Biochemistry Exam #3 Concepts And Pathways

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Dietary carbohydrate is...

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a source of energy, approximately 50% of our diet comes from carbs

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Various activities have..

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different metabolic energy requirements:

i.e. basal: 1400 kcal/day
trauma: 2700 kcal/day
major op: 4300 kcal/day
major burn: 9300 kcal/day

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The value for a dietary carobhydrate is

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4 Cal/gram

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Dietary carbohydrates

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starches
disaccharides
sucrose
maltose
lactose
monosaccharides: fructose and glucose

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functions of alpha amylase are:

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acting on glucose alpha 1-4 linkages, origins of salivary amylase are in the salivary glands and in the pancreas

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maltase

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intestinal mucosa, maltose alpha glucose 1-4 linkage

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isomaltase

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intestinal mucosa, glucose alpha glucose (1-6 linkages) in isomaltose and branched dextrins

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sucrase

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intestinal mucosa, glucose alpha 1-2 b fructose in sucrose

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lactase

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intestinal mucosa, gal B-1,4 linkage in lactose

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most of digestion of starch is carried out by...

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pancreatic amylase

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end products of digestion of starch by alpha amylase are

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maltose, maltotriose, and small branched oligosaccharides (limit dextrans)

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isomaltase sucrase

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is a complex consisting of two distinct polypeptide subunits, isomaltase and sucrase

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lactase insuffiency is..

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common among human populations, marked abdominal discomfort=lactose intolerance

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defiency in isomaltase sucrase leads to

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simultaneous loss of both activites of the enzymes, because polypeptides are originally synthesized as a single long precursor protein

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mammals lack the ability to digest...

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cellulose and other plant polysaccharides, which is known as dietary fiber, also include lignins: non-carbohydrate plant materials

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absorption of the monosaccharides occurs in the

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small intestine by means of simple diffusion: xylose and arabionse
facilated transport: fructose and mannose
active transport: glucose and galactose (Na+ glucose symport), note that it follows Michaelis Menten Kinetics, at the same time you also have an Na+/K+ ATPase

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defect in sodium glucose transporter leads to what??

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leads to fermentation by bacteria, evolving gas and metabolites, dehydration can be serious in new borns,

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how are sugars carried when they enter the bloodstream?

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-pass out of mucosal cells and into bloodstream via a membrane carrier in the basolateral part

-capillaries of portal circulation and travel directly to the liver

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livers role in regulation of glucose

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1). absorption of glucose from the portal system
2). Regulated release of glucose (from stored glycogen or new synthesis)

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a significant portion of glucose goes to the...

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general circulation, and is absorbed by cells throughout the body

muscle, fat, and WBC's: glucose absorption is stimulated by insulin

liver, erthryocytes, and brain cells: not stimulated by insulin!

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GLUT1

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Erythrocytes, Brian, Fetal Tissue, Placenta (not sodium linked nor responsive to insulin)

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GLUT2

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Liver, Pancreas B cells (not sodium linked or responsive to insulin

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GLUT3

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Brain, kidney and many other tissues

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GLUT5

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Brain, kidney and many other tissues

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GLUT4

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Skeletal and cardiac muscle, Fat cells, White Blood Cells (faciliattative, not sodium linked, insulin responsive)

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SLGLUT1

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Intestinal epithelium, mucosal surface, kidney epithelium

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Blood glucose concentration rises shortly after what....

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meals, but then physiological controls bring it back to a regulated basal level

mean value: 4.5 to 5.5 mM (80 to 90 mg/ 100 mL of plasma)

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Ingestion of a meal rich in carbs and rise in concentration of glucose is

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1). absorption into blood
2). Removal from blood by all tissues
3). conversion of glucose to lactate, glycogen, and fat in the liver
4)> conversion of non-glucose substrates to glucose in the liver
5). Return of glucose to blood from liver

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diabetes

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blood glucose regulation is altered, basal level of glucose is higher than normal

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glycemic index

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area underneath curve of post meal rise and fall in blood glucose concentration

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what happens to glucose when it is absorbed

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it gets phosphorylated, trapping the glucose inside of the cells

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hexokinase

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reaction it catalyzes is virtually irreversible in-vivo, it requires magnesium (Mg2++ ATP)

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enzymes that phosphorylate glucose are examples of

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isozymes: multiple enzymes that catalyze the same reaction, even though they differ in amino acid sequences

