7/23/2019 5690090404_ftp

1/5

133

THERMOD YN MIC SPEC TS OF GL YCOL YSIS

A T H E L C O R N I S H - B O W D E N

F ~ u ~ 1

D e p a r t m e n t o f B i o c h em i s t r y

U n i v e r s it y o f B i r m i n g h a m

B i r m i n g h a m , U K

The e xpe r ime n t s o f H a rde n a nd Y oung 1-3 in t he e a r ly ye a r s o f t h i s c e n tu ry a re a ll bu t

fo rgo t t e n by toda y ' s b ioc he mis t s a nd a re ba re ly me n t ione d in m od e m t e x tbooks . Y e t

the y w e re no t o n ly c ruc ia l i n t he de ve lopm e n t o f ou r u nde r s t a nd ing o f t he g lyc o ly t ic

pa thw a y ; t he y a l so p rov ide some o f t he mo s t i n s t ruc t ive e xa mple s fo r unde r s t a nd ing

biochem ica l equil ibr ia . Und erstanding them is easie r for us than i t was fo r Harden and

Y ou ng be c a use o f a ll o f t he kno w le dge a bou t t he c he mic a l de ta i ls o f g lyc o lys is t ha t ha s

be e n ga ine d s inc e the i r t ime . In mode m t e rmino logy , t he c ruc i a l obse rva t ions w e re

these:

(1) In t he p re se nc e o f a n a mp le supp ly o f i no rga n ic phospha te , a l c oho li c f e rme n ta t ion

of g lucose by yeast ext rac t proceeds unt i l a l l of the g lucose is consumed.t

(2) U nde r i no rga n ic phospha te l im i t a t i on f e rme n ta t ion be c om e s ve ry s low a nd

fruc tose 1 ,6-bispho sphate accumula tes. 2

(3) A dd i t i on o f a sma ll a mo un t o f i no rga n ic pho spha te t o ye a s t e x t ra c t f e rme n t ing

under p hosp hate l imi ta t ion causes a rapid increase in the ra te of fe rmenta t io n , dur ing

w h ic h 1 mo l C O 2 i s p roduc e d pe r m o l phospha te a dde d , a f t e r w h ic h v e ry s low

fe rme n ta t ion r e sume s . 2

(4) In the absence of inorganic phosp hate , add i t ion o f a trace of a rsena te causes a rapid

increase in fe rmen ta t ion , which ma y cont inue unt i l a ll of the g lucose is consum ed, and

analysis show s no percept ib le change in the concentra t ion of a rsena te a t any t im e. 3

To understand the f i rst three of these observ a t ions i t is suff icient to kno w the

glycolyt ic reac t ions and the i r s tandard Gibbs energies AG ' show n in F ig 1 . To

unde rs t a nd the fou r th i t i s a l so ne c e ssa ry to know tha t g lyc e ra lde hyde 3 -phospha te

dehyd rogenas e wi l l accept a rsena te as a subst ra te instead of inorganic phosph ate , b ut

tha t t he 1 -a r se no -3 -phosphog lyc e ra t e p re sume d to be p roduc e d i s uns t a b l e a nd i s

spon ta ne ous ly hyd ro ly se d to 3 -phosph og lyc e ra t e a nd a r se na te .

I fw e igno re fo r t he m om e n t t he e xpe r im e n t w i th a r sena t e, i t i s c le a r tha t t he r e a c t ion

c a t aly sed by g lyc e ralde hyde 3 -phospha te d e hydroge na se c a nno t p roc e e d in t he a bse nc e

o f i no rgan ic phospha te a nd tha t c onse que n tly t he w ho le p roc e ss m us t c e ase. B u t w hy

shou ld f ruc to se 1 ,6 -b i sphospha te a c c umulat e r a the r t ha n a ny o f t he o the r fou r

in te rmedia tes tha t occur before the b locked reac t ion? To understand th is we need to

e xa mine the A G ' va lues shown in Fig 1 .

