a
`multiply phosphorylated derivative of ribose plays key roles in the biosyn-
`
`PGR2020-00009
`Pharmacosmos A/S v. American Regent, Inc.
`Petitioner Ex. 1082 - Page 11
`
`

`

`0
`II
`-o -PO~C
`O
`H
`2
`I
`-o
`
`H
`
`H
`
`H
`
`o
`o
`II
`II
`o-1-o- 1-o-
`o-
`o-
`5-Phosphoribosyl-1-pyrophosphate
`(PRPP)
`
`0
`II
`C
`"-CH
`II
`.,,,.c-coo (cid:173)
`N
`
`HN/
`I
`O=C,
`
`0
`
`Orotate
`
`0
`II
`PP, - o-POCH
`\I) -6
`2
`
`HO
`OH
`Orotidylate
`
`471
`
`Figure 18-9
`The Nglycosidic linkage of pyrimidine
`nucleotides is formed by displacement
`of the pyrophosphate group of PRPP,
`the activated intermediate. The con(cid:173)
`figuration of the glycosidic ·bond is
`inverted in this reaction.
`
`HO
`
`OH
`
`theses of purine and pyrimidine nucleotides (p. 741) . The pyrophos(cid:173)
`phate group of5-phosphoribosyl-1-pyrophosphate (PRPP) is displaced by
`a nitrogen atom of a free pyrimidine (orotate) to form a pyrimidine
`nucleotide ( orotidyla~e) (Figure 18-9) .
`
`SUCROSE, LACTOSE, AND MALTOSE
`ARE THE COMMON DISACCHARIDES
`
`Disaccharides consist of two sugars joined by an O-glycosidic bond. Three
`abundant disaccharides are sucrose, lactose, and maltose (Figure 18-10).
`Sucrose (common table sugar) is obtained commercially from cane or
`beet. The anomeric carbon atoms of a glucose unit and a fructose unit are
`joined in this disaccharide; the configuration of this glycosidic linkage is
`a for glucose and /3 for fructose. Consequently, sucrose lacks a free reducing
`group ( an aldehyde or ketone end), in contrast with most other sugars. The hydro(cid:173)
`lysis of sucrose to glucose and fructose is catalyzed by sucrase ( also called
`invertase because hydrolysis changes the optical activity from dextro- to
`levorotatory).
`Lactose, the disaccharide of milk, consists of galactose joined to glucose
`by a {3-1,4 glycosidic linkage (see Figure 18-10 and Figure 18-11) . Lactose
`
`Galactose unit
`
`Figure 18-11
`Model of lactose. Galactose is linked to glucose by a /3-1,4 glycosidic bond.
`
`Glucose unit
`
`HOCH 2
`
`HO
`~
`
`O H HO~CH2 0
`H
`1
`2
`OH H
`H HO
`
`O
`
`CH 20H
`
`H
`
`OH
`
`OH H
`Sucrose
`(a-o-Glucopyranosyl-I1-2)(cid:173)
`/J-o-fructofuranoside)
`
`H~~ocH;
`O
`~HOCH2 O H
`
`; ~H H
`
`H
`
`1 O 4
`H
`
`~H H
`
`OH
`
`H
`
`OH
`
`H
`
`OH
`
`Lactose
`(/J-o-Galactopyranosyl-(1-4)(cid:173)
`a-o-glucopyranose)
`
`~
`' o '~
`~
`H~ ~OH
`H
`OH
`H OH
`
`Maltose
`(a-o-Glucopyranosyl-(1-4)-
`a-o-glucopyranose)
`
`Figure 18-10
`Formulas of three common disaccha(cid:173)
`rides: sucrose, lactose, and maltose.
`The a configuration of the anomeric
`carbon atom at the reducing end of
`maltose and lactose is shown here.
`
`PGR2020-00009
`Pharmacosmos A/S v. American Regent, Inc.
`Petitioner Ex. 1082 - Page 12
`
`

