Macro Final Exam - Proteins (Chapter 6)/Fed, Fasting, & Starvation

fed state (absorptive)

postprandial

• 3-4 hr period after meal

• transient increases in plasma glucose, AAs, and triglycerides

• elevated insulin & glucagon

• blood glucose from exogenous/diet

• glucose use by all tissues

• major fuel for brain: glucose

plasma levels in the fed state

• glucose: largest spike

• AAs remain stable

• TAGs: little to no spike; used to synthesize

• chylomicrons: lactate spikes due to glucose metabolism (SM & heart)

upregulated pathways in the fed state

• glycogen synthesis: muscle

• TAG synthesis: adipose tissue & liver

• TAG storage: SM & liver

• FA synthesis: converting excess CHOs to FA and storing as TAG (liver)

what is TAG synthesis

delivers FAs to the liver and adipose tissue in the fed state via chylomicrons

• glycerol-3-phosphate (de novo lipogenesis & FAs)

• liver, SM, & adipose tissue can synthesize TAGs

• regulation is not clear

fatty acid synthesis

• production of FAs from acetyl CoA

  *2 carbon addition in form of malonyl-CoA

• rate-limiting enzyme: acetyl-CoA carboxylase

• synthesized from simple precursors

• happens under excess caloric intake; excess carbons from CHOs can be used

citrate synthase

makes citrate in mitochondria

citrate lyase

cytosol; reforms acetyl CoA

palmitoleic acid

mainly from synthesis

• used as marker of FA synthesis

postabsorptive state

4-18 hrs after meal

• no exogenous macronutrients coming in

• carbons from gluconeogenesis via lactate (early) and alanine (late) in liver

upregulated pathways in the postabsorptive state

glycogenolysis in the beginning

• production of glucose from body glycogen stores via glycogen phosphorylase

gluconeogenesis in the end

• production of glucose from non-carb precursors

  • lactate (early) to alanine (late)

  • lactate released from SM & RBC

  • alanine from protein breakdown

lipolysis & FA uptake from lipolysis in adipose tissue

decreased protein synthesis

fasting state

18-48 hrs after mean

• insulin decreased; cortisol and glucagon increased

• blood glucose from glycogen hepatic gluconeogenesis

• glucose used by all tissue except liver

• major fuel for brain: glucose

upregulated pathways in the fasting state

FA oxidation (b-oxidation)

ketosis (late)

gluconeogenesis

protein breakdown

• increased AAs leaving SM

glycolysis

*no glycogenolysis as glycogen stores are only 24 hrs

where does the fasting state get its carbons for glucose

gluconeogenesis via alanine in liver

starvation

>48 hrs after meal

• increased glucagon; decreased insulin

• carbons for glucose from gluconeogenesis via glycerol (early) & lipolysis (late)

• blood glucose from: gluconeogenesis, hepatic & renal

• glucose used by brain & RBC

• major fuel for brain: glucose

upregulated pathways in starvation

gluconeogenesis (early)

ketosis (late)

lipolysis (fuel shift)

use of FAs as fuel

decreased protein breakdown

lipolysis

hydrolysis of stored TAGs

• breaking FA off glycerol by intracellular lipases

• remove all 3 FAs in sequential steps

• glycerol travels back to liver & FA go through b-oxidation

  • cAMP stimulates phosphorylation of enzymes; signals pKa

ATGL (adipose triglyceride lipase)

always happening; TAGs to DAGs

HSL (hormone sensitive lipase)

• cytosol

• phosphorylation; travels to lipid droplet

• releases FAs from DAG

MGL (monoacylglycerol lipase)

• always in lipid droplet

• cleaves off last FA on DAGs making MAGs

CATI (carnitine acyltransferase I)

• outer membrane

• acts as regulator or “gate keeper”

*adds carnitine to make fatty acyl carnitine

malonyl CoA

_______ ___ inhibits CATI since it is a product of FA synthesis

CATII (carnitine acyltransferase II)

