Metabolic Pathways, Energy Balance, and Obesity

Comprehensive vocabulary flashcards covering cellular metabolic pathways, bioenergetics, body composition, hunger and satiety regulation, and obesity pathophysiology and treatment.

Hexokinase

The glycolytic enzyme that catalyzes the irreversible first phosphorylation of glucose into glucose 6-phosphate using $1\,\text{ATP}$.

Phosphofructokinase

A key regulatory and irreversible enzyme in glycolysis that phosphorylates fructose 6-phosphate into fructose 1,6-bisphosphate by consuming $1\,\text{ATP}$.

Pyruvate Kinase

The terminal glycolytic enzyme that catalyzes the irreversible conversion of phosphoenolpyruvate (PEP) to pyruvate, transferring a phosphate group to $ADP$ to produce $ATP$.

Pyruvate Dehydrogenase (PDH) Complex

A multi-enzyme mitochondrial complex (comprising E1, E2, and E3 domains) that oxidatively decarboxylates pyruvate to form acetyl-CoA, $CO_2$, and $NADH + H^+$, linking glycolysis to the citric acid cycle.

Citrate Synthase

The citric acid cycle enzyme that catalyzes the Claisen condensation of oxaloacetate and acetyl-CoA with $H_2O$ to produce citrate and release CoA-SH.

Isocitrate Dehydrogenase

An enzyme of the citric acid cycle that catalyzes the oxidative decarboxylation of isocitrate to $\alpha$-ketoglutarate, producing $CO_2$ and $NADH$.

Cori Cycle

A metabolic pathway in which lactate generated by anaerobic glycolysis in active muscle is transported through the bloodstream to the liver to be converted back into glucose at the cost of $6\,\text{ATP}$, while yielding $2\,\text{ATP}$ in muscle.

Carnitine Palmitoyltransferase 1 (CPT1)

An outer mitochondrial membrane enzyme that converts fatty acyl-CoA and carnitine into acylcarnitine to facilitate the entry of fatty acids into the mitochondria for $\beta$-oxidation.

$\beta$-Oxidation

The repetitive, four-step mitochondrial pathway (acyl-CoA dehydrogenase, enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, and thiolase) that degrades fatty acyl-CoA into acetyl-CoA, $FADH_2$, and $NADH$.

Ketogenesis

The hepatic pathway in which excess acetyl-CoA is converted into ketone bodies—acetoacetate, $D\text{-}\beta\text{-hydroxybutyrate}$, and acetone—during periods of low carbohydrate availability or high fatty acid breakdown.

HMG-CoA Synthase

An enzyme in ketogenesis that condenses acetoacetyl-CoA with a second acetyl-CoA molecule to produce $\beta$-hydroxy-$\beta$-methylglutaryl-CoA (HMG-CoA).

Lipogenesis

The metabolic process through which excess dietary carbohydrates are converted into fatty acids, utilizing cytosolic acetyl-CoA cleaved from mitochondrial citrate by ATP citrate lyase (ACLY).

Glucogenic Amino Acids

Amino acids whose carbon skeletons can be converted into pyruvate or TCA cycle intermediates ($\alpha$-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate) to serve as precursors for glucose synthesis.

Ketogenic Amino Acids

Amino acids (strictly leucine and lysine, plus several that are both) whose carbon skeletons are degraded directly into acetyl-CoA or acetoacetyl-CoA for ketone body synthesis.

Total Energy Expenditure (TEE)

The total caloric output expended by the body each day, consisting of resting energy expenditure (REE), physical activity, and the thermic effect of food (TEF).

Basal Metabolic Rate (BMR)

The energy expended to maintain vital involuntary life-sustaining bodily functions (breathing, heart beat, liver, kidneys, brain) measured at least $12\,\text{hours}$ after the last meal upon waking.

Resting Energy Expenditure (REE)

The calories burned while sitting or resting that make up the vast majority of total calories burned in a day, measured under less restrictive conditions than basal metabolic rate.