Roct ion Enzyme AGm K c

kJ too l- i

GIc

A T P . ~ h x ok in Q se - 1 6 ' 7

8 S O

A OP

J

G6 P

I ~ h o l ~ t + 1 '7 0 5 0

IsonN~(ll~

F 6 P

I ruc to - 1 4 . 2 3 1 0

A D P k l n o H

F B P

aldo los + 23.8 6 .7 IO~M

t r l ~

D H A P ~ ; - G 3 P p h o q ~ a t e + 7 - 5 0 0 4 8

. ~ tsoml~i'QsG

N A D , ~ ,

9 YCemicm~Y

3 - 1 ~ t + 6 3 0 0 7 9 M =

NA DH, H+ ~ '~ d e h y d ro g e n o s(

B P G

A D P . ~ 3 - p h os p ho -

~ l lyCerote - 18 '8 a o o o

ATP ~ ~ k l i ~3Se

3 P G

Th e f irst seven reactions o f lycolysis

The fo l low ing abbrev ia tions are used: G/c, glucose; G6 P, g lucose 6-phosphate; F 6P , f ruc tose

6--phosphate; FB P, f ruc tose 1 ,6-b isphosphate; DH A P , d ihydrox),ace tone phosphate; G 3P ,

glyceraldehyde 3-.phosphate; Pi , inorganic phosphate; B I)G , 1 ,3-b isphosphoglycerate; 3P G ,

3--phosphoglFcerate.

B I O C H E M I C A L E D U C A T I O N 9 4 ) 1 98 1

7/23/2019 5690090404_ftp

2/5

134

A naive (and wrong ) interp retation of AG values com mon ly encountered states that

large negative values indicate that the forward reaction is strong ly favoured, whereas

large positive values indicate that the reverse reaction is strongly favoured. (To make

matters mor e precise I shoul d say that large means larger than about 11 kJ tool 1, and

strongly favoured means favoured by at least a factor of 100 ). This seems to explain

the result in the absence of inorga nic phosphate, because it indicates that the hexokinase

and phosphofructokinase equilibria favour the forward reactions (which is true), that

the hexose phosphate isomerase equilibrium does not strongly favour either direction

(which is also true), bu t the aldolase equilib rium strongly favours the reverse reaction,

ie accumulation of fructose 1,6-bisphosphate. This explan ation creates a worse

problem than the one it solves, however, because it suggests that the aldolase reaction

should never proceed forwards at all, whether in organic phosphate is available or not - -

yet fermentation

d o e s

proceed readily when phosphate is available.

To understand properly what is happening we must examine the relationship

between AG and the equil ibrium constant K for a reaction:

A G = - R T I n K ~ -

-5.71ogK (in kJ mol -l) at 25C

where R = 8.314 J mol -l K -I is the gas constan t and T is the absolu te temperature.

Thus at 25C each - 5 .7 kJ mol -l in the value of AG corresponds to a factor of 10 in

the equilibrium constant in favour of product formation. If we apply this relationship

for the first three reactions ofglycolysis we have: K = 850 for hexokinase, K = 0.50 for

hexose phosphate isomerase, and K = 310 for phosphofructokin ase, in good agreement

with the interpretation of the standard Gibbs energies given above. For aldolase,

however, we find that K = 6.7 10 -5, which qu ite wrong ly suggests that the reaction

will not readily proceed forwards, even though we know that it does. What has gone

wrong? Why should a calculation that works with the first three enzymes not give a

sensible result for aldolase?

The explanation lies in the fact that if we calculate an equilib rium constant K from the

definition of AG above we mus t get a dimensionless result, because the expression

contains InK (or logK), and only dimensionless numbers have logarithms. So each K

given above is a dimensionless numb er. This presents no probl em for the first three

because we would never expect them to have dimensions, as each refers to a reaction in

which the number of reactants is the same as the number of products. For aldolase,

however, we migh t expect to interpret the equilibri um constant as

[G3P]eqm[DHAP]eqm

K c =

[VBPleqm

in which the subscripts eqm indicate concentrations at equilibrium. No w this quantity

Kc cannot be the same as K, because it is not a dimensionless numb er b ut a

concentration.

What is the relationship between Kc and K?. The conv entio n that we use to make K

dimensionless so that we can take its logarith m and relate it to the th ermo dyn amic

quantity AG is to say that we are not defining K in terms of real concentrations

measured in m ol 1-1, but concen trations relative to a set of standards, thus:

([G3P]eqm/IG3P])([DHAP]cqm/[DHAP] )

K =

([FBP]eqm/[FBP] )

where the superscripts () indicate these standards. In principle we could choose any

values we liked for the standard concentrations, and they could be different for each

chemical species if we wished. But a chaotic set of standards wo uld be very d ifficult to

remem ber and so, with a very few exceptions such as the proton (see below) and water,

we choose the same standard conce ntra tion of 1 M for every species. Thu s we have, for

the aldolase equilibrium,

K = Kc/(1 M)

or, from the value we calculated for K,

K = 6.7 10 -s M

The appearance of

u n i t s

in this equation provides the key to understanding why

aldolase is special: it is certainly true that a mix tur e of the three reactants

i n t h e i r s t a n d a r d

c o n c e n t r a t i o n s of 1 M will tend to react in the reverse dlrection, i.e. fro m triose to

hexose. But w hy should a biochemist care what happ ens at 1 M? Much more

interesting is what happens at a physiologically realistic concentration, such as 50 IzM.