• strips off carnitine & adds CoA back

• reform fatty acyl transferase

genetic defect in CATI

individuals unable to do long-chain FA metabolism

genetic defect in CATII

more muscle specific

acyl CoA dehydrogenase defect

• fasting: problems begin to occur

• sudden infant death syndrome (goes to sleep & enters severe hypoglycemia)

*deficits can be severe depending on what is happening

polypeptides

• amino acids linked by peptide bonds

• carry out actions in the body

• 40% SM

• 25% body organs

• essential b/c of constituent amino acids

amino acids

• building blocks

• 4 parts

  • amino

  • carboxyl

  • R group

  • hydrogen

zwitterion

a molecule or ion having sperate positively & negatively charged groups but is overall neutral

branched chain amino acids

isoleucine, leucine, and valine

• SM takes up

• liver does not utilize as much

essential amino acids

must be provided by the diet

• phenylalanine

• valine

• threonine

• methionine

• tryptophan

• histidine

• isoleucine

• leucine

• lysine

conditionally essential

synthesis limited under special pathophysiological conditions

• premature infants

• cirrhosis

• inborn errors of metabolism

phenylketonuria (PKU)

• lack phenylalanine hydroxylase

• cannot convert phenylalanine to tyrosine

  • leads to buildup in blood causing neurological issues

• lack tyrosine

treatment for phenylketonuria

• decrease composition of phenylalanine

• formula that has no phenylalanine

• no breast milk/animal products

sources of amino acids

• exogenous

  • diet (animal sources)

• endogenous

  • proteins secreted into digestive tract from body

describe protein digestion in the stomach

HCl denatures proteins

zymogen pepsinogen is secreted

• activated by HCl leads to active pepsin

pepsin cleaves AAs into small AA sequences

describe digestion of proteins by trypsin in intestines

• secreted as trypsinogen from pancreas

• functions as endopeptidase

• cleaves the carboxyl end of AA (mostly lysine & arginine

• activates other zymogens

chymotrypsin

zymogen is chymotrypsinogen; cleaved by trypsin

• secreted by the pancreas

carboxypeptidase A, B aminopeptidases

zymogen is procarboxypeptidases; cleaved by trypsin

• secreted by pancreas

describe protein digestion in the intestines

• aminopeptidases secreted from intestinal cells cleave amino acids at the amino (N) terminus

• dipeptidases secreted from intestinal cells hydrolyze dipeptidases

• brush border peptidases are bound to enterocyte brush border

  • 1: cleaves at amino end

  • 2: look for 2 AA hooked together and separates them

  • 3: bound to enterocyte and cleaves AA from peptide bond

name the end products of protein digestion

• free AAs

• dipeptides

• tripeptides

  • all can be absorbed into intestinal cells

complete protein

contains all essential AAs

incomplete protein

does not contain all essential AAs

protein quality is determined by

• ability to provide essential AA; digestibility

• essential AA composition

• PDCAAS (protein digestibility corrected amino acid score)

  • 1 = high quality

protein quality scores

digestibility decreases once you get to majority of plant sources

• protein is bound to fibers

recommended protein and amino acid intakes

RDA for adults: 0,8 grams protein/1 kg

• for essential AA

• 10-33% recommended of total calories

  • in reality should be closer to 1 gram/kg

• especially for aging population - loss of muscle mass

describe the absorption of amino acids

• duodenum & upper jejunum

• requires specific carriers - transport proteins that take AA into enterocyte

• sodium dependent & independent mechanisms

describe the amino acid transport system

competition between different amino acids for the same carrier

• overconsuming 1 amino acid may lead to loss of transport function for another one

best source of protein

WHOLE PROTEIN SOURCES

• no need for supplements or collagen

signals the machinery for building muscle mass; extremely important for trying to bulk

leucine

proton/peptide symporter (PEP1)

• peptides absorbed into cell with hydrogen

• hydrogen pumped out of cell in exchange for sodium

• pumped into portal vein to travel to liver

describe the absorption of peptides in the intestinal brush border

• absorption of peptides actually GREATER than free amino acids

• hydrolyzed in cell cytosolic peptidases; generate free amino acids

• limiting factor: digestion

what happens to amino acids after exiting enterocytes

• pass through enterocyte to portal blood

• used for protein synthesis within enterocytes

• oxidized for energy

• undergo metabolic conversion to other AA or metabolites

50%

intestines use about ____ amino acids for energy

3 major sources of amino acids

• digestion of endogenous proteins from GI tract; enzymes digested or sloughed off cells (proteases)

