Glycolysis

Glycolysis is the first phase of a series of reactions for the catabolism of carbohydrates. Catabolism is the breakdown of larger molecules into its respective smaller constituents. Glycolysis is the first part of cellular respiration that generates pyruvate to be used in either anaerobic respiration in the absence of oxygen or in the TCA cycle in aerobic respiration which yields useable energy for cells. This will be a general outline of the steps in glycolysis.

The whole process can be broken down into an energy investment phase where ATP is being used and an energy payoff phase where ATP is being generated. Fructose-1,6-biphosphate is where the energy investment phase ends. That is where the last ATP has to be used for energy to drive glycolysis.

A simple equation can be remembered as a summary of glycolysis.

Glucose + 2 ADP + 2 phosphate ions + 2 NAD+ —-> 2 Pyruvate + 2 ATP + 2 NADH + 2 H20 + 2 H+.

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In the first step of glycolysis an addition of a high energy phosphate from ATP yields glucose-6-phosphate and ADP. This step is initialized by the enzyme hexokinase. G6P is more reactive than glucose.

In the next step, glucose-6-phosphate is converted to its isomer, fructose-6-phosphate by phosphoglucose isomerase.

In the third step of glycolysis, fructose-6-phosphate is converted to fructose-1,6-biphosphate by phosphofructokinase (PFK) and the addition of ATP. This is the committed step, meaning that fructose-1,6-biphosphate MUST be converted to pyruvate. This is also the end of the energy investment phase of glycolysis.

Fructose-1,6-biphosphate is converted to glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate catalyzed by aldolase. Glyceraldehyde-3 phosphate maintains a reversible reaction with dihydroxyacetone phosphate through triode phosphate isomerase. The resulting reaction generates two molecules of glyceraldehyde-3-phosphate. A key point going forward is that two molecules of each substrate are produced.

Each G3P molecule gains an inorganic phosphate and with the addition of NAD+ to form the energized carrier molecules NADH. The resulting reaction catalyzed by glyeraldehyde-3-phosphate dehydrogenase generates two molecules of 1,3-bisphophoglycerate which yield two high energy phosphates.

Through the addition of two low energy ADP molecules and the enzyme phosphoglycerate kinase, the two molecules of 1,3-bisphophoglycerate are converted to 3-phosphoglycerate and yields two molecules of ATP. This reaction is called the break even reaction because at this point the energy input is equal to the energy output. Two molecules of ATP were expended and at this step there was a generation of two ATP molecules.

In the next step the two molecules of 3-phosphoglyercate are converted to 2-phosphoglycerate through the enzymatic properties of phosphoglycerate mutase.

The molecules of 2-phosphoglycerate are converted to phosphoenolpyruvate catalyzed by enolase. This step yields H20 molecules.

In the final step of glycolysis, the molecules of phosphoenolpyruvate are converted to pyruvate catalyzed by pyruvate kinase. ATP is generated from the addition of ADP and the two high energy phosphates from the molecules of phosphoenolpyruvate.

Upon the completion of glycolysis, the pyruvate molecules can be oxidized to carbon dioxide in cellular respiration to generate 28 molecules of ATP.

The NADH that is produced is turned back into NAD+ to drive further glycolysis. There are two ways to accomplish this. In the presence of oxygen NADH passes it electrons into the electron transport chain, which regenerates NAD+ for use in glycolysis. In the absence of oxygen, cells regenerate NAD+ by undergoing fermentation.

 

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Renal Function Markers

Proper renal function is important in normal homeostasis as they excrete waste products and remove excess fluid through steps of excretion and reabsorption. The kidneys regulate the bodies electrolytes as well as produce hormones such as EPO that stimulates the bone marrow to produce erythrocytes. The kidneys produce a second hormone called renin from the juxtaglomerular cells located in the renal arteries. When renin is secreted it acts on angiotensinogen and converts it to angiotensin I. Angiotensin I is then converted to angiotensin II by angiotensin converting enzyme. Angiotensin II acts on blood vessels and causes vasoconstriction that raises blood pressure.

The kidneys play such an important role in normal physiology that its imperative that they are kept functioning properly. It is common to have screening tests done annually to evaluate renal function. It is not uncommon for a physician to order a renal function test to role out chronic kidney disease (CKD). There are few tests that are important and can paint a picture as to how well the kidneys are functioning. Blood urea nitrogen (BUN) provides a rough measurement of the GFR. Urea is formed in the liver as an end product to protein metabolism. It is a breakdown product from use of amino acids. In impaired renal function, the kidneys will inadequately excrete urea, which elevates blood BUN levels. Serum creatinine is another important indicator of renal health because it is solely excreted by the kidneys. Creatinine is a waste product created by muscle metabolism. Creatinine is synthesized via creatine, phosphocreatine, and adenosine triphosphate (ATP). Creatine is synthesized in the liver and is transported through blood to the other major organs where through phosphorylation is converted to phosphocreatine. Creatine becomes phosphocreatine through a catalytic reaction by creatine kinase. The by-product produced by that reaction is creatinine. Little to no tubular reabsorption of creatinine occurs so if there are elevated levels detected in the blood, it is an indicator of renal impairment. The creatinine levels in the blood and urine can be used to calculate the creatinine clearance which correlates to the GFR.

It is important to note that a creatinine concentration in urine may also be tested during a drug of abuse screen. Normal creatinine levels indicate a test sample is undiluted, therefore if there are decreased levels of creatinine it indicates a manipulated test and the test must be repeated.

The GFR describes the flow rate of filtered fluid through the glomerular capillaries into the Bowman’s capsule per unit time. Its important to note that a normal GFR level decreases with age so that must be taken into account when screening patients with suspected CKD, for example the reference range for GFR in adults age 20-29 is 116, in adults 60-69, the GFR should be around 85. A physician can also properly stage CKD based on ones GFR. A progressively decreasing GFR indicates disease progression and more aggressive treatment needs to be considered. GFR is measured typically using a patients creatinine level in accordance with there age, sex, and body size. There are multiple equations that can be used that have all been validated, but are slightly different. Certain physicians or hospitals may have standardized ways of calculating the GFR. There is the Bedside Schwartz equation which should be used for patients 18 years of age and younger. The Modification of Diet and Renal Disease (MDRD study equation and the Chronic Kidney Disease Epidemiology Collabortion (CKD-EPI) equation are the most commonly used for adults 18 and older.

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Physicians may order microalbumin testing to screen individuals who are at high risk for developing CKD, especially diabetics. A urine microalbumin test detects minute levels of albumin in the urine. Albumin is one of the first proteins that be detected in the urine when renal function becomes impaired. Albumin is part of the globular protein family whose main function is to regulate the colloid osmotic pressure. Albumin also serves as a protein carrier for hydrophobic molecules such as lipid-soluble hormones, unconjugated bilirubin, free fatty acids, as well as some types of particular drugs like warfarin and phenytoin.

The kidneys are arguably one of the most important organs in homeostasis and its important that they are functioning properly. There are number of tests that can be performed to test renal function with each one giving a little piece of the puzzle. Physicians can use these tests to rule out CKD, or stage a patients disease progression.

-Caleb