Nutritional Pathways Carb Part 1

Stereoisomerism

Compounds with the same structural formula but a different arrangement of atoms in space

chiral carbon

A carbon atom attached to four different atoms or groups of atoms.

Monosaccharides

glucose, fructose, galactose

reducing sugars

saccharides (sugars) that donate electrons resulting in the reduction (gain of electrons) of another molecule.

pentoses

Monosaccharides with five carbon atoms, e.g., ribose.

derivatives of monosaccharides

sugar phosphates, deoxy sugars, amino sugars, sugar alcohols, and sugar acids

maltose

glucose + glucose

Lactose

glucose + galactose

sucrose

glucose + fructose

trehalose

A disaccharide of two glucose molecules, but with a linkage different from maltose. Used as a food additive and sweetener.

Oligosaccharides

short chains of 3 or more monosaccharides (at least 10)

raffinose

indigestible oligosaccharide made of 3 monosaccharides (galactose-glucose-fructose)

Stachyose

indigestible oligosaccharide made of 4 monosaccharides (galactose-galactose-glucose-fructose)

Verbascose

- 5 sugar molecules: 3 galactoses, glucose and fructose

- found in legumes

Polysaccharides

Carbohydrates that are made up of more than two monosaccharides

polysaccharide structure

polymer of monosaccharides

1. starch

- amylose (linear, alpha 1, 4 bonds)

- amylopectin (branched, alpha 1,6 bonds & alpha 1,4 bonds)

2. glycogen (very branched, only alpha 1,6 bonds)

3. cellulose (beta 1,6 bonds, indigestible, very stable)

Amylose

simplest form of starch

Amylopectin

a soluble polysaccharide and highly-branched polymer of glucose found in plants as one of the two components of starch (the other being amylose).

Amylopectin structure

1. Branched Chains of alpha glucose

2. 1-4 and 1-6 glycosidic bonds

3. With Branches

Glycogen

An extensively branched glucose storage polysaccharide found in the liver and muscle of animals; the animal equivalent of starch.

Glycogen structure

Long, branched chain of alpha-glucose.

Similar structure to amylopectin, but has much more side branches.

Cellulose

A substance (made of sugars) that is common in the cell walls of many organisms

Cellulose structure

1. Long, unbranched chain of beta-glucose.

2. Bonds between the sugars are straight, so cellulose chains are straight.

3. Cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils.

high intensity sweeteners

group of nonnutritive sweeteners that are extremely sweet tasting compared to the same amount of sugar

salivary amylase

Enzyme in saliva that breaks down starch

pancreatic amylase

An enzyme secreted by the pancreas into the small intestine that digests any remaining starch into maltose.

resistant starch

the fraction of starch in a food that is digested slowly, or not at all, by human enzymes

alpha-amylase

cleaves randomly along the chain to yield shorter polysaccharide chains, maltose, and glucose

alpha-1,6-glucosidase

Cleaves the alpha-1,6 bond at the branch point, releasing a free glucose

Oligosaccharides

stachyose, raffinose, bacterial fermination

fibers

collagen, elastic, reticular

Fiber energy sources

colonic mucosal cells or absorbed into the hepatic portal vein

Disaccharide digestion

-Small intestine (sucrase, lactase, and maltase)

intestinal absorption of glucose and galactose

-Absorbed by both active and facilitated transport

•Relative contribution depends on amount of CHO consumed

•Facilitated transport contributes more after a high CHO meal

-Active transport

•Sodium-glucose tranpsorter1 (SGLT1) transports 2 molecules of Na and 1 molecule

of glucose into the enterocyte

•Na+ is "pumped" back out of the cell by the activity of Na+/K+-ATPase

-Facilitated transport

•Following a high CHO meal, GLUT2 translocates to the brush border membrane to

facilitate glucose uptake

-GLUT2 facilitates glucose transport across the basolateral membrane for

delivery into the hepatic portal vein

intestinal absorption of fructose

Into enterocyte: facilitated transport

GLUT5

Into blood: GLUT2

Limited in 60% of adults

Typically no fructose beyond the portal vein as it is efficiently absorbed by the liver

Absorbed slower than GLU and GAL but faster than sugar alcohols

post-absorption facilitated transport

- hepatic portal vein takes monosaccharides to liver

- galactose & fructose used for energy or converted to glucose

- glucose stored as glycogen or transported to tissues

GLUT isoforms

14 identified, with genes

Integral proteins

Each has specific combining site

Undergoes a conformational change upon binding the molecule

Can reverse this change when unbound

GLUT 1

RBCs, brain, cornea, placenta

GLUT 2

Important in synthesis of glucose to glycogen in the liver

GLUT 3

brain, placenta

GLUT 4

Important in muscle and adipose tissue for glucose transport across muscles and TG storage by lipoprotein lipase activation

GLUT 5

Fructose: spermatocytes, GI tract

GLUT 7

Transports glucose across the ER.

Pyruvate

Organic compound with a backbone of three carbon atoms. Two molecules form as end products of glycolysis

Alanine function

Primarily released from muscle to the liver during catabolism and is a precursor for gluconeogenesis.

Fate of Pyruvate

Depends on oxygen availability.

When oxygen is present, pyruvate is oxidized to acetyl-CoA which enters the Krebs cycle

Aerobic respiration

Without oxygen, pyruvate is reduced in order to oxidize NADH back to NAD+

Fermentation

Lactate dehydrogenase

converts pyruvate to lactate

pyruvate dehydrogenase

converts pyruvate to acetyl-CoA.

- stimulated by insulin

- inhibited by acetyl-CoA

NADH

the reduced form of NAD+; an electron-carrying molecule that functions in cellular respiration

pyruvate kinase

Enzyme converting phosphoenolpyruvate to pyruvate.

