Download Glucose

Overview of
Carbohydrate Digestion
and Metabolism
Carbohydrates
•Carbohydrates are called carbohydrates because they
are essentially hydrates of carbon (i.e. they are
composed of carbon and water and have a
composition of (CH2O)n.
•The major nutritional role of carbohydrates is to
provide energy and digestible carbohydrates provide
4 kilocalories per gram.
Photosynthesis: Sun’s energy becomes part of glucose molecule
energy
Carbon dioxide
Water
Chlorophyll
GLUCOSE
6 CO2 + 6 H20 + energy (sun)
C6H12O6 + 6 O2
120 grams of glucose / day = 480 calories
Simple Sugars -
Disaccharides
Complex carbohydrates


Oligosaccharides
Polysaccharides
Starch
 Glycogen
 Dietary fiber

Starch



Major storage
carbohydrate in higher
plants
Amylose – long straight
glucose chains (a1-4)
Amylopectin – branched
every 24-30 glc residues
(a 1-6)
Glycogen
Major storage
carbohydrate in animals
 Long straight glucose
G
chains (a1-4)
G
 Branched every 4-8 glc
residues (a 1-6)
a 1-4 link
 More branched than
starch
Easily mobilized

G
G
G
G
G
G
G
G
G
G
G
G
G
a 1-6 link
G
G
G
G
To ensure a readily available supply, liver and muscle
cells store carbohydrate as glycogen .
Glycogen is stored hydrated with water; thus the water
makes glycogen large, cumbersome, and unsuitable for
long-term energy storage. The 70-kg "average" man
stores only an IS-hour fuel supply as glycogen,
compared with a 2-month supply stored as fat. If all
human energy stores were glycogen,
humans would need to weigh 60 additional pounds
Approximately 150 g of glycogen is
stored in muscle, which can be increased fivefold with
physical training but is not available to maintain blood
glucose directly. It is the glycogen store in the human
liver (approximately 90 g) that is involved in the
hormonal control of blood sugar.
Digestion

Pre-stomach – Salivary amylase : a 1-4 endoglycosidase
G
a 1-4 link
G
G
G
G
G
G
G
G
G
G
G
G
G
G
amylase
a 1-6 link
G
G
G
G
G
G a Limit
G G
G
G G
G G
G
dextrins
maltotriose
G G
G
maltose
G
G
isomaltose
Stomach



Not much carbohydrate digestion
Acid and pepsin to unfold proteins
Ruminants have fore stomachs with extensive
microbial populations to breakdown and
anaerobically ferment feed
Small Intestine
Pancreatic enzymes
a-amylase

maltotriose
G G G
G G G G G
amylose
G G
a amylase
G G G G G
G G G G G G
amylopectin
+
maltose
G G G
G G G
a Limit dextrins
Small intestine
Portal for transport of virtually
all nutrients
Water and electrolyte balance
Enzymes associated with
intestinal surface membranes
i. Sucrase
ii. a dextrinase
iii. Glucoamylase (maltase)
iv. Lactase
v. peptidases
Carbohydrate absorption
Hexose transporter
apical
basolateral
Carbohydrate malabsorption



Lactose intolerance (hypolactasia),
Decline lactase with age
Lactose fermented in LI –



Not all populations

b 1-4 linkage
Gas and volatile FA
Water retention – diarrhea/bloating

Northern European – low incidence
Asian/African Americans – High
Metabolism – the chemical changes
that take place in a cell that produce
energy and basic materials needed for
important life processes
-millions of cells
-Multiple organs (liver, adipose, heart, brain)
-Thousands of enzymes
-Various conditions (fed, fasted, exercise, stress)
Carbohydrates