`

`is hydrolyzed to these monosaccharides by lactase in humans (by {3-
`galactosidase in bacteria). In maltose, two glucose units are joined by an
`a-1,4 glycosidic linkage. Maltose comes from the hydrolysis of starch and
`is in turn hydrolyzed to glucose by maltase. Sucrase, lactase, and maltase
`are located on the outer surface of epithelial cells lining the small intes(cid:173)
`tine. These cells have many fingerlike folds called microvilli that markedly
`increase their surface area for digestion and absorption of nutrients.
`
`472
`
`Part Ill
`
`METABOLIC ENERGY
`
`Figure 18-12
`Electron micrograph of a microvillus
`projecting from an intestinal epithe(cid:173)
`lial cell. Lactase and other enzymes
`that hydrolyze carbohydrates are
`present on the outer face of the
`plasma membrane. The filaments
`inside the microvillus contain actin,
`a contractile protein. [From
`M.S. Mooseker and L.G. Tilney.
`J Cell Biol. 67(1975):725.]
`
`MOST ADULTS ARE INTOLERANT OF MILK
`BECAUSE THEY ARE DEFICIENT IN LACTASE
`
`Nearly all infants and children are able to dige·sr,lactose. In contrast, a
`majority of the adults in the world are deficient in lactase, which makes
`them intolerant of milk. In a lactase-deficient adult, lactose accumulates
`ir- the lumen of the small intestine after ingestion of milk because there is
`no mechanism for the uptake of this disaccharide. The large osmotic
`effect of the unabsorbed lactose leads to an influx of fluid into the ·small
`intestine. Hence, the clinical symptoms oflactose intolerance are abdom(cid:173)
`inal distention, nausea, cramping, pain, and a watery diarrhea. Lactase
`deficiency appears to be inherited as an autosomal recessive trait and is
`usually first expressed in adolescence or young adulthood. The preva(cid:173)
`lence of lactase deficiency in human populations varies greatly. For ex(cid:173)
`ample, 3% of Danes are deficient in lactase, compared with 97% of Thais.
`Human populations that do not consume milk in adulthood generally
`have a high incidence of lactase deficiency, which is also characteristic of
`other mammals. Milk treated with lactase is available for consumption by ,
`lactose-intolerant people. The capacity of humans to digest lactose in
`adulthood seems to have evolved since the domestication of cattle some
`ten thousand years ago.
`
`GLYCOGEN, STARCH, AND DEXTRAN
`ARE MOBILIZABLE STORES OF GLUCOSE
`
`Animal cells store glucose in the form of glycogen. As will be discussed in
`detail in Chapter 23, glycogen is a very large, branched polymer of glu(cid:173)
`cose residues. Most of the glucose units in glycogen are linked by a-1,4-
`glycosidic bonds. The branches are formed by a-1,6-glycosidic bonds,
`which occur about once in ten units (Figure 18-13). These branches serve
`to increase the solubility of glycogen and make its sugar units accessible.
`They are released from the many nonreducing ends of this highly
`branched carbohydrate store.
`The nutritional reservoir in plants is starch, of which there are two
`forms. Amylase, the unbranched type of starch, consists of glucose resi(cid:173)
`dues in a-1,4 linkage. Amylopectin, the branched form, has about one a-1,6
`linkage per thirty a-1,4 linkages, and so it is like glycogen except for its
`lower degree of branching.
`
`PGR2020-00009
`Pharmacosmos A/S v. American Regent, Inc.
`Petitioner Ex. 1082 - Page 13
`
`