Phosphoenolpyruvate (PEP)

Three-carbon phosphorylated compound that is an important intermediate in glycolysis and is a reactant in the initial carbon fixation step in C4 and CAM photosynthesis.

Hepatocytes

liver cells; produce bile, absorb and secrete nutrients like glucose, and secrete plasma proteins

insulin in liver cells

1. attaches to receptor on cell surface membrane of target cells

2. activates enzymes that convert glucose into glucose 6 phosphate into glycogen (glycogenesis)

converting glucose to glucose 6 phosphate ensures a steep diffusion gradient is maintained

liver and glucose

epinephrine binds to receptors on the plasma membrane of ____ cells and release ___

(#0-40% of glucose)

monosaccharides and blood

glucose no galactose or fructose

euglycemia

normal (level of) sugar in the blood (within normal range)

70-110 mg/100 mL fasting state

insulin

controls cellular events when hyperglycemic

glucagon, epinephrine, and cortisol

- catabolic hormones

- trigger breakdown of stored triglycerides, glycogen, and body protein for energy

Insulin Production

produced by beta cells in pancreas

Insulin and GLUT4

Insulin binds to receptor and initiates the recruitment of GLUT4 to the cell surface by initiating a protein kinase cascade

insulin half life

3-5 minutes

40-50 units

Insulin-mediated glucose uptake

Insulin binding to the insulin receptor induces a signal transduction cascade which allows the glucose transporter (GLUT4) to transport glucose into the cell

Glucokinase

- converts glucose to glucose-6-phosphate

- present in pancreatic B-islet cells as part of glucose sensor

- responsive to insulin in the liver

Hexokinase

The enzymes that catalyzes the phosphorylation of glucose to form glucose-6-phosphate in the first step of glycolysis. This is one of the ain regulatory steps of this pathway. Hexokinase is feedback-inhibited by glucose-6-P.

glucokinase vs hexokinase

Glucokinase = In Liver

Hexokinase = Non-liver cells

insulin-sensitive tissues

Insulin binds to its receptor, causing a conformational change → tyrosine kinase induces autophosphorylation and tyrosine phosphorylation of cell molecules→ GLUT 4 translocation to plasma membrane

tyrosine kinase

An enzyme that catalyzes the transfer of phosphate groups from ATP to the amino acid tyrosine on a substrate protein.

autophosphorylation

the process by which one molecule of the tyrosine kinase receptor dimer phosphorylates its partner

What is one metabolic role of insulin?

Increases the uptake of glucose and amino acids.

How does insulin influence energy pathways?

It influences energy pathways within the cells.

What processes does insulin promote?

Promotes glycolysis, pentose phosphate pathway, glycogenesis (glycogen formation), and lipogenesis

What effect does insulin have on glucose formation?

Inhibits glucose formation.

What process does insulin inhibit that is related to glucose?

Inhibits gluconeogenesis.

What effect does insulin have on fat breakdown?

Inhibits fat breakdown. (lipolysis)

What does insulin promote in terms of protein?

Promotes protein synthesis.

What does insulin promote regarding energy?

Promotes storage of energy.

Glycolysis

the breakdown of glucose by enzymes, releasing energy and pyruvic acid.

Krebs Cycle

second stage of cellular respiration, in which pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions

Glycemic Index

a measure of how a particular food affects blood glucose levels

glycemic load

the amount of carbohydrate in a food multiplied by the glycemic index of the carbohydrate

blood barrier system

strictly regulates what substances can get from the bloodstream into the tissue fluid of the brain

Glucagon function and orgin

increases blood glucose levels by stimulating the liver to break down glycogen into glucose

from pancreas

What pathways does glucagon promote in the liver?

Gluconeogenic, glycogenolytic, and ketogenic pathways (in liver)

Gluconeogenesis is the process in which ________.

glucose is formed from noncarbohydrate precursors

Glycogenolysis

breakdown of glycogen to glucose

What type of activity does glucagon promote in various tissues?

Lipolytic activity

Does glucagon affect muscle glycogen breakdown?

No effect

Insulin-glycogen ratio

Balanced meal ratio 2.3:1

Glucose solution infusion ratio 25:1

Epinephrine metabolic effects

- Hyperglycemia

* stimulates gluconeogenesis and glycogenolysis

* inhibits insulin release

- lipolysis

* increased free fatty acids

Epinephrine B1 and B2

B1- Heart rate and contractions increased

B2- Smooth muscle relaxation, vasodilation in skeletal muscles and liver

Epinephrine characteristics

- Secreted naturally by the adrenal glands from tyrosine

- stress response

What is one effect of cortisol on carbohydrate metabolism?

Stimulates gluconeogenesis and glycogen synthesis

How does cortisol affect peripheral glucose uptake?

Inhibits peripheral glucose uptake

What effect does cortisol have on protein metabolism?

Stimulates muscle catabolism and amino acid release

What is the overall purpose of cortisol's metabolic effects?

To ensure an adequate supply of glucose to the brain

What is the effect of cortisol on fat metabolism?

Stimulates lipolysis

Cortisol characteristics

- STEROIDS

- produced in adrenal glands

- control blood sugar level

- regulate metab

- reduce inflammation (+memory)

- controls salt/water balance

- blood pressure

Tricarboxylic acid

Citric Acid Cycle (Krebs Cycle)

Pentose phosphate pathway

A metabolic process that produces NADPH and ribose 5-phosphate for nucleotide synthesis.

glycogenesis vs glycogenolysis

glycogenesis = building of glycogen

glycogenolysis: breakdown of glycogen