Biochemistry- Metabolism Flashcards
What metabolic processes occur in the mitochondria?
fatty acid oxidation (B-oxidation)
acetyl-CoA production
TCA cycle
oxidative phosphorylation
ketogenesis
What metabolic processes occur in the cytoplasm?
glycolysis
fatty acid synthesis
HMP shunt
protein synthesis (RER)
steroid synthesis (SER)
cholesterol synthesis
What metabolic processes occur both in the cytoplasm and mitochondria?
heme synthesis
urea cycle
gluconeogenesis
What does a kinase do?
uses ATP to add a high energy phosphate onto a substrate (e.g. phosphofructokinase)
What does a phosphorylase do?
adds inorganic phosphate onto substrate without using ATP
What does a phosphatase do?
removes phosphate groups
What does a dehydrogenase do?
catalyzes redox rxns
What does a hydroxylase do?
adds hydroxyl group (-OH) onto substrates
What does a carboxylase do?
transfers Co2 groups with the help of biotin
What does a mutase do?
relocates a functional group within a molecule
What is the rate determining step of glycolysis?
phosphofructokinase-1 (PFK-1)
Stimulate: AMP, fructose-2,6-bisphosphate
Inhibit: ATP, citrate
What is the rate determining step of gluconeogenesis?
fructose-1,6-bisphosphatase
Stimulates: ATP, acetyl-CoA
Inhibits: AMP, fructose-2,6-bisphosphate
What is the rate determining step of the TCA cycle?
isocitrate dehydrogenase
Stimulates: ADP
Inhibits: ATP, NADH
What is the rate determining step of glycogenesis?
glycogen synthease
+: glucose-6-phosphate, insulin, cortisol
-: epi, glucagon
What is the rate determining step of glycogenolysis?
glycogen phosphorylase
+: epi, glucagon, AMP
-: glucose-6-phosphate, insulin, ATP
What is the rate determining step of HMP shunt?
Glucose-6-phosphate dehydrogenase (G6PD)
+: NADP+
-: NADPH
What is the rate determining step of de novo pyrimidine synthesis?
carbamoyl phosphate synthetase II
+: ATP
-: UTP
What is the rate determining step of de novo purine synthesis?
glutamine-phosphoribosylphosphate (PRPP) amidotransferase
+: none
-: AMP, IMP, GMP
What is the rate determining step of the urea cycle?
carbamoyl phosphate synthetase I
+: N-acetylglutamate
What is the rate determining step of fatty acid synthesis?
acetyl-CoA carboxylase (ACC)
+: Insulin, citrate
-:glucagon, palmitoyl-CoA
What is the rate determining step of fatty acid oxidation?
carnitine acyltransferase I
-: Malonyl CoA
What is the rate determining step of ketogenesis?
HMG-CoA synthase
What is the rate determining step of cholesterol synthesis?
HMG-CoA reductase
+: insulin, thyroxine
-: glucagon, cholesterol
Describe ATP production
Aerobic metabolism of glucose produces 32 net ATp via malate-aspartate shuttle (heart and liver), 30 net ATP via glycerol-3-phosphate shuttle (muscle)
Anaerobic gycolysis produces only 2 net ATP per glucose molecule (arsensic causes glycolysis to produce 0 zero ATP)
What are some universal electron acceptors?
Nicotinamides (NAD+ from vitB3 and NADP+) and flavin nucleotides (FAD+ from vitB2)
NAD+ is generally used in catabolic processes to carry reducing equivalents away as NADH and NADPH is used in anabolic processes (steroid and fatty acid synthesis) as a supply of reducing equivalents
What are the functions of hexokinase and glycokinase
both are involved in phosphorylation of glucose to yield glucose-6-phosphate (1st step in glycolysis)
At low glucose conc, hexokinase sequesters glucose in the tissue and at high glucose conc, excess glucose is stored in the liver
Describe hexokinase
Location: most tissues, except liver and pancreatic B cells
Km: lower (increased affinity)
Vmax: lower (decreased capacity)
Induced by insulin: No
Feedback inhibited by G6P: Yes
Gene mutation associated with maturity onset of the young (MODY): No
Describe glucokinase
Location: liver, pancreatic B cells
Km: higher (decreased affinity)
Vmax: higher (increased capacity)
Induced by insulin: Yes
Feedback inhibited by G6P: No
Gene mutation associated with maturity onset of the young (MODY): Yes
net glucolysis eqn
Glucose + 2Pi + 2ADP + 2NAD+ -> 2 pyruvate + 2ATP + 2NADH + 2H+ + 2H2O
Eqn not balanced chemically, and exact balanced eqn depends on ionization state of reactants and products
What enzymes in glycolysis require ATP?