• dietary proteins

• intracellular protein turnover

2 themes for amino acid metabolism

• nitrogen theme

  • movement of amino groups

• carbon theme

  • fate of carbon skeletons

transamination

transfer of amino group to an a-keto acid

• N group goes to ca carbon skeleton to form a new amino acid

• catalyzed by aminotransferases

• pyridoxal 5-phosphate dependent

• can produce products to help fill TCA cycle (glucogenic amino acids

a-ketoglutarate

_________________________ is a widely used acceptor of amino groups; many of the new amino acids are glutamate

alanine aminotransferase

alanine to pyruvate (a-ketoglutarate as acceptor)

• liver function leaking into the blood = poor liver function

• gluconeogenesis of alanine in the fasting state; need to convert to pyruvate first

  • high activity in liver

aspartate aminotransferase

aspartate to oxaloacetate (a-ketoglutarate)

• common to assess heart damage

  • high activity in heart

deamination

removal of amino group to release ammonia & form a-keto acid

• glutamate dehydrogenase is major reaction

• glutamate to a-ketoglutarate

• OPPISITE OF TRANSAMINATION

deamidation

transfer of amide group to carbon skeleton

• glutamine & asparagine

• amide nitrogen can be released by glutaminase or asparagine

what is the purpose of the urea cycle

“fix” or excrete free ammonia from the body

sources of ammonia from the body

• deamination & deamidation chemical reactions

• ingestion & absorption of processed meats (nitrogen content)

• generation by bacterial lysis of urea and amino acids in the GI tract

enzymes involved in the removal of ammonia

• glutamate dehydrogenase

• glutamine synthetase

• carbamoyl phosphate synthetase I

  • found in high concentrations in liver & other tissues

glutamate dehydrogenase

uses ammonia and a-ketoglutarate to make glutamate

• reverse of deamination reaction

• abundant in liver

• scavenger for excess ammonia

• improves ability of mitochondrial glutamate for NAG synthesis

• allows formation of aspartate via AST

  • fixes ammonia

carbamoyl phosphate synthetase I

ammonia into carbamoyl phosphate

• citrulline production in urea cycle & for intestines

  • intestines funnels to kidneys

• expressed in liver & small intestine

urea cycle

• production of urea (major N-containing component in urine)

• occurs in liver

• critical pathway for removal of ammonia from the body

• TCA cycle intermediates can enter urea cycle

urea

• first N from ammonia

• second N from aspartate

• carbon # comes from CO2

describe the urea cycle

• carbamoyl phosphate synthetase forms a complex called carbamoyl phosphate

• citrulline leaves mitochondria

• urea travels to kidneys and intestines

what is the connection between TCA cycle & urea cycle

• TCA provides carbons

• OAA that can make aspartate

• CO2 from TCA material for urea

• urea provides fumarate which can feed back into TCA to make OAA

  • alanine can also feed

• TCA & urea have feedback loop

what happens to urea

• travels in blood to kidneys for excretion into urine

• 25% may be secreted into intestinal lumen & degraded by bacteria in intestine to yield ammonia

why is ammonia dangerous

free ammonia can diffuse across cells & into bloodstream without transporters

• can cause eventual diffusion into brain

how is the urea cycle regulated

• substrate availability (supply and demand/ammonia levels)

• allosteric activation (NAG acts as an allosteric site on carbamoyl phosphate synthetase I)

rate-limiting step of urea cycle

carbamoyl phosphate synthetase I

when is the urea cycle upregulated

• fed state because you are taking in more AAs

• starvation stated due to increased protein breakdown

hormones effecting urea cycle

• glucocorticoids (cortisol) released during illness or infection; driver of protein breakdown