Serve as primary source of energy in the cell
Central to all metabolic processes
Glucose
Cytosol - anaerobic
Hexokinase
Pentose
Phosphate
Shunt
Glucose-6-P
glycolysis
Pyruvate
Glc-1- phosphate
glycogen
cytosol
mitochondria
(aerobic)
Pyruvate
Aceytl CoA
FATTY ACIDS
Krebs
cycle
AMINO
ACIDS
Reducing
equivalents
Oxidative
Phosphorylation
(ATP)
No mitochondria
Glucose
Glycogen
Lactate
Glucose
Glucose
Glucose
The Full
Monty
Fasted State
Glucose
G-6-Pase
Pentose
Phosphate
Shunt
Hexokinase
Glucose-6-P
GNG
glycolysis
Pyruvate
Need 13.8 kJ/mol
ATP = -30 kJ/mol
-16.7 kJ/mol
Glc-1- phosphate
glycogen
Controlling Metabolic Flux
1. Control enzyme levels
2. Control of enzyme activity (activation or inhibition)
Control of enzyme activity
Rate limiting step
insulin
IR
P
Protein Kinase B
(inactive)
Glycogen synthase kinase
(active)
P
Glycogen synthase
(inactive)
OH
Protein Kinase B
(active)
P
OH
synthase kinase
P Glycogen(inactive)
OH
Glycogen synthase
(active)
Glycogen formation
Controlling Metabolic Flux
1. Control enzyme levels
2. Control of enzyme activity (activation or inhibition)
3. Compartamentalization
Fatty acid oxidation occurs in mitochondrial matrix
Fatty acid synthesis occurs in endoplasmic reticulum membrane exposed
to the cytoplasm of the cell.
4. Hormonal control
Glucose utilization
Stage 1 – postparandial
All tissues utilize glucose
Stage 2 – postabsorptive
KEY – Maintain blood glucose
Glycogenolysis
Glucogneogenesis
Lactate
Pyruvate
Glycerol
AA
Propionate
Spare glucose by metabolizing fat
Stage 3- Early starvation
Gluconeogenesis
Stave 4 – Intermediate starvation
gluconeogenesis
Ketone bodies
Stage 5 – Starvation
Glycemic Index
The glycemic index is used to rank
carbohydrates by
their ability to raise blood glucose levels as
compared
with a reference food
foods with a low glycemic index have
consistently shown beneficial effects on blood
glucose control in both the short
and long term in diabetic patients. The glycemic
index of
a diet has a predictable effect on blood glucose
levels
The glycemic load of a food is the
glycemic index of the carbohydrate divided
by 100 and
multiplied by its amount of available
carbohydrate content
(i.e. carbohydrates minus fiber) in grams.
Carbohydrate Regulation of Blood Lipids
Carbohydrate-induced hypertriglyceridemia can
result from consuming a high-carbohydrate diet.
The body regulates macro nutrient levels to
provide adequate fuel for body tissues. The brain
uses the most of the approximately 200 g
of glucose required per day. "When blood glucose
level falls to less than 40 mg/ dL,
counterregulatory hormones release
macronutrients from stores. "When blood glucose
level rises to more than 180 mg/dL, glucose is
spilled into the urine
High intakes of carbohydrate trigger large releases of
insulin for compensatory responses, including insulindependent glucose uptake by muscle or fat and active
synthesis of glycogen and fat. Blood glucose then drops to
a normal range.
Approximately 2 hours after a meal, intestinal absorption is
complete, but insulin effects persist and blood glucose falls.
The body interprets this hypoglycemic state as starvation
and secretes counterregulatory hormones to release free
fatty acids from fat cells. Fatty acids are packed into
transport lipoproteins (very-low-density lipoproteins
[VLDLs) in the liver, thereby elevating serum
triglycerides.
Carbohydrate Metabolism/
Utilization- Tissue Specificity





Muscle – cardiac and skeletal
 Oxidize glucose/produce and store glycogen (fed)
 Breakdown glycogen (fasted state)
 Shift to other fuels in fasting state (fatty acids)
Adipose and liver
 Glucose  acetyl CoA
 Glucose to glycerol for triglyceride synthesis
 Liver releases glucose for other tissues
Nervous system
 Always use glucose except during extreme fasts
Reproductive tract/mammary
 Glucose required by fetus
 Lactose  major milk carbohydrate
Red blood cells
 No mitochondria
 Oxidize glucose to lactate
 Lactate returned to liver for Gluconeogenesis