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L.A.B. Carbohydrate Metabolism
Transport of sugars across cell membrane
Catabolism of sugars for energy

• Pathways for glucose fermentation

Homolactic fermentation Mixed acid fermentation
Heterolactic fermentation
Synthesis of polysaccharides

• Pathways for other hexoses

• Disaccharides

• Pathways for pentoses

• Pathway for citrate

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Selected references

• Salminen, S. and A. vonWright. 1998. Lactic
  acid bacteria. Marcel Dekker, Inc., New York,
  NY. pp. 51-56 (sugar transport), pp. 22-43
  (degradation of sugars for energy).
• Marth, E.H. and J.L. Steele (eds.). 1998.
  Applied Dairy Microbiology. Marcel Dekker, Inc.,
  New York, NY. Pp. 178-187.

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Sugar Transport in L.A.B.
OUT
cell membrane
IN
Concentration Gradient (?S1 ?S2)
Energy from
PEP
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PEP-PTS
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Sugar Transport Systems are Sugar-Specific
Lactose
Glucose
Galactose
H
H
H
OUT
Lactose Permease
cell membrane
Glucose Permease
Galactose Permease
IN
H
H
H
Lactose
Glucose
Galactose
Also sugar specific Antiport permease
PEP-PTS EIIA and EIIBC
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Type of Transport System for a Sugar is Species
Specific
Example Lactose Transport
PEP-PTS Lactococcus Lactobacillus casei
Antiport (lactose/galactose) S.
thermophilus Lactobacillus delbrueckii subsp.
bulgaricus
Symport Lactobacillus helveticus Lactobacillus
acidophilus Leuconostoc Pediococcus
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Homolactic Fermentation of Glucose
(Embden-Meyerhof Pathway)
(Glycolysis LDH)
Glucose
Pyruvate key intermediate
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Two roles for PEP
Transport (PEP-PTS) or ATP generation
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Mixed Acid Fermentation Alternative endproducts
for pyruvate
Glucose
2 ATP
2 ADP
(2) NAD
(2) H2PO4-
(2) NADH (2) H
(2) ADP
(2) ATP
(2) ADP
(2) ATP
(2) Pyruvate
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Homolactic vs. Mixed Acid Fermentation
Substrate availability and the nature of the
substrate determine which pathway is used..
Homolactic fermentation prevails when glucose is
abundant.
Mixed acid fermentation prevails when sugars are
limited semi-starvation.
Mixed acid fermentation prevails during growth on
galactose as the primary sugar source.
Homolactic fermentation prevails under aerobic
conditions because the pyruvate formate lyase
(PFL) enzyme is oxygen sensitive.
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Heterolactic Fermentation of Glucose (Pentose
Phosphate Pathway)
--- Aerobic conditions
Glucose
O2
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Heterolactic Fermentation of Glucose ---
Anaerobic conditions
Glucose
ATP
ADP
Glucose-6-phosphate
6-phospho-gluconate
CO2
Ribulose-5-phosphate
Xylulose-5-phosphate
H2PO4-
Phosphoketolase
Acetyl-phosphate
Glyceraldehyde-3-phosphate
NAD
NADH H
1,3-Diphosphoglycerate
ADP
ATP
3-Phosphoglycerate
2-Phosphoglycerate
(2) H2O
Phosphoenolpyruvate (PEP)
ADP
ATP
Pyruvate
NADH H
LDH
NAD
Lactate
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Differentiation of L.A.B. Genera
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Hexoses other than glucose
Fructose, mannose and galactose enter the major
pathways at the level of glucose-6-phosphate or
fructose-6-phosphate after isomerization and
phosphorylation steps
(when galactose is transported by permease
Leloir next slide)
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Galactose metabolism pathway depends on transport
system used
H
Galactose
Galactose
Galactose Permease
H
PEP
Galactose
Galactose-6-PO4
Pyruvate
EI
EI
Mixed acid fermentation dominates if galactose
is the most abundant sugar available
Homolactic or mixed acid pathway
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Some L.A.B. cannot metabolize galactose
Example S. thermophilus and Lb. delbrueckii
subsp. bulgaricus
Galactose is exported via antiport system
Galactose
Lactose
OUT
cell membrane
Permease
IN
Lactose
Galactose
Glucose
Homolactic, mixed acid or heterolactic pathway
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Disaccharides broken into monosaccharides before
metabolized
lactose
sucrose
maltose
glucose
glucose
fructose
glucose
galactose
glucose
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Lactose Breakdown depends on transport system