hexokinase/glucose (glucose to G6P) (glucose-6-P inhibits hexokinase and fructose-6-P inhibits glucokinase)
PFK-1 (+AMP, fructose-2,6-bisphosphate; -ATP, citrate)
What enzymes in glycolysis produce ATP?
conversion of 1,3-BPG to 3-PG using phosphoglycerate kinase and
conversion of PEP to pyruvate via pyruvate kinase (fructose-1,6-bisphosphate+; -ATP, alanine)
While PFK-2 catalyzes conversion of fructose-6-P to fructose-1,6-BP in glycolysis, what catalyzes the reverse rxn to promote gluconeogenesis?
FBPase-2 (fructose bisphosphatase-2)
FBPase-2 and PFK-2 are essentially the same enzyme whose function is reversed by what?
phosphorylation by protein kinase A (= more FBPase-2)
What does fasting promote in relation to glucose?
fasting increases glucagon which increases cAMP which increases PKA which promotes increased FBPase-2 and less PFK-2, leading to gluconeogenesis
What does the fed state promote in relation to glucose?
increased insulin leading to decreased cAMP= less PKA= less FBPase-2, more PFK-2= glycolysis
Describe the pyruvate dehydrogenase complex
Mitochondrial enzyme complex linking glycolysis and TCA cycle. Differentially regulated in fed/fasting states (active in fed state)
Rxn: pyruvate + NAD+ + CoA -> acetyl-CoA + CO2 + NADH
What cofactors are needed for the pyruvate dehydrogenase complex?
pyrophosphate (B1, thiamine, TPP)
FAD (B2, riboflavin)
NAD (B3, niacin)
CoA (B5, pantothenic acid)
Lipoic acid
Activated by exercise, which promotes increased NAD+/NADH ratio, increased ADP, and increased Ca2+
What inhibits lipoic acid and how does that present clinically?
Arsenic, leading to vomiting, rice-water stools, garlic breath
What does pyruvate dehydrogenase complex deficiency result in?
causes a buildup of pyruvate that gets shunted to lactate (via LDH) and alanine (via ALT)
X-linked disease
How does pyruvate dehydrogenase complex deficiency present?
neurologic defects, lactic acidosis, increased serum alanine starting in infancy
How is pyruvate dehydrogenase complex deficiency tx?
increase intake of ketogenic nutrients (e.g. high fat content or more lysine and leucine)
Lysine and Leucine are the only purely ketogenic AAs
What 4 things can primarily happen to pyruvate?
What is the purpose of the Cahill cycle? What cofactor is needed for ALT?
Alanine carries amino groups from muscle to the liver
needs B6
What cofactor does pyruvate carboxylase use?
biotin
What cofactors does pyruvate dehydrogenase use?
B1, B2, B3, B5, and lipoic acid
What cofactor does lactic acid dehydrogenase (LDH) use?
B3 (note that conversion of pyruvate to lactic acid is the end of the anaerobic glycolysis (the major pathway in RBCs, WBCs, kidney medulla, lens, cornea, and testes)
What are the products of the TCA cycle?
3NADH, 2Co2, 1GTP, 10ATP, 1FADH2 per Acetyl-CoA (2x per glucose molecule)
Occurs in the mitochondria
1st step in TCA cycle
conversion of pyruvate (3C) to acetyl-CoA via PDH (-ATP, Acetyl-CoA, NADH)
2nd step in TCA cycle
conversion of oxaloacetate (4C) to citrate (6C) by adding acetyl CoA (2C) and via citrate synthase
3rd step in TCA cycle
conversion of citrate (6C) to cis-aconitate to isocitrate (6C)
4th step in TCA cycle
conversion of isocitrate to a-KG (5C) via isocitrate dehydrogenase (giving off Co2+ NADH) (-ATP, NADH; +ADP)
5th step in TCA cycle
conversion of a-KG (5C) to succinyl-CoA (4C) via a-KG dehydrogenase (giving off NADH +CO2) (-succinyl CoA, NADH, ATP)
6th step in TCA cycle
conversion of succinyl CoA (4C) to succinate (4C) giving off GT + CoA
7th step in TCA cycle
conversion of succinate to fumarate giving of FADH2
8th step in TCA cycle
conversion of fumarate (4C) to malate (4C)
9th step in TCA cycle
conversion of malate (4C) to oxaloacetate (4C) giving off NADH
Describe the electron transport chain and oxidative phosphorylation
NADH electrons from glycolysis enter mitochondria via the malate-aspartate of glycerol-3-phosphate shuttle. FADH2 electrons are transferred to complex II (at a lower energy level than NADH).