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hexokinases (I,II, III)

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distribution: all tissues, regulated by product (G-6-P)
Km<2.0 mM

I: predominant in brain
II: predominant in muscle

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glucokinase

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-distribution: liver, pancreatic B islet cells

10-20 mM

-regulation: substrate concentration and amount of enzyme

i). increasing glucose uptake liver
ii). acting as a part of glucose sensor system in B cells of pancreas

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counteracting regulation occurs through

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action of alpha cells and its secretion of glucagon

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glucokinase dynamics

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has a Km greater than the physiological blood glucose concentration. As blood glucose level rises, the glucokinase reaction is accelerated (shows cooperativity)

Shows greatest sensitivity to substrate concentration near normal blood glucose concentrations

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Glucokinase activity is regulated by

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Variations in the amount of enzyme as well as by substrate concentration

liver: insulin and glucagon stimulate gene transcription of glucokinase and inhibit it respectively

pancreatic b cells: glucose regulates enzyme concentration post transcriptionally

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Glucokinase gene and MODY

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Mature onset diabetes of the Young (MODY): mutations in gene for glucokinase, and mutations in other sensor response system factors

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Fructose is phosphorylated by

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Hexokinase: Fructose-6 phosphate
Fructokinase: Fructose 1-phosphate

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mannose is phosphorylated by

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hexokinase: mannose 6 phosphate

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galactose is phosphorylated by

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galactokinase: galactose 1 phosphate

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glycolysis

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underanearobic or aerobic pathway, glucose 6-P enters the Embden Meyerhof pathway, universal biological process

Anerobic glycolysis overall reaction:

Glucose+2ADP+ 2Pi-->2 Lactate + 2 ATP+2 H20

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erhtryocytes and skeletal muscle are especially by...

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deficiences of glycolytic enzymes, most frequent is pyruvate kinase deficiency

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skeletal muscle

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partial deficiency of a glycolytic enzyme may not affect the function of a muscle at rest

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glycolysis is a...

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multistep process with 2 stages: input of 2 moles of ATP to carry out 2 phosphorylations, to generate 2 pyruvates with 4 moles of ATP.

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glucose 6 phosphate is converted to fructose 6 phosphate by

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phosphohexase isomerase: readily reversible, ene-diol intermediate

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fructose 6 phosphate is phosphorylated to give

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fructose 1,6 BP, which is done by PFK-1, first comitted step uniquely to glycolysis

inhibited by excess ATP, citrate, and fatty acids

highly activated by Fructose 2,6 BP

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PFKII

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does reaction of Fructose 6 phosphate +ATP-->Fructose 2,6 BP+ADP

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PFK I and PFKII catalyzed reactions are...

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IRREVERSIBLE, only bisphosphatases will convert back to phosphate forms

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Fructose 1,6-bisphosphate is cleaved to....

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to triose phosphates: DHAP, and GA3-P, by triose phosphate isomerase, favor DHAP formation

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G-3P is converted to

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1,3 BPG by glyceraldehyde 3-phosphate dehydrogenase, electrons are accepted by NAD+ from B vitamin niacin, reduced coenzyme must be recycled to NAD+

This Enzyme is an example of negative cooperativity

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Conversion of 1,3 BPG to 3-PG yields ATP

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perfomed by phosphoglycerate kinase

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2,3 BPGH is an intermediate in the conversion of 3-PG to to 2-PG

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converted by phosphoglycerate mutase

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Dehydration of 2-PG yields a high energy phosphate bond in PEP

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caused by enolase

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Phosphate transfer from PEP to ADP forms....

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ATP and pyruvate, caused by pyruvate kinase, it's irreversible!

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Pyruvate kinase regulation

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stimulated by F-1,6 bisphosphate,

inhibited by alanine, NADH, ATP, fatty acids, and succinyl coA

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pyruvate kinase deficiency causes...

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hemolytic anemia, erthryocytes depend on glycolysis for ATP production

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reduction of pyruvate to lactate

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regenerates NAD+ allowing glycolysis to occur under anaerobic conditions

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pyridine nucleotide coenzymes

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NAD+: accept electrons
NADH: donate electrons

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G-3-P dehydrogenase demonstrates how NAD+ can be used for oxidation

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NAD+ is used to oxidize hydroxyl so that a thioester is formed.

