Biochemistry: Monosaccharides & Disaccharides

Questions and Answers

What is a physiological implication of high galactose levels in the context of metabolism?

It indicates healthy liver function.

It triggers enzyme activation.

It results in low blood sugar levels.

It may lead to cataracts. (correct)

Which test is used as a confirmatory assay for galactose metabolism issues?

Lactate dehydrogenase assay

Complete blood count

Blood glucose level measurement

Galactose 1-Phosphate Uridyl Transferase (correct)

What dietary change is typically recommended for patients with galactosemia?

Increased intake of dairy products

Inclusion of fruits and vegetables

Diet high in sugar

Galactose-free diet (correct)

Which hormone inhibits protein B synthesis during pregnancy?

Progesterone Signup and view all the answers

What is lactose classified as?

Disaccharide Signup and view all the answers

What symptom could indicate the onset of liver damage related to galactose metabolism?

Mild jaundice Signup and view all the answers

Which sugar is produced from the synthesis of lactose?

Galactose Signup and view all the answers

How does high reducing sugar levels without glucose relate to galactose metabolism?

It confirms the presence of galactosemia. Signup and view all the answers

Which enzyme is responsible for lactose synthesis?

Lactose synthase Signup and view all the answers

What is the primary dietary management for galactosemia?

Removing galactose and lactose from the diet Signup and view all the answers

Which of the following is a common symptom in an infant with galactosemia?

Failure to thrive Signup and view all the answers

In the diagnosis of galactokinase deficiency, which laboratory finding is indicative?

Presence of reducing sugars in urinalysis Signup and view all the answers

What is a potential outcome of galactose accumulation in infants?

Cataracts Signup and view all the answers

Which enzyme deficiency is primarily associated with galactosemia?

Galactose 1-Phosphate Uridyl Transferase Signup and view all the answers

Which condition results from the effect of aldose reductase on galactose?

Formation of galactitol leading to osmotic damage Signup and view all the answers

What major action does therapy aim to achieve in managing galactosemia?

Rapid diagnosis and removal of dietary galactose Signup and view all the answers

What is a likely laboratory result for a patient with galactosemia?

High levels of galactose in urine and serum Signup and view all the answers

What can be a long-term risk in female patients with galactosemia?

Premature ovarian failure Signup and view all the answers

What is the primary role of UDP-glucuronate in polysaccharide biosynthesis?

It serves as a donor for the glucuronyl moiety in some polysaccharides. Signup and view all the answers

Which statement accurately describes the consequence of a defect in xylulose reductase?

Xylulose can accumulate and lead to significant urinary excretion. Signup and view all the answers

In the context of the uronic acid pathway, which drug is noted to elevate glucose conversion rates to glucuronate?

Chlorobutanol Signup and view all the answers

What can xylulose, as a reducing sugar, cause when measured using alkaline copper reagents?

False positive results for glucose. Signup and view all the answers

Which polysaccharide biosynthesis component is formed by oxidizing UDP-glucose?

UDP-glucuronate Signup and view all the answers

What condition is characterized by the presence of excess xylulose in urine?

Essential Pentosuria Signup and view all the answers

Which enzyme primarily phosphorylates fructose in the liver?

Fructokinase Signup and view all the answers

What is the product when fructose is phosphorylated by fructokinase?

Fructose 1-phosphate Signup and view all the answers

Which statement is true regarding hexokinase's affinity for fructose?

It has a high Km for fructose. Signup and view all the answers

In which tissue is fructokinase predominantly active?

Liver Signup and view all the answers

Which substrate do all hexokinases, including hexokinase itself, primarily phosphorylate?

Glucose Signup and view all the answers

How does the activity of fructokinase respond to changes in feeding and fasting states?

It remains unchanged in feeding-fasting cycles. Signup and view all the answers

Which compound serves as the phosphate donor for the fructokinase reaction?

ATP Signup and view all the answers

Which of the following sugars is considered the most common monosaccharide consumed by humans?

Glucose Signup and view all the answers

Which statement best describes the activity of hexokinase in relation to fructose and other hexoses?

Hexokinase can phosphorylate several hexoses including fructose. Signup and view all the answers

What is the main function of protein A (β-D-Galactosyltransferase) in lactating mammary glands?