If we put b oth [G3P] and [DHAP] to this value (ignor ing for the mom en t the

complication that in the presence of triose phosphate isomerase they will equilibrate to

unequal concentrations) we can readily calculate the concentration of fructose

1,6-bisphosphate at equilibrium with them as

[FBP] = 5 10 -5 5 10-5/(6 .7 10 -5 ) = 3.7 10-5M

which is a little smaller than the concentration s of the t wo triose phosphates, despite the

large positive value of AG .

B I O C H E M I C A L E D U C A T I O N 9 4 ) 1981

7/23/2019 5690090404_ftp

3/5

135

To d e t e r m i n e t h e d i r e c t i o n i n wh i c h a n y p a r t i c u la r m i x t u r e o f re a c t a nt s wi l l b e a b l e

t o r e a c t t h e m o s t c o n v e n i e n t q u a n t i t y t o c o n s i d e r i s n o t AG ' b u t AG, i e n o t t h e

s t a n d a r d Gi b b s e n e r g y b u t t h e Gi b b s e n e r g y , wh i c h i s d e f i n e d a s

( [G 3 P ] /[ G 3 P ] ) ( [ D H A P ] / [ D H A P ]

A G = A G ' + R T I n

( [ F B P I / I F B P D

( As in t h e d e f i n it i o n o f K a b o v e , w e o b t a i n a d i m e n s i o n l e ss n u m b e r b e f o r e t a k i n g

l o g a r i t h m s b y d i v i d i n g e a c h c o n c e n t r a ti o n b y t h e c o r r e s p o n d i n g s t a n d a rd . I n p ra c t ic e

we u s u a l l y o m i t t h e s t a n d a r d c o n c e n t r a t i o n s f r o m s u c h e x p r e s s i o n s , wh i c h i s n o t

s t r ic t l y c o r r e c t b u t a c c e p t a b le p r o v i d e d we r e m e m b e r t h a t i t is t h e s e ra t i o s t h a t we m e a n

wh e n w e wr i t e t h e c o n c e n t r a t io n s . N o p r o b l e m s a r is e i f a ll c o n c e n t r a t i o n s a r e m e a s u r e d

i n M, b u t i f o t h e r u n i t s a re u s e d th e c o n c e n t r a t i o n s m u s t a l wa y s b e c o n v e r t e d t o M

b e f o r e c a l c u la t in g AG. ) T h e v a l u e o f AG d i r e c tl y a n s we r s t h e q u e s t i o n , h o w f a r i s t h e

s y s t e m f r o m e q u i l ib r i u m a n d i n wh i c h d i r e ct i o n ? I f AG i s n e g a t iv e t h e r e a c t i o n m u s t

p r o c e e d f o r wa r d s t o r e a c h e q u i l ib r i u m ; i f i t is p o s i t i v e i t m u s t p r o c e e d b a c k wa r d s ; i f i t is

zero the sy s tem i s a t equi l ibr ium. I f w e put a l l three o f the conc ent ra t ion s in the a ldolase

equi l ibr ium to 50 v .M, we have

AG = 23 .8 + 5 .71og(5 x 10 - s ) = -0 .7 2 kJ mol - t

Th i s s m a l l n e g a t i v e v a lu e s h o w s t h a t s u c h a m i x t u r e i s c lo s e t o e q u i l i b r iu m b u t n e e d s t o

r e a c t f o r wa r d s t o a sm a l l d e g r e e to r e a c h e q u i li b r i u m , a re s u l t in g o o d a g r e e m e n t wi t h

o u r c a l c u l a t io n a b o v e t h a t t h e e q u i l i b r i u m c o n c e n t r a t i o n o f fr u c t o s e 1 , 6 - b i s p h o s p h a t e i s

3 7 ~ M wh e n t h e t w o t r i o s e p h o s p h a t e s a r e p r e s e n t a t c o n c e n t r a ti o n s o f 5 0 v ~M.