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G-3-P dehydrogenase can be inhibited by things such as...

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organic mercury compounds, trivalent arsenic, or iodoacetic acid (completely inactivated)

pentavalent (acts differently) no net gain in ATP, but the process continues

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Pyruvate has several alternative metabolic paths

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Can be converted to lactate (anaerobic), alanine (protein synthesis), OAA, or Acetyl coA (for fatty acid synthesis)

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allosteric regulation is a major mechanism of control of glycolysis

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hexokinase: inhibitor G-6-P
PKI: Activator: F-2,6 BP, ADP, AMP
Inhibitor: ATP, citrate, fatty acids

PK: activator: F-1,6 BP
Inhibitor: ATP, NADH, Alanine, Fatty Acids, Succinyl coA

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Regulation by enzyme phosphorylation

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pyruvate kinase (inactive when phosphorylated)

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Regulation by induced enzyme synthesis

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glucokinase: inhibited by high carbohydrate diet and insulin, decreased synthesis in low carbohydrate diet, starvation

pyruvate kinase: induced by high carbohydrate diet and insulin; decreased synthesis in low carbohydrate diet, starvation

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Key features of glycolysis

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Net yield of ATP from glycolysis

A series of reactions allow unfavorable reactions to proceed

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three reactions of glycolytic pathway are irreversible

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hexokinsae
phosphofructokinase
pyruvate kinase

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intermediates of glycolysis are used in other pathways

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1. G-6P for biosynthesis of glycogen and other polysaccharides, pentoses
2. F-6-P and GA3P can be produced to pentoses
3. DHAP can be reduced to glycerol phosphate
4. Serine can be synthesized from 3-PG
5. 2,3 BPG to regulate hemoglobin can be made from 1,3, BPG
6. Pyruvate can be converted or produced from the amino acid alanine by reversible transamination
7). Pyruvate can be oxidized to acetylcoA

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toxic substances inhibit glycolysis

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1. 2-deoxyglucose
2. Pentavalent arsenate, no net gain of ATP
3. Mercury compounds and trivalent arsenicals
4. Fluoride ion inhibits enolase reaction

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tissues differ in the physiological role and properties of glycolytic path

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RBC's: lack mito's and use glysis for energy production
Skeletal muscle at rest: carries glycolysis at a much less than maximal rate than possible. Hard excercise, rates of glycolysis increases greatly.

Cardiac muscle: glycolysis doesn't proceed at a rate taht exceeds capacity for oxidative metabolism of pyruvate (LDH is inhibited as pyruvate concentration increases)

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LDH in heart muscle

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Has four H subunits,

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Metabolism of fructose in liver uses

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fructose is preent in honey, and is digested by the enzyme sucrase, fructokinase catalyzes the formation of fructose 1-phosphate from fructose and ATP

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aldolase type A and B

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aldolase type B: fructose 6-phosphate and fructose 1,6-bisphosphate
aldolase type A: shows a very high preference for the bisphosphate

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hereditary fructose intolerance reveals...

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toxicity of fructose 1-phosphate, a deficiencey in aldosalbe B leads to a marked sensitivity to to dietary fructose

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hereditary fructosuria

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doesn't result in clinical abnormalities, because it doesn't result in an accumulation of fructose 1-phosphate

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mannose

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is phosphorylated and converted to fructose 6-phosphate

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galactose metabolism

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galactose is phosphorylated to galactose 1 phosphate by galactokinase.

Cell must first make a glucose nucleotide, uridine diphosphoglucose.

Phosphoglucomutase makes Glucose 1-phosphate from G-6-P.

Need to form UDP galactose by transfer of UDP from UDP glucose to galactose 1-phosphate

Galactose in UDP galactose is epimerized to UDP glucose by UDP galactose 4 epimerase, the reaction is reversible! you can make glycolipids and proteins even when no galactose is provided in diet

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Released glucose 1-phosphate in galactose metabolism

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Was originally galactose 1-phosphate

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disorders of galactose metabolism

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deficiency of galactokinase
galactose 1 phosphate uridyltransferase
UDP-galactose 4-epimerase

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galactokinase deficiency

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not disturbance with growth or development, but have eleveated levels, some of the galactose is reduced to the sugar galactitol, cataracts??