Thermic Effect of Food (TEF)

The energetic cost of digesting, absorbing, transporting, metabolizing, and storing nutrients from food, accounting for approximately $10\%$ of total meal calories burned and slightly higher for dietary protein.

Body Mass Index (BMI)

A standardized clinical indicator of body fat calculated as weight in kilograms divided by height in meters squared ($\frac{\text{weight in kg}}{(\text{height in m})^2}$).

BMI Weight Classifications

Standard clinical BMI strata: underweight ($< 18.5\,\text{kg/m}^2$), normal weight ($18.5\text{--}24.9\,\text{kg/m}^2$), overweight ($25\text{--}29.5\,\text{kg/m}^2$), and obese ($> 30\,\text{kg/m}^2$).

Osteoporosis

A medical condition characterized by reduced bone mass and density leading to increased risk of fractures, often associated with underweight status ($BMI < 18.5\,\text{kg/m}^2$), older age, female sex, tobacco use, and physical inactivity.

Essential Fat

The structural and physiological lipid stores required for normal organ and systemic function, representing a higher percentage of body mass in women than in men.

Storage Fat

Lipid reserves found in adipose tissue, subcutaneously beneath the skin, and around internal organs to cushion and insulate the body.

Android Body Fat Distribution

An 'apple-shaped' body fat distribution characterized by visceral fat accumulated around abdominal organs, which carries a significantly increased risk for chronic metabolic and cardiovascular diseases.

Gynoid Body Fat Distribution

A 'pear-shaped' body fat distribution characterized by subcutaneous fat deposited over muscles and under the skin around the hips and thighs.

Set Point Theory

The hypothesis proposing that human body weight is biologically regulated around a predetermined target weight through homeostatic adjustments in energy expenditure and appetite.

Adipocytes

Fat-storing cells that can alter in both total number and cell diameter during periods of positive energy balance and weight gain.

Leptin

A satiety hormone secreted by adipocytes in proportion to fat stores that signals the hypothalamus to reduce food intake and increase energy expenditure.

Ghrelin

An orexigenic peptide hormone produced by P/D1 cells in the stomach lining when empty; its circulating concentration rises before meals and increases with weight loss.

Neuropeptide Y (NPY)

A neurotransmitter synthesized by hypothalamic neurons that stimulates hunger and food intake, increases blood glucose, promotes lipid storage, and decreases anxiety, pain, and energy expenditure.

Agouti-Related Peptide (AgRP)

An orexigenic neuropeptide released by hypothalamic neurons alongside neuropeptide Y to stimulate appetite and promote food consumption.

Cholecystokinin (CCK)

A satiety peptide hormone released from duodenal I cells that triggers gallbladder contraction for bile secretion, pancreatic digestive enzyme release, and gastrointestinal motility.

Glucagon-Like Peptide-1 (GLP-1)

An incretin hormone released from intestinal L cells that enhances glucose-dependent insulin secretion, slows gastric emptying, and decreases appetite.

Peptide YY

An anorexigenic peptide secreted by intestinal L cells in response to feeding that acts on the brain to signal fullness and reduce food intake.

Gastrin

A gastrointestinal hormone secreted by gastric G cells that stimulates hydrochloric acid ($\text{HCl}$) production and promotes stomach churning motility.

Secretin

An enteroendocrine hormone secreted by intestinal S cells that stimulates the exocrine pancreas to secrete bicarbonate to neutralize acidic chyme.

Bariatric Surgery Candidates

Individuals with a $BMI > 40\,\text{kg/m}^2$, or a $BMI > 35\,\text{kg/m}^2$ accompanied by severe obesity-related health comorbidities.

Vertical Sleeve Gastrectomy

A restrictive surgical procedure in which a substantial portion of the stomach is excised, leaving behind a narrow, sleeve-like tube that restricts food intake.

Roux-en-Y Gastric Bypass

A dual restrictive and malabsorptive bariatric surgery where a small gastric pouch is created from the upper stomach and attached directly to a bypassed segment of the small intestine.

Childhood Obesity Treatment Recommendation

A clinical guideline advising weight maintenance rather than severe caloric restriction, allowing overweight children to maintain their weight while growing in height.