Th i s c a l c u la t io n , t h e n , a l l o ws u s t o u n d e r s t a n d wh y f e r m e n t a t i o n c a n p r o c e e d u n d e r

n o r m a l c o n d i t i o n s , b u t i t l e av e s o p e n t h e q u e s t i o n o f w h y f r u c t o s e 1 , 6 - b i s p h o s p h a t e

( r a t h e r t h a n , f o r e x a m p l e , d i h y d r o x y a c e t o n e p h o s p h a t e o r g l y c e r a l d e h y d e 3 -

p h o s p h a t e ) a c c u m u l a t e s wh e n t h e s u p p l y o f i n o r g a n i c p h o s p h a t e i s c u t o f f . W h e n t h e

r e a c t i o n c a t a l y s e d b y g l y c e r a l d e h y d e 3 - p h o s p h a t e d e h y d r o g e n a s e i s b l o c k e d , t h e p o o l

o f al l three a ldolase reac tant s m us t increase , so th a t 50 ~M ceases to be a rea li s ti c

c o n c e n t r a t i o n t o c o n s i d e r . As t h e c o n c e n t r a t i o n s i n c r e a s e , AG m u s t b e c o m e p o s i t i v e

and then increase s t eadily , unless the equi l ibr ium shi f ts in favo ur of f ruc tose

1 , 6 - b i s p h o s p h a t e , a n d t h e m o r e t h e c o n c e n t r a t i o n i n c r e a s e s t h e m o r e t h e r e v e r s e

r e a c t io n i s f a v o u r e d . Th u s i t is f r u c to s e 1 , 6 - b i s p h o s p h a t e t h a t a c c u m u l a t e s , n o t t h e t wo

t r iose phosphates .

Actua l ly the s i tua t ion in the ce l l i s a l i t t l e more comphcated than I have indica ted ,

b e c a u s e d i h y d r o x y a c e t o n e p h o s p h a t e a n d g l y c e r a l d e h y d e 3 - p h o s p h a t e a r e n o t a t e q u a l

concent ra t ions a t equi l ibr ium, because they a re in te rconver t ib le by a reac t ion ca ta lysed

b y t r i o s e p h o s p h a t e d e h y d r o g e n a s e , wh i c h h a s AG ' = 7 . 5 k J m o l - I . Th i s c o r r e s p o n d s

t o t h e r at i o [ G3 P ] / [ DH AP ] = 0 . 0 4 8 a t e q u i l i b r iu m a n d s o i f we c o n s i d e r a c o n c e n t r a t i o n

o f 5 0 ~ M f o r g l y ce r a ld e h y d e 3 -p h o s p h a t e w e m u s t a s s u m e a c o n c e n t r a ti o n o f

1 . 0 4 m M f o r d i h y d r o x y a c e t o n e p h o s p h a t e b e f o r e c a l c u la t i n g t h e e q u i li b r i u m

c o n c e n t r a t i o n o f f r u c t o s e 1 , 6 - b is p h o s p h a t e a s d e s c r i b e d a b o v e . Th e r e s u l ts o f t w o s u c h

ca lcula t ions a re i l lus t ra ted schemat ica l ly in Fig 2 . When the concent ra t ion of

g l y c e r a l d e h y d e 3 - p h o s p h a t e i s 2 0 I ~ M ( a r e a l i s t i c v a l u e u n d e r o r d i n a r y c o n d i t i o n s in

vivo4 , t h e e q u i li b r i u m c o n c e n t r a t i o n o f f r u c t o s e 1 , 6 - b i s p h o s p h a t e i s 0 . 1 2 m M , a n d

al tho ugh th i s i s grea te r than the conce nt ra t ion o f g lyce ra ldeh yde 3-ph osp hat e i t is st il l

n o t t h e p r e d o m i n a n t s pe ci es . Bu t i f t h e c o n c e n t r a t i o n s o f t h e t r io s e p h o s p h a t e s a r e

increased 10- fo ld there is a 100- fo ld increase in the co ncen t ra t ion o f f ruc tose

1 , 6 - b i s p h o s p h a t e a n d i t b e c o m e s t h e p r e d o m i n a n t c o m p o n e n t o f t h e e q u i li b r i u m

m i x t u r e . Th i s i s e f f e ct i ve l y wh a t h a p p e n s i n t h e f e r m e n t a t i o n b y y e a s t e x tr a c t wh e n t h e

s u p p l y ~ o f i n o r g a n i c p h o s p h a t e i s c u t o f f a n d t h e r e a c t i o n c a t a ly s e d b y g l y c e r a l d e h y d e

3 - p h o s p h a t e d e h y d r o g e n a s e i s b l o c k e d .