`

`HOCH2
`
`-0
`
`H
`
`OH
`
`HOCH2
`
`-0
`
`H
`
`a-1,6 linkage
`_ _ _ , - - between two
`1
`glucose units
`~
`
`I s CH 2
`
`Chapter 18
`
`473
`
`CARBOHYDRATES
`
`Figure 18-13
`A branch in glycogen is formed by an
`a-1,6 glycosidic linkage.
`
`0-
`
`H
`
`OH
`
`(
`
`H
`
`OH
`
`a-1,4 linkage
`between two
`glucose units
`
`More than half the carbohydrate ingested by humans is starch. Both
`amylopectin and amylose are rapidly hydrolyzed by a-amylase, which is
`secreted by the salivary glands and the pancreas. a-Amylase, an endogly(cid:173)
`cosidase, hydrolyzes internal a-1,4 linkages to yield maltose, maltotriose, and
`a-dextrin. Maltose consists of two glucose residues in a-1,4 linkage
`(p. 471), and maltotriose of three such residues. a-Dextrin is made up of
`several glucose units joined by an a-1,6 linkage in addition to a-1,4 link(cid:173)
`ages. Maltose and maltotriose are hydrolyzed to glucose by maltase,
`whereas a-dextrin is hydrolyzed to glucose by a-dextrinase. Malt contains
`{3-amylase, an enzyme that hydrolyzes starch into maltose by sequential
`removal of disaccharide units from nonreducing ends. Malt derived from
`barley or other grains is used to make beer.
`Dextran, a storage polysaccharide in yeasts and bacteria, also consists
`only of glucose residues, but differs from glycogen and starch in that
`nearly all linkages are a-1,6. Occasional branches are formed by a-1,2,
`a-1,3, or a-1,4 linkages, depending on the species.
`
`CELLULOSE, THE MAJOR STRUCTURAL POLYMER OF PLANTS,
`CONSISTS OF LINEAR CHAINS OF GLUCOSE UNITS
`
`Cellulose, the other major polysaccharide of plants, serves a structural
`rather than a nutritional role. Cellulose is one of the most abundant organic
`compounds in the biosphere. Some 1015 kg of cellulose is synthesized and
`degraded on earth each year! It is an unbranched polymer of glucose
`residues joined by {3-1,4 linkages. The {3 configuration allows cellulose to
`form very long straight chains (Figure 18-14). Each glucose residue is
`related to the next by a rotation of 180°, and the ring oxygen atom of one
`is hydrogen-bonded to the 3-OH group of the next. Fibrils are formed by
`
`Contemporary drawing depicting a
`16th century German brewery. Barley
`was steeped in water for several days
`and then allowed to germinate in a
`warm damp room. /3-Amylase and
`maltase produced by the sprouts then
`digest the starch into glucose, which
`is fermented. [Der Bierbreuwer, by Jost
`Ammon.]
`
`Figure 18-14
`Schematic diagram showing the con(cid:173)
`formation of cellulose. The structure
`is stabilized by hydrogen bonds be(cid:173)
`tween adjacent glucose units in the
`same strand. In fibrils of cellulose,
`hydrogen bonds are formed between
`different strands as well.
`
`a'--
`
`Cellulose
`(,8-1,4 linkages)
`
`PGR2020-00009
`Pharmacosmos A/S v. American Regent, Inc.
`Petitioner Ex. 1082 - Page 14
`
`

`

`N- H
`I
`O= C
`I
`CH3
`
`N-H
`I
`O= C
`I
`CH3
`
`Chitin
`(NAG-/3(1-4)-NAG repeat)
`
`A Sally lightfoot crab. The exoskele(cid:173)
`ton of this arthropod is rich in chitin,
`one of the most abundant biopolymers
`on earth.
`
`coo-
`
`vt>t
`_JHA
`
`0-
`
`H NHCOCH3
`
`Chondroitin 6-sulfate
`
`Figure 18-15
`Structural formulas of the r epeating
`disaccharide units of some major gly(cid:173)
`cosaminoglycans. Negatively charged
`groups are shown in red, and amino
`groups in blue.
`
`parallel chains. The a -1,4 linkages in glycogen and starch produce a very
`different molecular architecture. A hollow helix is formed instead of a
`straight chain. These differing consequences of the a and /3 linkages are
`biologically important. The straight chain formed by /3 linkages is ojJtimal for
`the construction of fibers having a high tensile strength. In contrast, the open helix
`formed by a linkages is well suited to forming an accessible store of sugar.
`Mammals lack cellulases and therefore cannot digest wood and vegeta(cid:173)
`ble fibers. However, some ruminants harbor cellulase-producing bacteria
`in their digestive tracts and thus can digest cellulose. Fungi and protozoa
`also secrete cellulases. In fact, the digestion of wood by termites depends
`on protozoa in their gut, a mutually beneficial association.
`The exoskeletons of insects and crustacea contain chitin, which consists
`of N-acetylglucosamine residues in {3-1,4 linkage. Chitin forms long
`straight chains that serve a structural role. Thus, chitin is like cellulose
`except that the substituent at C-2 is an acetylated amino group instead of
`a hydroxyl group.
`
`GLYCOSAMINOGLYCANS ARE ANIONIC POLYSACCHARIDE
`CHAINS MADE OF REPEATING DISACCHARIDE UNITS
`
`A different kind of repeating polysaccharide is present on the cell surface
`and in the extracellular matrix of animals. Many glycosaminoglycans are
`made of disaccharide repeating units containing a derivative of an amino
`sugar, either glucosamine or galactosamine. At least one of the sugars in
`the repeating unit has a negatively charged carbox:ylate or sulfate group. Chon(cid:173)
`droitin sulfate, keratan sulfate, heparin, heparan sulfate, dermatan sul(cid:173)
`fate, and hyaluronate are the major glycosaminoglycans (Figure 18-15).
`Heparin is synthesized as a nonsulfated proteoglyca n , which is then de(cid:173)
`acetylated and sulfated. Incomplete modification leads to a mixture of
`variously sulfated sequences. One of them acts as an anticoagulant by
`binding specifically to antithrombin, which accelerates its sequ