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galactose 1 phosphate uridyltransferase deficiency

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autosomal recessive, galactose 1-phosphate cannot be utilized, and it accumulates to the tissues and gets reduced to galactitol-1-phosphate, lack of growth, diarrhea, dehydration in new born. Still can make glycoproteins and glycoproteins however by using UDP glucose formation and epimerase to make UDP galactose

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UDP galactose 4-epimerase

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leads to high amounts of blood cell galctose 1-phosphate, but no clinical problems, a limited amount of galactose has to be supplied

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alcohol absorption

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peak blood ethanol concentrations are observed approximately after 1 hr of drinking, the longer it remains in the stomach, the slower it is absorbed

Ethanol is highly water solubule and is less absorbed into fat

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alcohol metabolism

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remains in blood until excreted or metabolized, 90% of alchol is metabolized in liver. 5% is excreted in lungs, another 5% is excreted into urine.

Average person metabolize about 10 grams of alcohol per hour

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elimination of alcohol

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first step in metabolism of alcohol is the oxidation of ethanol to acetaldehyde by Alcohol dehydrogenase, MEOS, or Catalse

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fate of acetylaldehyde

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reacts with amino groups so they can interact with proteins, can lead to cross linking

aldehyde dehydrogenase leads to production of acetic acid

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acidosis

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excess acetate can lead to lactic acidosis, pyruvate +NADH leads to lactic acidosis

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gluconeogensis

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use of NADH by alcohol and aldehyde dehydrogenases generate NADH at the expense of NAD+, leads to a deficiency

demand for pyruvate also reduces its availabiility for gluconeogenesis

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malate+NAD--->oxaloacetate+NADH

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gluconeogenesis, happens and the TCA cycle is a potential source of OAA. Excess NADH again decreases gluconeogenesis.

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alcoholics

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Can have vitamin deficiency, can interfere with absorption or storage, in the case of pyridoxine, acetaldehyde causes degradation by displacing it from its carrier protein

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poisons

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moonshine and antifreeze

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TCA cycle

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Complete oxidation takes place in 3 stages

1). Oxidation of one molecule of pyruvate to CO2
2). 2 more CO2 are formed and the electrons are transferred to NAD+ and FAD
3). electrons are transported through a series of carriers and then ultimately transferred to O2 and lead to the formation of water

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pyruvate is converted to Acetyl CoA by...

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pyruvate dehydrogenase complex (3 enzyme components and 5 coenzymes are required)

pyruvate+NAD+ + CoASH---> AcetylcoA + NADH + H+ + CO2

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Thiamine pyrophosphate

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Coenzyme formed from Vitamin B1

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Beri Beri

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Deficiency of Thiamine

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Lipoate

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thiol ester linkage, another coenzyme

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CoASH

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accepts the acyl group from lipoate

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FAD

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derived from the B vitamin lipoate

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NAD+

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accepts electrons from FADH2

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activity of PDH

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inhibited by acetyl coA
and NADH and phosphorylation of E1,

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TCA cycle

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pyruvate-->acetyl coA-->citrate-->cis acitonate-->isocitrate-->alpha ketoglutarate-->succinyl coA--->succinate-->fumarate-->malate-->OAA

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acetyl coA gets converted to citrate by

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citrate synthase (inhibited by NADH and succinyl coA)

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citrate gets converted to isocitrate by action of

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aconitase... it makes a cis-aconitas

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isocitrate dehydrogenase

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converts isocitrate to alpha ketoglutarate (inhibited by NADH and ATP), stimulated by AMP and ADP

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alphaketoglutarate dehydrogenase

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converts alpha ketoglutarate to succinyl coA (similar to PDC complex)

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succinyl coA gets converted to succinate by

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succinyl coA synthetase which carries out substrate level phosphorylation

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succinate gets converted to fumarate by

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succinate dehydrogenase

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fumarate gets converted into malate by

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fumarase

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malate gets converted back to OAA by

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malate dehydrogenase

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acetyl coA

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used for fatty acid and steroid biosynthesis, formed by oxidation of all fatty acids

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citrate

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exported to cytoplasm for fatty acid biosynthesis

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alpha ketoglutarate

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reversibly interconverted with glutamate

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succinyl coA

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precursor for heme biosynthesis

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fumarate

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formed from the breakdown of tyrosine and phenylalanine in the urea cycle

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malate

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exported to the cytoplasm for glucose synthesis

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OAA

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formed by pyruvate carboxylase from pyruvate