Fst/AN/hn 761

Overview of Carbohydrate Digestion and Metabolism

FST/AN/HN 761

• FST 761
• Dr. Jeff Firkins – Carbohydrates
• Dr. Josh Bomser – Lipids
• TA- Amy Long, MS
• Reading / Writing Assignments
• Text - Biochemical and Physiological Aspects of Human Nutrition- Martha H. Stipanuk.
• Today – Overview of carbohydrates (Jan 7)

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. No single carbohydrate is essential, but carbohydrates do participate in many required functions in the body.

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 (Dr. Firkins)

Starch

• Major storage carbohydrate in higher plants
• Amylose – long straight glucose chains (a1-4)
• Amylopectin – branched every 24-30 glc residues (a 1-6)
• Provides 80% of dietary calories in humans worldwide

Glycogen

G

G

G

G

G

G

G

G

a 1-4 link

G

G

G

G

G

a 1-6 link

G

G

G

G

G

G

• Major storage carbohydrate in animals
• Long straight glucose chains (a1-4)
• Branched every 4-8 glc residues (a 1-6)
• More branched than starch
• Less osmotic pressure
• Easily mobilized

Digestion

• Pre-stomach – Salivary amylase : a 1-4 endoglycosidase

G

G

G

G

G

G

G

G

a 1-4 link

G

G

G

G

a 1-6 link

G

G

G

G

G

G

G

G

G

G

G

G

G

G

G

maltose

G

G

G

isomaltose

amylase

maltotriose

G

G

G

G

a Limit dextrins

Stomach

• Not much carbohydrate digestion
• Acid and pepsin to unfold proteins
• Ruminants have forestomachs with extensive

microbial populations to breakdown and

anaerobically ferment feed

Small Intestine

• Pancreatic enzymes

a-amylase

G

G

G

G

G

G

G

G

G

G

G

G

G

G

G

G

amylose

amylopectin

G

G

G

G

G

a amylase

+

G

G

G

G

G

maltotriose

maltose

a Limit dextrins

G

Oligosaccharide digestion..cont

G

G

G

G

G

G

G

G

G

G

G

G

Glucoamylase (maltase)

or

a-dextrinase

G

G

G

G

G

a-dextrinase

G

G

G

G

G

G

G

maltase

sucrase

a Limit dextrins

G

Small intestine

Portal for transport of virtually

all nutrients

Water and electrolyte balance

Enzymes associated with

intestinal surface membranes

• Sucrase
• a dextrinase
• Glucoamylase (maltase)
• Lactase
• peptidases

Carbohydrate absorption

Hexose transporter

apical

basolateral

Glucose and galactose absorption

• Read Chapter 5 and answer the questions on page 102 of Stipanuk. Be prepared to discuss them on Friday

Carbohydrate malabsorption

• Lactose intolerance (hypolactasia), page 100.
• Decline lactase with age
• Lactose fermented in LI –
• Gas and volatile FA
• Water retention – diarrhea/bloating
• Not all populations
• Northern European – low incidence
• Asian/African Americans – High

b 1-4 linkage

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

Glucose

Glucose-6-P

Pyruvate

Hexokinase

Pentose

Phosphate

Shunt

glycolysis

• Serve as primary source of energy in the cell
• Central to all metabolic processes

Glc-1- phosphate

glycogen

Cytosol - anaerobic

Pyruvate

cytosol

Aceytl CoA

mitochondria

(aerobic)

Krebs

cycle

Reducing

equivalents

Oxidative

Phosphorylation

(ATP)

AMINO

ACIDS

FATTY ACIDS

No mitochondria

Glucose

Glucose

Glucose

The Full

Monty

Glucose

Glycogen

Lactate

Fasted State

Glucose

Glucose-6-P

Pyruvate

Hexokinase

Pentose

Phosphate

Shunt

glycolysis

Glc-1- phosphate

glycogen

Need 13.8 kJ/mol

ATP = -30 kJ/mol

-16.7 kJ/mol

GNG

G-6-Pase

Controlling Metabolic Flux

1. Control enzyme levels

2. Control of enzyme activity (activation or inhibition)

Control of enzyme activity

Rate limiting step

Glycogen synthase

(active)

OH

P

Glycogen synthase

(inactive)

Glycogen formation

Glycogen synthase kinase

(active)

OH

IR

insulin

P

P

Protein Kinase B

(active)

Protein Kinase B

(inactive)

OH

P

Glycogen synthase kinase

(inactive)

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

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