The passage of electrons results in the formation of a proton gradient that, coupled to oxidative phsophorylation, drives the production of ATP
How many ATP are produced per NADH? per FADH2?
NADH: 2.5ATP
FADH2: 1.5ATP
What drug inhibits complex I of the ETC? complex III? IV?
I: Roterone
III: Antimycin
IV: Cyanide, CO
Oxidative phosphorylation poisons: Electron transport inhibitors
directly inhibit electron transport, causing a decreased proton gradient and block of ATP synthesis
Rotenone, cyanide, antimycin A, CO
Oxidative phosphorylation poisons: ATP synthase inhibitors
directly inibit mitochondrial ATP synthase, causing an increased proton gradient.
Oligomycin
Oxidative phosphorylation poisons: Uncoupling agents
Increase permeability of membrane, causing a decreased proton gradient and increased O2 consumption. ATP synthesis stops, but electron transport continues. Produces heat
2,4-Dinitrophenol (used for weight loss), aspirin (fever often occur after aspirin OD), thermogenin in brown fat
What are the irreversible enzymes of gluconeogenesis?
Pyruvate carboxylase
Phosphoenolpyruvate carboxykinase
Fructose-1,6-bisphosphatase
Glucose-6-phosphatase
What does Pyruvate carboxylase do?
in mitochondria, converts pyruvate to oxaloacetate
requires biotin, ATP
Activated by acetyl-CoA
What does PEP carboxylkinase do?
In cytosol. converts oxaloacetate to PEP (requires GTP)
What does fructose-1,6- bisphosphatase do?
in cytosol. converts fructose-1,6-bisphosphate to fructose-6-phosphate
+citrate; -fructose-2,6-bisphosphate
What does glucose-6-phosphatase do?
In ER. converts G6P to glucose
Where does gluconeogenesis occur?
primarily in the liver and serves to maintain euglycemia during fasting
Enzymes are also found in the kidney and intestinal epithelium (muscle cannot participate in gluconeogenesis because it lacks G6P)
More about gluconeogenesis
Odd-chain fatty acids yield 1 propionyl-CoA during metabolism which can enter the TCA as succinyl CoA, and serve as a glucose source
Even-chain fatty acids cannot produce new glucose, since they yield only acetyl-CoA equivalents
What is the HMP shunt (pentose phosphate pathway)?
Provides a source of NADPH from abundantly available glucose-6-P (NADPH is required for reductive rxns, e.g. glutathione reduction inside RBCs, fatty acid and cholesterol biosynthesis)
Additionally, this pathway yields ribose for nucleotide sytnhesis and glycolytic intermediates. 2 distinct phases (oxidative and nonoxidative), both of which occur in the cytoplasm. No ATP is need or produced.
What are some body sites where the HMP shunt (pentose phosphate pathway) occurs?
lactating mammary glands, liver, adrenal cortex, (sites of fatty acid or steroid synthesis)
Describe the oxidative (irreversible) part of the HMP shunt (pentose phosphate pathway)
G6P to Co2, 2NADPH, and ribulose-5-P via glucose-6-P dehydrogenase
Describe the nonoxidative (reversible) part of the HMP shunt (pentose phosphate pathway)
ribulose-5-P to ribose-5-P, glucose-3-P, and fructose-6-P via phosphopentose isomerase transketolases (requires B1)
What does G6PD deficiency result in?
NADPH is needed to keep glutathiona reduced, which in turn detoxifies free radicals and peroxides. Decreased levels of NADPH in RBCs leads to hemolytic anemia due to poor RBC defense against oxidizing agents, (e.g. fava beans, sulonamides, primaquine, antiTB drugs)
Infection can also precipitate hemolysis (free radicals generated via inflammatory response can diffuse into RBCS and cause oxidative damage)
Describe the glutathione pathway
G6PD conevrts G6P to 6-phosphogluconate as NADP+ is reduced to NADPH
glutathione reductase then reduces GSSG to 2GSH as NADPH is oxidized back to NADP+
Glutathiona peroxidase then reduces H2O2 to 2H2O as GSH is oxidized back to GSSG
What are the main findings of G6PD deficiency
x-linked recessive disorder; most common enzyme deficiency
offers increased malarial resistance
Heinz bodies (denatured hemoglobin precipitates within RBCs due to oxidative stress) (below)
Bite cells- result from the phagocytic removal of Heinz bodies by splenic macrophages
What causes essential fructosuria?
involves a defect in fructokinase. AR
described as a bening, asymptomatic condition, since fructose is not trapped in cells
How does essential fructosuria present?
fructose appears in blood and urine
What causes fructose intolerance?