W e h a v e s e e n f r o m t h is d i s c u s s io n t h a t t h e q u a n t i t y t o b e e x a m i n e d i s n o t A G ' b u t

AG i f we w a n t t o k n o w wh i c h d i r e c t io n o f r e a c t io n wi l l b e p o ss i b le u n d e r a n y s e t o f

c o n d i t io n s . W h y , t h e n , d o w e s e e m t o g e t th e r i g h t a n s we r i f we l o o k a t AG ' f o r t h e

f i r st three reac t ions of g lycolys i s? Thi s i s because w e have , by imp l ica t ion , assum ed tha t

we a r e d e a li n g wi t h e q u a l ( t h o u g h n o t n e c e s s a ri l y s ta n d a r d ) c o n c e n t r a t i o n s o f r ea c t a n ts

a n d p r o d u c t s , e g f o r h e x o s e p h o s p h a t e i s o m e r a s e :

[ r ~ P ]

A G = A G ' + 5.71og - - - A G ' = 1 . 7 k J m o 1 - 1 i f [ F6 P] = [ G6 P ]

[G6P]

Un d e r c e ll u la r c o n d i ti o n s , h o w e v e r , t h e se c o n c e n t r a t io n s a r e n o t l i k e l y t o b e e q u a l ( a n d

i f t h e y we r e g l y c o l y s i s c o u l d n o t p r o c e e d b e c a u s e t h is v a l u e i s, t h o u g h s m a l l, p o s i ti v e )

a n d s o AG i s n o t e x a c t l y e q u a l t o AG ' . I n f a c t, u n d e r g l y c o l y t i c c o n d i t i o n s i n t h e

h u m a n e r y t h r o c y t e Mi n a k a m i a n d Y o s h i k a w a 4 f o u n d [ G 6 P] = 8 3 la uM, [ F6 P] =

14 ILM, so

A G - - A G ' + 5 . 7 1 o g (1 4 / 83 ) = 1 . 7 - 4 . 4 = - 2 . 7 k J m o l - I

B I O C H E M I C A L E D U C A T I O N 9 (4 ) 1 9 81

7/23/2019 5690090404_ftp

4/5

136 l

Figure 2

Dihydroxyocetone

phot

Th e po ol of aldolase reactants

Th e figure i l lustrates schematical ly how the relat ive proport ions o fructose 1 ,6-bisphosphate ,

dihydroxyacetone phosphate and glyceraldehyde 3-phosphate change when the react ion catalysed

by glyceraldehyde 3-phosphate dehydrogenase is blocked but the early s teps o fglycolysis continue.

A real ist ic ini tial s tate is indicated by the shad ed squares, with areas proport ional to the

c o nc en tr a ti on s , [ G3 P ] = 2 0 I ~ M, [ D H A P ] = 0 . 4 2 m M , [ F B P ] = 0 . 1 2 r a M. I f e q ui li b ri um

is maintained, both between fructose 1 ,6-bisphosphate and the two tr iose phosphates catalysed

by aldolase) and between the triose phosph ates triose phosph ate isomera se), a lO-fold increase in

the concentrat ion of glyceraldehyde 3-phosphate is accompanied by a lO-fold increase in the

concentrat ion of dihydroxyacetone phosphate , but a lO0-fold increase in the concentrat ion of

fructose 1 ,6-bisphosphate . Th is second s tate is indicated by the areas of the com plete squares.

Altho ugh this correction is not large it is crucial because it shows that u nder glycolytic

condition s AG is negative, as it must be if glycolysis is to be possible.

Most of the other glycolytic reactions are free from the sort o f complication that had

to be considered for aldolase, because they have equal numbers of molecules

participating in the forward and reverse directions. Glyceraldehyde 3-phosphate

dehydrog enase does need to be considered, however: if we include H + as a product we

do have equal number s of reactant and product molecules, but if we omit H + from

consideratio n we have an excess o f reactants over products. In principle, we could treat

H + jus t like any other reactant, and that is what chemists typically do. But a standard

state of 1M for H + (corresponding to p H 0) is extremely in conv enien t for most

biochemical purposes, and biochemists customarily use a standard concentration of

0.1 ~M for H + (corresponding to pH 7); they write AG (rather than AG) to indicate

this, and they work with buffered solutions and consequently do not have to worry

about changes in the H + concentratio n as a reaction proceeds.

