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What enzymes are secreted to break down starches and where? What do they do?
- Mouth: alpha amylase hydrolyzes alpha-1,4 bond of starches
- Stomach: alpha-amyalse to continue breaking down starch - mostly short chain polysaccharides
- Small intestine: pancreatic alpha-amylase and alpha dextrinase - secreted into the lumen
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What is the enzyme activity in the brush border?
- Digestion must occur before absorption can
- Lactase: breaks down lactose into galactose and glucose - hydrolyze the galactose beta (1-4) glucose bond
- Maltase: hydrolyzes alpha-1,4 bond in maltose
- Isomaltase: hydrolyzes alpha-1,6 bond in dextrines to form 2 glucoses
- Alpha dextrinase: hydrolyzes alpha-1,4 of dextrines
- Sucrase and Invertase: hydrolyze the glucose alpha (1-2) fructose bond
- There are no enxymes that cleave bonds in dietary fibre (beta-1,4 between glucoses)
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How are monosaccharides absorbed?
- Very efficient
- Nearly all absorbed by the end of the jejunum
- Active transport: used for glucose and galactose - uses SGLT1 (sodium glucose transporter)
- Facilited transport: for frutose as maintains [] gradient therefore uses downward gradient to head to the liver
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What are the 3 functions of carbs?
- 1. Glucose = primary source of energy for cells: especially brain and RBC
- 2. carbs spare proteins: prevents breakdown of proteins for energy and allows protein to concentrate on building, repairing, and mainting body tissues
- 3. Carbs prevent ketosis: when carbs are limited, fat can be broken down for energy whic creates ketone bodies and amkes the body acidic - partial breakdown can of fat can be used short term by the body
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What are the key metabolic pathways of CHO use and storage?
- Glycogenesis
- Glyconeogenesis
- Glycogenolysis
- Glycolysis
- TCA cycle
- Hexose monophosphate shunt
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What is glycogenesis? How and where does it occur?
- Synthesis of glycogen from glucose to maintain homeostasis
- alpha-1,4 with alpha-1,6 branches
- Use enzymes glycogen synthase and a branching enzyme (makes alpha-1,6 bonds)
- Requires energy
- Major sites: liver, skeletal muscle
- Hormonal control: insulin - released to stimulate glycogen synthesis
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What is glucogenolysis? What does it involve? Where does it occur?
- Breakdown od glycogen into glucose units
- Uses the enzymes glycogen phosphorylase and and debranching enzyme
- Hormonal control (eg. glucagon - high during fasting therefore signals glucogenolysis)
- Muscle does not contribute to blood glucose - it breaks down its own glycogen and uses it locally
- Liver contributes free glucose and has blood glucose control
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What is glycolysis? What is produced? How does this affect RBC? What are the 2 different kinds?
- Initial reactions for compelte oxidation of glucose to O2 and H2O
- Glycolytic enzymes in cytoplasm
- Substrate level phosphorylation yeilds minor amount of ATP
- RBC don't have mitochondria therefore this is important
- Endpoints depend on available O2
- Aerobic: pyruvate goes to mitochondira for complete oxidation
- Anaerobic: lactate is formed
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What is the hexose monophosphate shunt? Where does it have low and high activity?
- Important for NADPH production and ribose synthesis
- high acitivty in liver, adipose tissues, lactating mammary gland
- Low acitivty in skeletal muscles
- Leads to production of pentose phosphates
- Different steps lead to NADPH synthesis (important in FA synthesis)
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What is pyruvate dehydrogenation?
- Deydrogenation of pyruvate to form acetyl CoA which is a meeting point for many metabolic processes
- Uses pyruvate dehydrogenase complex
- Negative regulation by ATP and NADH
- Micronutrients drive the reaction: thiamine, niacin, riboflavin, pantothenic acid
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What is th Krebs cycle?
- Over 90% of the energy in food released here
- A common and final catabolic pathway b/c products of CHO, fat and protein feed into the cycle
- Occurs in mitochondiral matric
- Centre of energy metabolism
- Provides reduceed cosubstrates NADH and FADH2 that yield ATP through their oxidation via ETC
- Not all substrates entering cycle are fully oxidized
- Produces: 3NADH, 1FADH2, 2CO2, 1ATP and 1 Acetyl CoA
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What is gluconeogenesis and waht can be used to do this?
- Glucose synthesis from non-CHO sources
- Pyruvate, some a.a., lactate, glycerol
- Only uses glucogenic a.a.
- All get converted to pyruvate which then needs a bypass step
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When is gluconeogenesis needed? What is done with muscle lactate?
- When blood glucose drops - need hormonal control
- occurs in the liver and also in kidneys (starvation)
- Muscle adipose tissue lack needed bypass anzymes
- Cori cycle: muscle lactate transported to liver - glucose synthesis - back to muscle
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What is a key step in gluconeogenesis?
- the bypass of the pyruvate kinase reaction involves formation of oxaloacetate
- moving oxaloacetate out of the mitochondira (conversion to malate first) and into the cytoplasm
- Oxaloacetate to PEP by PEP carboxykinase (a cytoplasmic enzyme)
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What can acetyl CoA be used for? What about oxaloacetate-malate?
- Can be used for fatty acid synthesis or in krebs cycle. BUT can not be used to make glucose, as it provides no net increase in cycle intermediates
- Allows carbon skeletons of various amino acids to enter gluneogenesis
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