AR inherited deficiency of aldolase B
What happens with aldolase B deficiency in fructose intolerance?
Fluctose-1-P accumulates, causing a decrease in available phosphate, which results in inhibition of glycogenolysis and gluconeogenesis. Symptoms then present following consumption of fruit, honey, or juice and consist of hypoglycemia, jaundice, cirrhosis, and vomiting
How is fructose intolerance diagnosis and tx?
diagnosis: urine dipstick will be negatie
reducing sugar can be detected in the urine (nonspecific test for inborn errors of carb metabolism)
tx: decrease intake of both fructose and sucrose (glucose + fructose)
Describe fructose metabolism
Fructose is converted to fructose-1-P via fructokinase (producing ADP from ATP)
Fructose-1-P is converted to glyceraldehyde and dihydroxyacetone-P via aldolase B
What happens to glyceraldehyde during fructose metabolism?
can either:
be converted to glycerol (NADH to NAD+)
or be converted to glyceraldehyde-3-P via triose kinase (ATP to ADP) (simialrly, dihdyroxyacetone-P is converted directly to glyceraldehyde-3-P). this end product then enters glycolysis
What would hereditary deficiency in galactokinase cause?
galactokinase deficiency, marking accumulating galactitol when galactose is present in the diet
relatively mild condition (AR)
How does galactokinase deficiency present?
galactose appears in blood and urine, infantile cataracts
may present as failure to track objects or to develop a social smile
What does absence of galactose-1-phosphate uridyltransferase cause?
Classic galactosemia
Describe Classic galactosemia
AR syndrome marked by damage induced by accumulation of toxic substances (including glactitol, which accumulates in the lens of the eyes), resulting in symptoms of:
failure to thrive, jaundice, hepatomegaly, infantile cataratcts, intellectual disability, and increased risk of E. coli sepsis i neonates
How is Classic galactosemia tx?
exclude galactose and lactose (galactose + glucose) from diet
Describe galactose metabolism
galactose can be converted to either galactitol via aldose reductase or galactose-1-P via galactokinase (ATP to ADP)
Galactose-1-P is converted to glucose-1-P via uridyltransferase as UDP-Glu is converted to UDP-Gal via 4-epimerase
Describe the sorbitol pathway
glucose converted to sorbitol via aldose reductase (NADPH) and then to fructose via sorbitol dehydrogenase (NAD+)
Note that the schwann cells, retine, and kidneys have only aldose reductase and similar for the lens
What is the purpose of making sorbitol?
It is an alternative method of trapping glucose in the cell by converting it to its alcohol counterpart, sorbitol, via aldose reductase. Some tissues then convert sorbitol to fructose using sorbitol dehydrogenase and tissues lacking this enzyme are at risk of accumulating sorbitol, which causes osmotic damage (e.g. cataracts, retinopathy, and peripheral neuropathy seen with chronic hyperglycemia in diabetes)
What is the result of lactase deficiency?
insufficient lactase leads to dietary lactose intolerance (note that lactase is a brush-border enzyme used to break down lactase to glucose and galactose)
Primary, secondary, and congenital lactase deficiency
Primary: age-dependent decline after childhood (due to absence of lactase-persistent allele), common in Asians, Africans and Native Americans
Secondary: loss of brush border due to gastroenteritis (e.g. rotavirus), autoimmune disease, etc.
Congenital: rare, due to defective gene
T or F. Only L-amino acids are found in protein
T.
What are the essential glucogenic AAs?
methionine (met), valine (Val), and histidine (His)
What are the essential glucogenic/ketogenic AAs?
isoleucine (IlE), phenylalanine (Phe), thronine (Thr), and trptophan (Trp)
What are the essential ketogenic AAs?
leucine (Leu), lysine (Lys)
What are the acidic AAs?
aspartic acid (Asp) and glutamic acid (Glu). Negatively charged at body pH
What are the basic AAs?
arginne (Arg; most basic), lysine (Lys), and histidine (His)
His no charge at body pH
Note: Arg and His are required during growth periods and Arg and Lys are in histones