With this conv entio n we can ignore H + as a reactant and treat the reaction catalysed

by glyceraldehyde 3-phos phate dehydrogenase as one in which there are three reactants

but only two products. The practical equilibrium constant is consequently not

K = 7.9

x 1 0 - 2

but

Kc = 7.9

x 1 0 - 2 M - 1

which is a reciprocal concentration. The effect o f dilution thus wo rks in the opposite

direction for this reaction from the w ay it works for the aldolase reaction: the aldolase

equilib rium is by no means as unfavourable to the forward reaction as it appears at first

sight; this reaction is much m ore unfavou rable to the for ward direction than it appears

at first sight. How, then, is it able to proceed when cells undergo glycolysis? The

explanation is partly that the [NAD +]/ [NA DH] ratio is typically maintain ed at a value

much greater than unity, and partly that the reaction is followed in glycolysis by a

reaction with AG = -1 8. 8 kJ mo1-1, a large negative value that ensures a very low

concentration of 1,3-bisphosphoglycerate.

If we put [NAD+]/ [N AD H] = 240, [G3P] = 19 p,M, [BPG] = 0.6 ~M and [Pi] =

1 IzM (as given by Minaka mi and Yoshikawa4), we obtain

0.6 1 0 - 6

AG = 6.3 x 5.71og = +1.3 kJ mol -I

240 x 0.001 x 19

x 1 0 - 6

As this value is positive it cannot be quite right, b ut it is close enoug h to zero for us to

believe that the discrepancy can be accounted for by errors in measuring the

concentrations of the various metabolites in the cell.

This last result raises one further question: ifa very favour able reaction can overco me

a very unfavourable equilibrium by ensuring that the concentration of the product of

r u c t o s e

1 6-bisphosphatc

B I O C H E M I C A L E D U C A T I O N 9 4 ) 1 98 1

7/23/2019 5690090404_ftp

5/5

Study questions

eferences

137

the un fa vou ra b le r ea c t ion is ve ry low , w hy c a n th is so lu t ion no t be u se d to ov e rc om e

ma ny o the r un fa vo u ra b le e qu il ib ri a? F rom the the rm odyn a mic po in t o f v i e w the re is no

ob je c t ion to t hi s: a n y un fa vou ra b le e qu i li b r ium c a n inde e d be pu l l e d ove r by r e mo v ing

the produ ct . But k ine t ica l ly there i s a se r ious objec t ion . A ny spec ies tha t i s present in

ve ry low c onc e n t ra t i ons m us t r e a c t s low ly in se c ond-o rde r r e a c tions ( suc h a s b ind ing to

an enzyme) because i t wi l l take a f in i te t ime for the o ther par t ic ipant in the reac t ion to

' f i nd ' a r e ac t a n t i n l ow c onc e n t ra t ion . N o ma t t e r ho w e f f ic i e nt a n e nzyme m a y be a s a

ca ta lyst i t cannot reac t faste r than the d i ffusion l imi t , which corresponds to a

se c ond-o rde r r a t e c ons t a n t o f a bou t 10s M - s - . So i f bo th e nzym e a nd subs tr a t e a re

present a t concentra t ions less than 1 ~M there is no wa y in which a ra te grea te r than

a bou t 10 -4 M s -1 o r 0 .1 mM s - I c a n be a c h ie ve d , a nd e ve n th i s p re suppose s a

perfec t ly e ff ic ient enzyme. Thus in genera l i t i s not desi rable to have in te rmedia tes in

ma jo r pa thw a ys suc h a s g lyc o lys i s p re se n t a t e x t re me ly low c onc e n t rat i ons .

