L.A.B.: Carbohydrate Metabolism ransport of sugars across cell membran Catabolism of sugars for energy Synthesis of polysaccha rides Homolactic fermentation Mixed acid fermentation Heterolactic fermentation Pathways for glucose fermentation • Pathways for other hexoses • Pathways for pentoses • Pathway for citrate • Disaccharides
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lab carbohydrate metabolism BY SOHAIL SARWAR from University of LHH
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• 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.
Sugar Transport in L.A.B.ce
llm
embr
ane
OUT
IN
ConcentrationGradient
(S1 & S2) PEP
Sugar
Sugar
H+
H+
Symport
Permease
Sugar
Sugar-PO4
PEP-PTSAntiport
Sugar1
Sugar1
Sugar2
Sugar2
Permease
PEP Pyruvate
EIIBCS
EIIAS
HPr
EI
ATP
H+
H+
ADP
F1F0 ATPase
PMF(pH + )
Energyfrom:
PEP-PTS
Sugar Transport Systems are Sugar-Specific ce
llm
embr
ane
OUT
IN
Lactose
Lactose
H+
H+
LactosePermease
Glucose
Glucose
H+
H+
GlucosePermease
Galactose
Galactose
H+
H+
GalactosePermease
Also sugar specific…… Antiport permease PEP-PTS – EIIA and EIIBC
Example: Lactose Transport
Type of Transport System for a Sugar is Species Specific
Heterolactic Fermentation of Glucose --- Anaerobic conditions
NAD+
NADH + H+
Xylulose-5-phosphate
H2PO4-
NAD+
NADH + H+CO2
Acetyl-phosphate
Acetyl-CoA
Acetaldehyde
Ethanol
NAD+
NADH + H+
H2PO4-
CoA
NAD+
NADH + H+
CoA
Products:1 ATP1 CO2
1 Lactate1 Ethanol
Key enzymes:PhosphoketolaseLactate dehydrogenase
Phosphoketolase
Differentiation of L.A.B. Genera
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
Glucose
Glucose-6-phosphate
Fructose-6-phosphate
HOMOLACTIC &
MIXED ACID
Glucose
Glucose-6-phosphate
6-phospho-gluconate
HETEROLACTIC
Galactose
Galactose-1-P
Glucose-1-P
Fructose
(when galactose is transported by permease – Leloir – next slide)
Galactose metabolism pathway depends on transport system used
GalactoseH+
H+
GalactosePermease
Galactose
Galactose
Galactose-6-PO4
PEP
PyruvateEIEI
LeloirPathway
Galactose-1-PO4
Glucose-1-PO4
Glucose-6-PO4
ATP
ADP
Homolactic, *mixed acid, or heterolactic pathway
Tagatose-1,6-diPO4
ATP
ADP
(2) Glyceraldehyde- 3-phosphate
Dihydroxyacetone-phosphate
TagatosePathway
Tagatose-6-PO4
Homolactic or *mixed acid pathway
* Mixed acid fermentation dominates if galactose is the most abundant sugar available
Some L.A.B. cannot metabolize galactose
Lactose
Lactose
Galactose
Galactose
Permeasecell
mem
bran
e
OUT
IN
Example: S. thermophilus and Lb. delbrueckii subsp. bulgaricus
Galactose is exported via antiport system
Glucose
Homolactic, mixed acid or heterolactic
pathway
Disaccharides: broken into monosaccharides before metabolized
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
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
Lb. rhamnosus
Lactobacillus
Homolactic and facultatively heterolactic – no CO2 from glucose, FDP aldolase presentObligately heterolactic – CO2 from glucose, phosphoketolase present
Lb. sanfranciscoLb. kefir
Citrate Transport
CitrateH+
CitrateH+
CitratePermease
Citrate permease is pH dependent – only functions pH 5 – 6. Optimum = pH 5.2