• glucagon

  • both promote AA degradation and increase urea cycle enzymes

diet effecting urea cycle

high/low protein diets

• increase or decrease urea cycle enzymes

hyperammonemia

urea cycle defects resulting in high blood ammonia

ornithine transcarbamoylase

• most common

• ammonia can diffuse across blood-brain barrier

• brain synthesizes glutamine to compensate

• can result in brain swelling or encephalopathy

treatment for urea cycle disorders

• low protein diet

• drugs that acidify GI tract and promote diffusion of ammonia out of blood into GI tract

• antibiotics

others at risk of urea cycle deficiency

hepatic encephalopathy

• cirrhosis

• liver disease

• liver cancer

positive protein balance

taking more protein in than we are breaking

• athletes, children, pregnant women

negative protein balance

excreting more protein than taking in

• illness, injury, fasting/starvation, diabetes/obesity, and cancer

  • diabetes results in increased protein breakdown due to inability to release insulin

  • chemotherapy drugs can induce negative protein balance

describe the metabolism of carbon skeletons

*main goal of protein to build mass and replace what we are losing

carbons skeletons are used for:

• energy

• glucose (alanine)

• ketone bodies

• cholesterol

• fatty acids

  • about 10-15% of AAs oxidized for energy

describe glucose production from proteins

gluconeogenesis

• liver & kidney

• glucogenic AAs

  • must yield pyruvate or TCA cycle intermediates

describe ketone production from proteins

*must generate acetyl CoA or acetoacetate

• some AAs both gluconeogenic and ketogenic

describe cholesterol & FA synthesis from proteins

*must generate acetyl CoA

• leucine is the only AA that generates HMG CoA (intermediate in cholesterol synthesis)

• acetyl CoA can be used to make FAs

describe intestinal amino acid metabolism

• first cells to receive AA in fed state

• uses 30-40% of essential AAs from diet

  • energy production (50% from AAs)
    protein synthesis (happens in every tissue)

  • synthesis of N-containing compounds (glutathione & carnosine)

  • AA metabolism

• some will go to portal vein & liver

• increases in systemic circulation

• all tissues in the fed state will do protein synthesis

AA used by the intestine from the diet

• glutamine

• glutamate

• aspartate

• arginine

AA released by intestine

• alanine

• proline

  ** used for the liver

• portal blood will also be enriched with citrulline

glutamine

most abundant in circulation

• serves as vehicle for ammonia transport via action of glutamine synthetase

  * cells of GI tract & immune cells rely on energy production and health

• used by cell with hypercatabolic conditions

• sepsis (infection)

• trauma (burns)

• surgery

• starvation

glutamine

glutamate from aminotransferases is used to make ___________

carbamoyl phosphate synthetase I

• found in intestines

• creates citrulline which eventually flows to kidneys

ornithine production

• some released into portal blood

• used with carbamoyl phosphate to make citrulline

  *citrulline needed by kidney to make arginine

  • loss of intestinal function can decrease citrulline production; makes arginine a conditionally essential AA

• liver uses some ornithine to make citrulline

skeletal muscle & amino acids

• major site of protein synthesis & breakdown

• efflux of AAs from SM supports AA pool in the blood

• rich in BCAAs

• alanine & glutamine: 50% of all AAs released from SM

  • muscle stores turn over every 3 months; break down and go to pool

skeletal muscle in the fasting state

• we need alanine for gluconeogenesis

• AA metabolism & releases a lot

glutamine metabolism in the SM

• uses ammonia to form glutamine (glutamine synthetase)

• glutamine released in blood

  • ammonia formed

  • protein metabolism - glutamate formation

  • AMP deaminase - triggered by muscle contraction

alanine metabolism in the SM

conditions:

• fasting: need for gluconeogenesis

• illness: increased glucose need

generate alanine from BCAA metabolism (glutamate to pyruvate to form a-keto acid & alanine)

• alanine travels to liver to make glucose via alanine glucose cycle

leucine in the SM

required to drive protein synthesis in the SM

• need certain amounts at each meal to drive

• most abundant BCAA in SM

  *found in both plants and animals

nitrogen loss

due to breakdown of muscle protein & synthesis of glucose through hepatic gluconeogenesis