What is an enzyme?
A biological catalyst that increases reaction rate without being consumed.
What is the substrate in enzyme catalysis?
The substance acted on by an enzyme.
Where does the substrate bind on an enzyme?
At the enzyme’s active site.
What does the lock-and-key model state about the active site?
It is already complementary to the substrate.
What happens in the induced-fit model when substrate binds?
The enzyme’s conformation changes.
How many enzyme classes are there?
Six enzyme classes.
Name three of the six enzyme classes.
Oxidoreductases, transferases, and hydrolases.
Name the other three enzyme classes besides oxidoreductases, transferases, and hydrolases.
Lyases, isomerases, and ligases.
What characterizes competitive inhibition in enzymes?
An inhibitor competes for the active site and can be overcome by more substrate.
How does noncompetitive inhibition affect enzyme activity?
It involves another site and reduces catalytic activity.
What is a holoenzyme?
An active enzyme made of an apoenzyme plus its cofactor or coenzyme.
What is the optimum temperature for most human enzymes?
Around 37°C.
What happens to enzymes at excessive temperatures?
They become denatured.
What is glycolysis?
A cytosolic pathway of 10 reactions converting glucose to pyruvate.
What is the net ATP yield of glycolysis per glucose?
2 ATP molecules are produced per glucose.
What is the net NADH yield of glycolysis per glucose?
2 NADH molecules are produced per glucose.
Which enzyme is the major regulatory and rate-limiting step in glycolysis?
Phosphofructokinase-1 (PFK-1).
How does ATP affect PFK-1 activity in glycolysis?
ATP inhibits PFK-1 activity.
How does ADP affect PFK-1 activity in glycolysis?
ADP stimulates PFK-1 activity.
What is the final product of the ten glycolytic reactions?
Pyruvate is the final product.
Why do red blood cells rely on anaerobic glycolysis?
Because they lack mitochondria and produce lactate from pyruvate.
What is gluconeogenesis?
Formation of glucose from non-carbohydrate precursors.
Which three bypasses occur in gluconeogenesis?
Pyruvate to PEP, fructose-1,6-bisphosphate to fructose-6-phosphate, glucose-6-phosphate to glucose.
What cofactor does pyruvate carboxylase require?
Biotin.
Where does pyruvate carboxylase act and what activates it?
In mitochondria and activated by acetyl-CoA.
What energy molecules does pyruvate carboxylase use and what does PEP carboxykinase use?
Pyruvate carboxylase uses ATP; PEP carboxykinase uses GTP.
What is the energy cost formula for producing one glucose in gluconeogenesis?
2 pyruvate + 4 ATP + 2 GTP + 2 NADH → glucose.
How many high-energy phosphate bonds are required to produce one glucose?
Six high-energy phosphate bonds.
How does fructose-2,6-bisphosphate affect glycolysis and gluconeogenesis?
Increased stimulates glycolysis; decreased stimulates gluconeogenesis.
What is glycogen in animals?
The highly branched storage form of glucose.
Which bonds form glycogen's linear chains and branches?
α(1→4) bonds in chains and α(1→6) bonds at branches.
What enzymes convert G6P to glycogen in glycogenesis?
Phosphoglucomutase, UDP-glucose pyrophosphorylase, and glycogen synthase.
What is the role of liver glycogen?
To maintain blood glucose and release glucose.
Why can't muscle glycogen supply glucose to the blood?
Muscle lacks glucose-6-phosphatase.
What causes Von Gierke disease?
Glucose-6-phosphatase deficiency.
What causes McArdle disease?
Muscle glycogen phosphorylase deficiency.
Where does the TCA cycle occur in the cell?
In the mitochondrial matrix.
What types of molecules does the TCA cycle oxidize?
Carbohydrates, fats, and amino acids.
Name one intermediate regenerated in the TCA cycle.
Oxaloacetate.
Which enzyme in the TCA cycle produces FADH₂?
Succinate dehydrogenase.
What energy molecule is produced from succinyl-CoA to succinate conversion?
GTP.
What does the pyruvate dehydrogenase complex irreversibly convert pyruvate into?
Acetyl-CoA, CO₂, and NADH.
Which five cofactors are required by the pyruvate dehydrogenase complex?
TPP, lipoic acid, CoA, FAD, and NAD⁺.
What activates PDH kinase to inhibit the pyruvate dehydrogenase complex?
ATP, NADH, and acetyl-CoA.
What effect does PDH kinase activation have on the pyruvate dehydrogenase complex?
It causes PDH phosphorylation and inhibition.