I t r e ma ins t o c ons ider t he e f fe c t o f a r se nat e d i sc ove re d by H a rde n a nd Y oung . A s I

ha ve sugge s t e d , t h i s i s a c onse que nc e o f t he a b i li t y o f g lyc e ra lde hyde 3 -phosp ha te

de hydroge na se to a c c e p t a r se na t e a nd the e x t re me l a b i l i t y o f t he p roduc t ,

1 -a r se no -3 -phosphog lyc e ra t e . Th i s p re sume d spe c i e s i s hyd ro ly se d to 3 -

phosphog lyc e ra t e a nd a rse na te a s soon a s i t is fo rme d . Th e f o rm e r c on t inue s t h r ough

the la te r s tages of fe rmenta t ion w hereas the a rsena te i s imme dia te ly ava i lable for a

fu r the r c yc l e o f t he g lyc e ra ldehyde 3 -phospha te d c hydroge na se r e a c tion . T hus e ve n

though fe rme n ta t ion r e qu ire s r e a ge n t qua n ti t ie s o f i no rga n ic phosp ha te , i t c a n p roc e e d

in the presence of ca ta ly t ic amounts of a rsena te .

(1) T he ' i r reversib le ' reac t ions of g lycolysis a re bypassed by hyd rolyt ic reac t ions in

g luc one oge ne s i s , bu t t he re i s no bypa ss fo r t he r e a c t ion c a t a ly se d by 3 -

phosphog lyc e ra t e kina se , e ve n though i ts va lue o f A G ' = -1 8 . 8 k J mo1-1 i s one o f t he

most nega t ive in g lycolysis . How is i t possib le for th is reac t ion to proceed in the

direc t ion o f g luconeogenesis?

(2) In t he a bse nc e o f i no rga n ic phospha te , H a rde n a nd Y o ung obse rve d a l ow ra t e o f

fe rmenta t ion , not a zero ra te? How can th is low ra te be expla ined?

(3) Th e usua l experim enta l prac t ice i s to assay g lycera lde hyde 3-p hos pha te

de hydroge na se w i th a r se na t e a s subs t r a t e i n p l a c e o f phospha te . W hy i s t h i s

advantageous?

i Harden. A and Young, W J (1906)

Proceedinj~sof the RoFal Society Series B 77 405-420

2 Harden, A and Young, WJ (1908) D0c

Roy Soc Se t B M

299-311

3 Harden. A and Young, WJ (1911) Dec

Roy Soc Se t B U

451-475

4 Minakami, S and Yoshikawa, H (1965) Biachem Biophys Res C arom lg, 345-349

_ n n o u n c e m e n t

on~ard all abstracts and requests or information

t o

D r S S a ha , P r o g r a m C h a i r m a n

Fi r st Sou the rn B iome d ic a l Eng ine e r ing C onfe re nc e

D e p a r t m e n t o f O r t h o p a e d i c S u r g e r y

Lou i s ia na S ta te U n ive r s i t y Me d ic a l C e n te r

PO B ox 339 32

Shrevep ort , Louisiana 71130 , US A

FIRST SOUT HER N BIOMEDICAL ENGINEERING

CONFERENCE

LOUISIANA STA TE UNIVER SITY MEDICAL CENTER

SHRE VEPOR T LO UISIA NA

7-8June 1982

A u t h o r s a r e in v i t e d t o s u b m i t p a p e r s o n a n y a s p e c t o f B i o e n g i n e er i n g

i n c l u d i n g t h e f o l l o w i n g a r e a s :

A r t if i ci a l O rga ns /Pros the t i c D e v ic e s , B ioc he mic a l Eng ine er ing , B ioc on t ro l Sys t e ms ,

Bioe lec t r ic i ty , Bioinst rumenta t ion , Biomater ia ls , B iomechanics, Biomedica l Educa-

t i on , B iophys ic a l Me a su re m e n t s , B ios ignal P roc e ssing , C a rd io logy /H a e mo dyna m ic s ,

C l ini c al Eng ine e ring , C o mp u te r s i n Me d ic ine , Ima g ing , Mic roc i rc u la t ion , Mode l l i ng

and Simula t ion , Occupat iona l In jury and Safe ty , Rehabi l i ta t ion Engineer ing, Thermal

Regula t ion in Biologica l Systems, Ul t rasonics in Medic ine .

Deadl ine for rece ip t of Abstrac ts (500 words) 5 Dec em ber 1981.

Papers wi l l be reviewed. Accepted papers wi l l be inc luded in a proceedings to be

p u b li s he d b y P E R G A M O N P R E S S w h i c h w i l l b e a va i la b le a t t h e t im e o f t h e m e e t in g .

B I O C H EM I C A L E D U C A T I O N 9 4) 1 ~ 1