?-galactosidase
Leloir pathway
Homolactic, mixed acid, or heterolactic pathway

Phospho-?-galactosidase
Tagatose pathway
Homolactic, mixed acid, or heterolactic pathway
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Leuconostoc Group III Lactobacillus
S. thermophilus, Lb. delbrueckii, Lb.
Helveticus, Lb. lactis
Lactococcus Lb. casei
Summary of Lactose Metabolism in L.A.B.
?-galactosidase
phospho-?-galactosidase
How many ATPs from one lactose?
CO2
S. thermophilus, and Lb. delbrueckii do not
metabolize the galactose part of lactose. They
export galactose from the cell.
(Figure from Fox et al. 1990. Critical Reviews in
Food Science and Nutrition. 29237-253.)
Tagatose Pathway
Leloir Pathway
Heterolactic Pathway
Homolactic Pathway
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Pentoses
Pentose
Pentose
ATP
ADP
Pentose-PO4
isomerization
Heterolactic fermentation pathway
Xylulose-PO4 or Ribulose-PO4
Pentoses cannot enter the homolactic or mixed
acid pathways
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Heterolactic Fermentation Pentose-PO4 entry
Glucose
O2
ATP
NADH H
NADH oxidase
ADP
NAD
Glucose-6-phosphate
H2O2
NADH H
NADH oxidase
NAD
CO2
2 H2O
Ribulose-5-phosphate
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L.A.B. groups based on fermentation pathways

• Obligately homofermentative
• Obligately heterofermentative
• Facultatively heterofermentative

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Obligately homofermentative
Group I Lactobacillus species and a few other
species

• Hexoses are fermented by homolactic fermentation
  
• pathway (glycolysis LDH)
• Can do mixed acid fermentation of hexoses under
  certain conditions
• Do not ferment pentoses
• Have FDP aldolase enzyme
• Do not have phosphoketolase enzyme

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Homolactic Fermentation of Glucose (Glycolysis
LDH)
Glucose
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Heterolactic Fermentation of Glucose --- Aerobic
conditions
Glucose
O2
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Obligately heterofermentative
Group III Lactobacillus species, Leuconostoc,
Oenococcus

• Hexoses are fermented by heterolactic
  fermentation
• pathway (phosphoketolase pathway)
• Pentoses are fermented by heterolactic
  fermentation
• pathway (phosphoketolase pathway)
• Have phosphoketolase enzyme
• Do not have FDP aldolase enzyme

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Facultatively heterofermentative
Group II Lactobacillus species, Lactococcus,
Pediococcus, Streptococcus thermophilus

• Hexoses are fermented by homolactic fermentation
  
• pathway (glycolysis LDH)
• Can do mixed acid fermentation of hexoses under
  certain conditions
• Pentoses are fermented by heterolactic
  fermentation
• pathway (phosphoketolase pathway)
• Have both FDP aldolase and phosphoketolase
• enzymes

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Lactobacillus
Lb. rhamnosus
Lb. sanfrancisco
Lb. kefir
Homolactic and facultatively heterolactic no
CO2 from glucose, FDP aldolase present Obligately
heterolactic CO2 from glucose, phosphoketolase
present
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Citrate Transport
H
Citrate
1.5 mg/ml citrate in milk
Citrate Permease
H
Citrate
Citrate permease is pH dependent only functions
pH 5 6. Optimum pH 5.2
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Citrate Metabolism
citrate lyase
Acetate
CO2
citrate
Pathway does not generate ATP, but regenerates
NAD.
Leuc. mesenteroides subsp. cremoris Lc. lactis
subsp. lactis biovar. diacetylactis
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Lactate Export
Antiport
Malate-
(Not common)
Sugar
Lactate-
2H
Lactate-
2H
Symport
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Exopolysaccharide Synthesis
Some strains of Lc. lactis, S. thermophilus,
Leuconostoc, Lactobacillus
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Exopolysaccharide types and applications

• Capsular yogurts (thickener), cheeses (increase
  moisture)

(Perry et al. 1997. Journal of Dairy Science.
80799-805.)

• Ropy some fermented milks such as viili and
  långfil

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L.A.B. Carbohydrate Metabolism
Transport of sugars across cell membrane
Catabolism of sugars for energy

• Pathways for glucose fermentation

Homolactic fermentation Mixed acid fermentation
Heterolactic fermentation
Synthesis of polysaccharides

• Pathways for other hexoses

• Disaccharides

• Pathways for pentoses

• Pathway for citrate