Which vitamin deficiency impairs PDH and α-ketoglutarate dehydrogenase?
Vitamin B1 deficiency.
What diseases are caused by vitamin B1 deficiency affecting PDH?
Beriberi and Wernicke-Korsakoff syndrome.
What does the pentose phosphate pathway produce in the cytosol?
NADPH and ribose-5-phosphate.
What is the nature of the oxidative phase in the pentose phosphate pathway?
It is irreversible.
How many NADPH molecules are produced per glucose-6-phosphate in the oxidative phase?
Two NADPH molecules.
Name one function supported by NADPH.
Fatty-acid synthesis.
What effect does G6PD deficiency have on NADPH levels?
It lowers NADPH levels.
What condition results from lowered reduced glutathione due to G6PD deficiency?
Oxidative damage and hemolysis.
Which triggers can worsen hemolysis in G6PD deficiency?
Oxidant drugs, severe infection, or fava beans.
What are the four steps in each β-oxidation cycle?
Dehydrogenation producing FADH₂, hydration, second oxidation producing NADH, and thiolysis producing acetyl-CoA.
How do long-chain fatty acids enter the mitochondrial matrix?
Through the carnitine shuttle.
What molecule inhibits CPT-I in fatty acid oxidation?
Malonyl-CoA.
What does each β-oxidation cycle produce?
One acetyl-CoA, one NADH, and one FADH₂.
How many acetyl-CoA molecules does palmitate yield after β-oxidation?
Eight acetyl-CoA molecules.
What is the net ATP yield from palmitate after activation cost?
129 ATP.
What condition does MCAD deficiency cause related to fatty acid oxidation?
Impaired medium-chain fatty-acid oxidation.
Why should fasting be avoided in MCAD deficiency?
Because it causes hypoglycemia and hypoketonemia.
What are the three ketone bodies?
Acetoacetate, β-hydroxybutyrate, and acetone.
When does ketogenesis occur in the liver?
During fasting, prolonged exercise, or low-carbohydrate conditions.
What causes ketogenesis in these conditions?
Fatty-acid oxidation raises acetyl-CoA and oxaloacetate is diverted to gluconeogenesis.
Which enzyme is the rate-limiting step of ketogenesis?
Mitochondrial HMG-CoA synthase.
Why can't the liver use ketone bodies?
Because it lacks thiophorase.
Why can't red blood cells use ketone bodies?
Because they lack mitochondria.
What are the effects of increased ketone bodies in uncontrolled type 1 diabetes?
Ketonemia, ketonuria, and metabolic acidosis.
What causes fruity breath in uncontrolled type 1 diabetes?
Acetone production.
What does the urea cycle convert toxic ammonia into?
Urea
Where does the urea cycle mainly occur in the body?
In the liver
From which molecules do the two urea nitrogens originate?
One from ammonia and one from aspartate
What is the carbon source for urea in the urea cycle?
CO₂ or HCO₃⁻
Name the five enzymes of the urea cycle.
CPS-I, ornithine transcarbamoylase, argininosuccinate synthetase, argininosuccinate lyase, and arginase
Where do the first two urea-cycle reactions occur?
In the mitochondrial matrix
Where do the last urea-cycle reactions occur?
In the cytosol
Which urea-cycle enzyme uses 2 ATP and is activated by N-acetylglutamate?
CPS-I
Which vitamins are fat-soluble and stored in liver or adipose tissue?
Vitamins A, D, E, and K are fat-soluble and stored in liver or adipose tissue.
Which vitamins are water-soluble?
The B-complex vitamins and vitamin C are water-soluble.
What early deficiency sign indicates lack of vitamin A?
Night blindness is an early sign of vitamin A deficiency.
How is vitamin D activated in the body?
Vitamin D is activated from skin D3 to liver 25-hydroxy-D and then kidney 1,25-dihydroxy-D.
What is vitamin K required for in blood clotting?
Vitamin K is required for carboxylation of clotting factors.
What deficiency symptoms result from lack of vitamin K?
Vitamin K deficiency causes bleeding and hypoprothrombinemia.
What are the active forms of vitamins B1, B2, and B3?
TPP is B1, FMN/FAD are B2, and NAD⁺/NADP⁺ are B3 active forms.
What roles does vitamin C play in the body?
Vitamin C is required for collagen hydroxylation, wound healing, and increased iron absorption.
Test your knowledge with 50 questions on Biochemistry Master Exam Review.
1. What distinguishes an enzyme from a substrate during a chemical reaction?
2. Where does the substrate bind on an enzyme?
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