To transfer galactose from UDP-galactose to N-acetyllactosamine Signup and view all the answers

Which hormone's elevation stimulates the synthesis of galactosyl transferase and β-lactalbumin?

Prolactin Signup and view all the answers

Where is protein B, α-lactalbumin, predominantly found?

In lactating mammary glands Signup and view all the answers

What type of linkage is formed between galactose and glucose in lactose production?

β-1,4 linkage Signup and view all the answers

What role does lactase play in the digestion of lactose?

It hydrolyzes lactose into glucose and galactose Signup and view all the answers

Which form of lactase is typically present in adults?

Mature lactase Signup and view all the answers

What is a common result of lactose intolerance?

Inability to hydrolyze lactose effectively Signup and view all the answers

Which compound does protein A preferentially transfer galactose to in non-lactating tissues?

N-acetyl-D-glucosamine Signup and view all the answers

What triggers the synthesis of prolactin after birth?

Significant drop in progesterone Signup and view all the answers

Which of the following components is formed when galactose is transferred by protein A in lactating tissues?

Lactose Signup and view all the answers

Study Notes

Introduction to Metabolism of Monosaccharides and Disaccharides

Glucose is the most commonly consumed monosaccharide in humans.
Fructose and galactose serve as significant cellular energy sources.
Metabolism pathways involve various enzymes influencing monosaccharide metabolism.

Phosphorylation of Fructose

Hexokinase:
- Phosphorylates glucose in all cells; acts on various hexoses.
- Low affinity for fructose (high Km), resulting in minimal direct conversion.
- Converts a small amount of fructose to fructose 6-phosphate (F6P) in muscle.
Fructokinase (Ketohexokinase):
- Primary enzyme for fructose phosphorylation.
- Converts fructose to fructose 1-phosphate using ATP as a phosphate donor.
- Located mainly in the liver, kidney, and small intestinal mucosa.
- Its activity remains constant regardless of nutritional state or insulin levels.

Galactose Metabolism and Disorders

Galactosemia:
- Results from enzyme deficiencies leading to elevated galactose levels in blood and tissues.
- Symptoms include vomiting, diarrhea, jaundice, and overall failure to thrive in infants.
Galactokinase Deficiency:
- Accumulation of galactose leads to cataract formation due to reduced galactitol by aldose reductase in the lens.
Aldose Reductase Effects:
- Present in many tissues but physiologically insignificant unless galactose levels become excessively high.
Diagnosis:
- Urinalysis shows reducing sugars without glucose.
- Elevated levels of galactose in urine and serum.
- Galactose 1-Phosphate Uridyl Transferase assay confirms enzyme deficiency.
Treatment:
- Galactose-free diet; avoidance of milk and dairy products as the patient matures.

Lactose Metabolism

Lactose Synthesis:
- Primarily involves lactose synthase, which transfers galactose from UDP-galactose to glucose.
- Involves protein A (β-D-Galactosyltransferase) and protein B (α-lactalbumin) in mammary glands.
Hormonal Control:
- Before and during pregnancy, mammary glands synthesize N-acetyllactosamine.
- Progesterone inhibits protein B synthesis during pregnancy, but prolactin levels rise post-birth, stimulating lactose production.
Lactose Utilization:
- Lactase enzyme hydrolyzes lactose into galactose and glucose in the intestines.
- Two forms of lactase exist, specific to infants and adults.
Lactose Intolerance:
- Occurs due to insufficient lactase enzyme leading to digestive issues when consuming lactose.

UDP-Glucuronate Metabolism

Important for synthesizing polysaccharides like heparin.
UDP-glucuronate is derived from UDP-glucose and serves as a donor for polysaccharide components.

Essential Pentosuria

Rare hereditary condition marked by excessive xylulose in urine due to xylulose reductase deficiency.
Can lead to false positives in urinary glucose tests.
Certain drugs can enhance glucose entry into the uronic acid pathway, increasing conversion rates to glucuronate and other compounds.

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Description

Explore the metabolism of monosaccharides and disaccharides in this biochemistry quiz. Delve into the critical processes such as D-phosphorylation of fructose and the role of specific enzymes like hexokinase. Test your knowledge and understanding of essential metabolic pathways.